K E Akoda

A Guédon-Gracia

J.-Y Delétage

B Plano

H Frémont

Impact of temperature on the corrosion of lead-free solder alloy during salt spray test

This paper presents the impact of temperature on the corrosion behaviour of lead-free solder alloy during neutral salt spray (NSS) test. The focus is made on the Sn-Ag-Cu (SAC305) solder alloy Ball Grid Array components. Components were aged during 24, 48, 72, and 96 hours in a salt spray chamber Ascott at 35°C with 5% sodium chloride (NaCl) solution at temperatures ranging from 25°C to 45°C to investigate the effect of temperature on the corrosion. Two observables corrosion criteria were defined for this analysis. Solder balls deterioration is visible after 72, 48, and 24 hours, respectively, at 25°C, 35°C, and 45°C. This shows that the temperature has a significant effect on the corrosion behaviour of lead-free solder balls. An increase in temperature from 25°C to 35°C and 45°C led to an earlier onset of corrosion. Tin (Sn), oxygen (O), and chloride ion (Cl) are found on solder balls. Finally, thermal "Arrhenius Law" is used to estimate an acceleration factor (AF) and an indicative value of the activation energy (Ea) after 96 hours of exposure. The kinetic of the corrosion failure mechanisms is being investigated in order to predict lead-free solder balls lifetime.

Introduction

Electronic assemblies of some equipment are exposed to a salt environment. However, few data have been published on the impact of temperature on the corrosion of Sn-Ag-Cu (tinsilver-copper) solder, solder joint materials, and microelectronic assemblies under these conditions [START_REF] Guédon-Gracia | Corrosion study on BGA assemblies[END_REF]. During electronic devices' operations, heat and corrosive agents can affect the solder and decrease the expected lifetimes. As an example, harsh marine environment can seriously affect the properties of the solder alloys during operations [START_REF] See | Corrosion Behaviour of Corroded Sn-3.0Ag-0.5Cu Solder Alloy[END_REF].

The reliability of the solder is important to determine the performance and lifetime of electronic assemblies when subjected to temperature and corrosive environments [START_REF] Affendy | Corrosion of Sn-9Zn Solder Joints: A Review[END_REF][START_REF] Mohamed | Polarization Study of Sn-9Zn and Sn-37Pb Solders in Hydrochloric Acid Solution[END_REF]. Thus, knowledge of the effect of temperature on the corrosion behaviour of the solder alloy in a salt environment is essential to prevent failure and malfunction of an electronic device [START_REF] Rosalbino | Electrochemical corrosion study of Sn-3Ag-3Cu solder alloy in NaCl solution[END_REF].

Many studies have been conducted to investigate the effect of corrosion and study the physical process mechanism on the Sn96.5-Ag3.0-Cu0.5 (SAC305) solder alloy. Most studies have applied the potentiodynamic polarization and electrochemical impedance spectroscopy [START_REF] Rosalbino | Electrochemical corrosion study of Sn-3Ag-3Cu solder alloy in NaCl solution[END_REF][START_REF] Li | Corrosion characterization of tin-lead and lead-free solders in 3, 5 wt. % NaCl solution[END_REF][START_REF] Rosalbino | Electrochemical corrosion behaviour of Sn-Ag-Cu (SAC) eutectic alloy in a chloride containing environment[END_REF][START_REF] See | Corrosion Behavior of Corroded Sn-3.0Ag-0.5Cu Solder Alloy[END_REF][START_REF] Wang | Corrosion behavior of Sn-3.0Ag-0.5Cu lead-free solder joints[END_REF][START_REF] Wang | Microstructure induced galvanic corrosion evolution of SAC305 solder alloys in simulated marine atmosphere[END_REF][START_REF]Electrochemical Corrosion Behavior of Pb-free SAC 105 and SAC 305 Solder Alloys : A Comparative Study[END_REF][START_REF] Qiao | In-situ EIS study on the initial corrosion evolution behavior of SAC305 solder alloy covered with NaCl solution[END_REF] to determine the corrosion resistance and assess the corrosion behaviour of a solid-state device. Fewer studies have applied the salt spray test to study corrosion behaviour and propose corrosion models [START_REF] Guédon-Gracia | Corrosion study on BGA assemblies[END_REF][START_REF] Meinshausen | Dynamical IMC-growth calculation[END_REF][START_REF] Weide-Zaage | Harsh Marine Environment-Toward Corrosion Simulation[END_REF][START_REF] Guédon-Gracia | Effects of salt spray test on lead-free solder alloy[END_REF]. However, acceleration factors, physical mechanisms, and a corrosion model based on experimental data are still unknown. Furthermore, none of these studies investigated the impact of temperature on the corrosion of lead-free solder during salt spray test.

The purpose of this study is to investigate the evolution of corrosion kinetics on the SAC305 solder and then to calculate the acceleration factor (AF) in the salt spray test by changing the temperature. The ageing tests were performed on BGA (Ball Grid Array) components in an Ascott salt spray chamber.

Experimental setup

Materials

Experiments were carried out on the Sn96.5-Ag3.0-Cu0.5 (SAC305) solder balls of commercial BGAs components provided by Amkor. They contain 128 balls per BGA, and measure 12x12 mm2. The circuits were aged to examine the impact of temperature on the corrosion of the alloy. The non-aged BGAs are referred to as "as-received sample" (ARS) hereafter.

Methods

To investigate the influence of temperature on corrosion, ARSs were aged during 24, 48, 72, and 96 hours in a salt spray chamber (Fig. 1) according to ASTM B117 standard (5% NaCl at T=35°C), and at 25°C and 45°C with the standard salinity. The advantages of the salt spray chamber are its temperature homogeneity (T±2°C) and the possibility to study the corrosion of various metals and surfaces without interference in chemical reactions. So, the test conditions are reproducible. Moreover, this equipment is the most used for industrial tests. The chosen temperature range may seem very narrow.

However, this kind of equipment is designed to perform a standard robustness test at 35°C and does not permit tests at higher temperatures.

Nevertheless, the effect of temperature in accelerating saline corrosion phenomena on SAC is evaluated for the first time in the present study.

After being removed from the chamber, samples were washed in deionized water and dried with a stream of clean, compressed air. They were then mounted in resin, grounded using SIC papers (P1000-P1200), and finely polished using diamond liquids (9µm-6µm-3µm-1µm). A cross-section of the treated samples was examined by optical microscopy (OM), scanning electron microscopy (SEM).

For each testing condition, 18 solder balls were observed and, statistical information on the percentage of corroded solder was obtained for each exposure time. Two observable corrosion criteria were defined for the OM and SEM analyses. The percentage of corroded solder balls was defined as the ratio of the number of solder balls corroded after the salt spray test to the total number of solder balls. The chemical composition and distribution of elements in the corroded area were determined by energy dispersive X-ray (EDX) analyses.

Results and discussion

Definition of corrosion criteria

Fig. 2 shows the cross-section of solder balls before and after the test. Compared to the solder ball at the initial state, the corrosion is measured by evaluating the eroded area. Image J analysis software is used to evaluate the percentage of the eroded area. For instance, in the picture (Fig. 2b), the corroded area is about 22.5% of the total solder surface. A ball is considered as corroded if at least 2.5 % of the material has disappeared.

Corrosion is also detectable by a change in color. The color changing (silver gray) of a part of the ball from its initial state (Fig. 2a) to dark gray allows us to highlight the corroded state of that ball (Fig. 2b). Song and Lee observed similar corrosion behaviour [START_REF] Song | Corrosion of Sn-Ag-Cu lead-free solders and the corresponding effects on board level solder joint reliability[END_REF].

Analyses and observations

Fig. 3 shows the corrosion morphology observation and the corroded area behaviour for OM analyses of the ARS solder balls exposed to a 5% NaCl salt spray at 25°C (Fig. 3a), 35°C (Fig. 3b), and 45°C (Fig. 3c) as a function of exposure time (0, 24, 48, 72 and 96 hours). Corroded areas are visible on the lead-free solder balls after 72h at 25°C, 48h at 35°C, and only 24 h, at 45°C. At 35°C, after 48 h, samples have some corroded areas. These areas become larger after 72 h. After 96 h, 67% of the observed surface is corroded. Song and Lee [START_REF] Song | Corrosion of Sn-Ag-Cu lead-free solders and the corresponding effects on board level solder joint reliability[END_REF] have observed a similar evolution of the corrosion on Sn-Ag-Cu solder balls in the same condition (5% NaCl at 35°C).

At 45°C, after 24 h, samples have some corroded areas. These areas become larger after 48 h and 72 h. After 96 h, 78 % of the observed surface is corroded. An increase in temperature leads to an earlier onset of corrosion. So, these experiments clearly demonstrate that temperature has a significant effect on the corrosion behaviour of lead-free solder balls.

Another observed phenomenon concerns the corrosion location. The ball surface is not uniformly attacked. There is only one corrosion product site per ball. The corrosion site is different from one ball to another. Fig. 4 shows EDX results of the corroded area sample after 48 hours of exposure in salt spray following the ASTM B117 standard condition (5% NaCl at T=35°C). According to the EDX analysis, the corrosion product mainly contains Sn, O, and Cl.

Acceleration factor estimation

Even if our results are only obtained by sampling at regular times and not by real-time evolution measurement, they allow us to have a first estimation of the accelerating factor. The relatively large number of observed solder balls (18 by measurement) allows, however, to have some confidence in the results.

The acceleration factor (AF) model that links the failure time (TTF) to temperature is based on the Arrhenius equation:

TTF = A.exp (Ea/kT) (1)
Despite the fact that corroded surfaces may be from 2.5% to 48% and despite the small amount of sampling times, a first estimation of the activation energy is proposed.

Fig. 5 shows the percentage Ρc of corroded solder balls as a function of temperature and the different exposure times defined as:

Pc = Nc/Nt (2)
Nc is the number of corroded solder balls after the test. Nt is the total number of solder balls. The data obtained at 25°C shows non-linearity, which can be explained either by the difficulty to actually assess the exact percentage of corroded areas, as at this temperature the corrosion is less severe. Or it is due to a different chemical corrosion mechanism. As a first approximation, we select the first hypothesis and linearize all the graphs according to the dashed lines in figure 5. Then we calculated the AF and Ea, using the following equation:

t50 = A.exp (Ea/kT) ( 3
)
where t50 is the time after fifty percent of the sample are corroded. So now we can use it to calculate the Ea by plotting lnt50 versus 1/ (T °C + 273.16). This will yield an Ea estimate of 0.86 eV. The regression parameter R² is low, due to the imprecision previously described. This value is obviously the first estimate. However, according to the corrosion mechanism orders of magnitude for Ea (0.3 eV to 1.1 eV) [START_REF]Power Semiconductor Reliability Handbook Alpha and Omega Semiconductor[END_REF], this Ea value is promising. The kinetic of the corrosion failure mechanisms is being investigated to predict the lifetime of the leadfree solder balls.

Discussion

The discussion hereafter focuses on the influence of temperature on the corrosion behaviour of Sn-Ag-Cu (SAC305) solder alloy of ball grid array components during salt spray test. Based on results of [START_REF] Qiao | In-situ EIS study on the initial corrosion evolution behavior of SAC305 solder alloy covered with NaCl solution[END_REF], a descriptive mechanism is proposed to explain the role played by temperature.

Just after assembly, SAC305 solder exposure to indoor or outdoor environment are gradually oxidized by oxygen to form a protective SnO2 film on the solder surface. It is believed to be a result of direct chemical reaction between Sn and O2 see equation ( 4):

Sn + O2 = SnO2 (4)
This protected film degrades in a short period once in contact with NaCl solution. The anodic and cathodic processes are described, respectively with the following equations:

Sn = Sn 2+ + 2e - (5) 2H2O + O2 +4e -= 4OH - (6)
The hydrolysis of dissolved Sn 2+ under the effect of concentrated Cl -forms the corrosion product, Sn3O(OH)2Cl2, which is a complex oxide chloride hydroxide of tin according to the following reaction [START_REF] Li | Corrosion characterization of tin-lead and lead-free solders in 3, 5 wt. % NaCl solution[END_REF][START_REF] Qiao | In-situ EIS study on the initial corrosion evolution behavior of SAC305 solder alloy covered with NaCl solution[END_REF]:

3Sn + 4OH -+2Cl -= Sn3O(OH)2Cl2 +H2O (7)
Most of the defect-related failures and wearout mechanisms can be accelerated by using temperatures higher than the temperature in operation. The induced failure usually involves some chemical or physical reactions [18]. Our study shows similar accelerated ageing processes on the solder balls when the temperature is increased from 25°C to 45°C during the NSS test.

Conclusion

We have presented an experimental study of the impact of temperature on the corrosion behaviour of BGA samples in a NaCl humid atmosphere. Observable corrosion criteria were defined. From our test results, the following conclusions are drawn:

• The ball surface is not uniformly attacked.

• There is only one corrosion product site per ball, which varies from ball to ball. • The corroded area observed in the BGAs solder balls mainly contain Sn, O, and Cl.

Moreover, the temperature has a significant effect on the corrosion behaviour of lead-free solder balls. An increase in temperature from 25°C, 35 °C to 45°C results in an earlier onset of corrosion. A first estimation of the activation energy was calculated. Raman and XRD characterization and X-ray inspections will be performed before and after the spray test to determine the corrosion product composition and examine the presence and evolution of voids or porosities of microshrinkage on the surface of solder joint balls.

In addition, other investigations are currently being carried out on the solder joint of other components in order to:

• compare and confirm BGAs results • investigate more precisely the corrosion kinetics to finally predict the lifetime of leadfree assemblies in saline corrosive environments • develop a model of the corrosion phenomena for simulation.

Fig. 1 .

 1 Fig. 1. Salt spray chamber

Fig. 2 .

 2 Fig. 2. Cross-section of (a) initial state and (b) corroded state of the BGA as-received samples

Fig. 3 .

 3 Fig. 3. Corrosion morphology observation change of the as-received SAC305 solder balls At 25°C, after 72 h of ageing, samples have some corroded areas. These areas become larger

Fig. 5 .

 5 Fig. 5. Percentage of corroded balls as a function of the exposure time of the BGAs as-received samples

Fig. 6 .

 6 Fig. 6. Lnt50 as a function of the 1/ (T °C + 273.16)


Gilles Grolleau

Deborah Peterson

Marjorie Tendero
email: marjorie.tendero@essca.fr

Changing the World with Words? Euphemisms in Climate Change Issues

Keywords: climate change, climate policy, euphemisms, hypocrisy, words

Words matter when talking about climate change. They influence thinking and ultimately behaviors. We contend that certain kinds of words frequently used in climate change communication, namely euphemisms, can undermine the objectives of raising climate change awareness and changing behaviors to reduce emissions. We characterize euphemisms related to climate change issues and show how they are often manipulated to serve vested interests opposing climate change action. In particular, we highlight euphemistic names of astroturfing organizations that aim to persuade consumers or citizens. We conclude by suggesting some practical ways to prevent or avoid detrimental consequences associated with euphemisms and draw several policy implications.

Introduction

Words can seem unimportant when compared with the urgency and magnitude of climate change. Yet, recent research emphasizes that even subtle words changes can significantly influence opinion and behavior, well beyond what is usually assumed [START_REF] Drews | Degrowth: A "missile word" that backfires?[END_REF][START_REF] Clot | Payment vs. compensation for ecosystem services: Do words have a voice in the design of environmental conservation programs[END_REF][START_REF] Farrow | What in the word! The scope for the effect of word choice on economic behavior[END_REF][START_REF] Schuldt | Global warming" or "climate change"?: Whether the planet is warming depends on question wording[END_REF][START_REF] Shi | #Climatechange vs. #Globalwarming: Characterizing two competing climate discourses on Twitter with semantic network and temporal analyses[END_REF], which is well understood by politicians and journalists. In 2017, The Guardian revealed that staff at the US Department of Agriculture were instructed to blacklist terms such as "climate change", "climate change adaption", "reduce greenhouse gases" and "sequester carbon". They were instructed to use "weather extremes", "resilience to weather extremes", "build soil organic matter, increase nutrient use efficiency" and "build soil organic matter" [START_REF] Milman | US federal department is censoring use of term 'climate change', emails reveal[END_REF]. In 2019, the same journal -one of the world's most read newspaper websites -announced several word changes (Table 1) in favor of terms that they considered to more accurately describe the "environmental crises" (Zeldin-O'Neill, 2019). The Guardian justified these changes by "the urgency of climate crisis" requiring "robust new language to describe it" [START_REF] Chadwick | The urgency of climate crisis needed robust new language to describe it[END_REF].

Table 1. Word changes The Guardian recommended to its journalists and editors (Source: Zeldin-O' Neill, 2019) Previous expressions Recommended expressions "Climate change" "Climate emergency", "climate crisis" "Climate sceptic" "Climate science denier", "climate denier" "Global warming" "Global heating" "Carbon emissions", "carbon dioxide emissions" "Greenhouse gas emissions" "Biodiversity" "Wildlife" "Fish stocks" "Fish populations"

Words are not neutral [START_REF] Drews | Degrowth: A "missile word" that backfires?[END_REF][START_REF] Clot | Payment vs. compensation for ecosystem services: Do words have a voice in the design of environmental conservation programs[END_REF], and the effect of different labels (such as climate change versus climate crisis) is complicated. Their impact depends on factors such as the gender, worldview and the individual's values [START_REF] Hung | Comparing the effects of climate change labelling on reactions of the Taiwanese public[END_REF]. To increase policy effectiveness, beyond the objective policy content, we posit word choices in climate policy and communication should be carefully considered to elicit the desired behavioral changes. Indeed, words constitute an often-neglected lever that can be mobilized to undermine or reinforce policy on a number of levels such as numbing or awakening people, weakening, or reinforcing policy support, or delaying, or driving change.

After examining their considerable potential to support emissions reduction, we focus on euphemisms -words or phrases used to avoid another word or phrase that may be unpleasant or give offence1 -that are pervasive in climate change communication.

Why words matter when communicating on climate change?

Language and thought are intertwined. Subtle variations in wording can cause significantly affect judgment, memory and even behavior [START_REF] Clot | Payment vs. compensation for ecosystem services: Do words have a voice in the design of environmental conservation programs[END_REF][START_REF] Farrow | What in the word! The scope for the effect of word choice on economic behavior[END_REF][START_REF] Hauser | Semantic prosody and judgement[END_REF]. Policy words are not judged only on their objective meaning or content as predicted by rational models. Words also trigger automatic processes (such as heuristics and biases) that can make policies more or less effective. For instance, recent studies show that Covid-19 related policies may have been far more effective in achieving their desired outcomes by simply changing some of the words used [START_REF] Ajzenman | More than words: Leaders' speech and risky behavior during a pandemic[END_REF][START_REF] Padan | Coronavirus: On crisis, emergency, and the power of words[END_REF]Miller, 2020). According to Padan (2020), the term "crisis" did not encourage the adoption of preventive measures while the term "emergency" did.

Similarly, climate policy objectives can be better achieved by carefully selecting words that are more likely to achieve desired outcomes. The words chosen to describe a phenomenon are highly influential. For instance, [START_REF] Woods | The use of religious metaphors by UK newspapers to describe and denigrate climate change[END_REF] illustrated how unsympathetic religious metaphors used in 122 U.K. newspaper articles were used to misrepresent and denigrate scientific consensus about climate change [START_REF] Woods | The use of religious metaphors by UK newspapers to describe and denigrate climate change[END_REF]. [START_REF] Atanasova | Metaphors in Guardian Online and Mail Online opinionpage content on climate change: War, religion, and politics[END_REF] showed how the Guardian Online and Mail Online used the "war" metaphor to communicate the urgency of action to address climate change. [START_REF] Flusberg | Metaphors for the war (or race) against climate change[END_REF] investigated how skeptical attitudes toward climate change can be modified using metaphors. They found that the war metaphor is more effective than a "race" frame: it conveys a feeling of urgency and pushes participants to express a greater willingness to increase conservation behaviors. Rather than basing word choice only on scientific and legal grounds, policymakers should inform their policy framing and formulation with well-crafted studies and experiments at an early stage to understand how various formulations of a similar policy will affect thinking and behaviors of citizens and other targets. In tough budgetary times, words can offer a potentially less costly lever likely to provoke first-order consequences.

In the following sections, we examine a particular type of words and expressions that have become pervasive in climate change discussions, namely euphemisms. Although euphemisms can be beneficial or detrimental to climate-related changes, we focus on the most pervasive ones used by corporate and political actors to deflate climate change action.

Euphemisms are also used to preserve the recipient sensibilities such as with those of ecoanxious individuals, i.e., individuals anxious about climate emergency or who have persistent worries about environmental issues [START_REF] Whitcomb | Therapists are reckoning with eco-anxiety[END_REF]. However, this motivation is less frequent in the cases of corporate or political messages about climate change. Indeed, the previously mentioned uses correspond more to a characterization of euphemisms as "the language of evasion, hypocrisy, prudery, and deceit" (Holder, 2008, p. vii).

Euphemisms in the context of climate change

Three characteristics are fundamental to delineate euphemisms. First, euphemisms are based on word substitution. The substitution replaces one (or several) plain term(s) that are considered as undesirable with other terms that are judged more appropriate [START_REF] Casas Gómez | Towards a new approach to the linguistic definition of euphemism[END_REF]. This substitution often leads to euphemistic expressions that are often longer than their plain counterpart. For example, acid rain is referred to as "atmospheric deposition of anthropogenically derived substances". Word substitution typically mobilizes two kinds of framing effects: (i) the equivalency effect, where using different, but logically equivalent, expressions cause individuals to alter their preferences, and (ii) the emphasis effect where a subset of considerations is emphasized to push individuals to focus on some aspects of a situation, relegating other considerations to the background [START_REF] Druckman | The implications of framing effects for citizen competence[END_REF][START_REF] Farrow | Let's call a spade a spade, not a gardening tool': How euphemisms shape moral judgement in corporate social responsibility domains[END_REF]. We argue that most euphemisms used in climate change issues can be analyzed as examples of the emphasis framing effect used to push people to prioritize a certain set of meanings. For example, "fossil fuels" have been rebranded in a press release of the US Department of Energy as "molecules of US freedom" [START_REF] Ellsmor | Trump administration rebrands fossil fuels as "molecules of U.S. freedom[END_REF]. One approach to measuring the effect of such euphemisms could be asking study participants to express (and compare) their immediate thoughts, attitudes and behavioral intentions (e.g., willingness to act) when faced with plain terms and corresponding euphemisms (see, for example, [START_REF] Farrow | Let's call a spade a spade, not a gardening tool': How euphemisms shape moral judgement in corporate social responsibility domains[END_REF]. [START_REF] Kreps | Doing well by doing good? Ambivalent moral framing in organizations[END_REF] argued that individuals have idealized cognitive templates.

When a real-world situation matches these moral templates, individuals are alerted that they should use a moral or ethical frame to respond to the situation. This matching can be (dis)activated by subtle changes in framing. For instance, a plainer term may trigger moralization (e.g., smoggy areas) while a euphemistic alternative (e.g., ozone non-attainment area) may completely focus the attention to other considerations. Euphemisms are frequently used to prevent a match with an available moral template and facilitate moral disengagement [START_REF] Bandura | Moral disengagement in the perpetration of inhumanities[END_REF][START_REF] Tenbrunsel | Ethical fading: The role of self-deception in unethical behavior[END_REF].

In a well-cited paper, Lakoff (2010) emphasized that framing is both powerful and pervasive -all thought and communication involves framing. He defined environmental frames as "the (typically unconscious) conceptual structures that people have in their brain circuitry to understand environmental issues" (p. 74). While recognizing that words themselves are not frames, he emphasized that words can be selected to activate desired frames. Lakoff suggests that in environmental realm, interest groups that have understood the power of frames for a long time have applied consistent effort to build frames that can be easily activated through the use and repetition of key words and phrases. Frequent repetition of pre-existing dominant environmental frames by the media serve to reinforce their effectiveness. [START_REF] Lakoff | Why it matters how we frame the environment[END_REF] argued "What is needed is a constant effort to build up the background frames needed to understand the crisis, while building up neural circuitry to inhibit the wrong frames." (p. 74). We contend that words used to communicate messages about climate change often activate pre-existing frames that will not serve the pursued goals nor contribute to building new and adequate frames. The issue is complex, and we note that some authors (e.g., [START_REF] Matthes | What's in a frame? A content analysis of media framing studies in world's leading communication journals, 1990-2005[END_REF] criticize the misuse of framing theory while others in the context of environmental issues (e.g., [START_REF] Ytterstad | Framing global warming: Is that really the question? A realist, gramscian critique of the framing paradigm in media and communication research[END_REF] propose to abandon an overreliance on framing and advocate that "global warming is an ethical challenge which communication scholars can best help solve with a combination of natural realism and political advocacy." (p.

1).

Second, euphemisms frequently reduce the emotional content by creating distance and serve to soften what is conveyed by the plain terms [START_REF] Casas Gómez | Towards a new approach to the linguistic definition of euphemism[END_REF]. Euphemisms are a linguistic manipulation to make some communication about difficult topics (more) socially acceptable or to reframe some issues. While this sometimes comes at the expense of clarity, reframing can also support development of understanding, a positive attitude, better ability to recall the concept and reduced anxiety related to sought-after behavioral change. Substituting the more dramatic phrase "global warming" to "climate change" is an example [START_REF] Lineman | Talking about climate change and global warming[END_REF].

Third, euphemisms are frequently used in a social interaction where two or more parties communicate. Frequently used in self-interest, they can also be other-oriented, such as when they are employed to preserve the sensibilities of eco-anxious individuals [START_REF] Whitcomb | Therapists are reckoning with eco-anxiety[END_REF]. Euphemisms can also be used for self-deception. For instance, an individual can engage in unethical behaviors and preserve a positive self-image by relabeling his/her behaviors in sugar-coated terms that create emotional distance [START_REF] Bandura | Moral disengagement in the perpetration of inhumanities[END_REF][START_REF] Tenbrunsel | Ethical fading: The role of self-deception in unethical behavior[END_REF]. For example, "routine exceedances" refer to an industrial plant's regular violation of clean air or water standards [START_REF] Beans | Eco euphemisms that confuse our understanding of environmental destruction[END_REF].

Euphemisms are also distinguished on a variety of dimensions such as valence (positive versus negative), areas of application, degree of consciousness, geographical, cultural, and historical contexts [START_REF] Burridge | Euphemism and language change: The sixth and seventh ages[END_REF]. A repeatedly used euphemism often loses its euphemistic power. The euphemism can become tainted over time, leading to a euphemistic treadmill where a new euphemism replaces the tainted one and so forth [START_REF] Burridge | Euphemism and language change: The sixth and seventh ages[END_REF]. Several euphemisms have been devised intentionally and consciously by climate change denial promoters to reduce the emotional appeal and deflate the concerns related to climate change issues. In the same vein, euphemisms can reduce (or even counteract) the impact of fear-based campaigns [START_REF] Tannenbaum | Appealing to fear: A meta-analysis of fear appeal effectiveness and theories[END_REF], to raise climate change awareness and actions. Moreover, the true intent of the euphemism user can remain hidden. Following the classification of euphemism functions by [START_REF] Burridge | Euphemism and language change: The sixth and seventh ages[END_REF], we apply them to climate change issues using real-world examples (Table 2). To act as an in-group trademark "Clean coal" or "ethical oil" among promoters of fossil fuels

Ludic

To have fun and entertain "A $100,000 fine was not a fine but (…) a "donation" to the region's air quality"

Going further, we introduce a two-dimensional framework: (i) whether the euphemism reinforces the truth or available evidence or undermines the truth or available evidence and

(ii) the likely effects of the euphemisms on climate change (positive versus negative). diminishing the perceived danger of situations, increasing the perceived moral permissibility of an action, and reducing public accountability for unethical behaviors [START_REF] Gladney | Euphemistic text affects attitudes, behavior[END_REF][START_REF] Rittenburg | The effects of euphemism usage in business contexts[END_REF]. 2 An interesting feature of well-crafted euphemisms is their ability to avoid lies and contradictory statements when addressing various stakeholders (La [START_REF] Cour | Euphemisms and hypocrisy in corporate philanthropy[END_REF][START_REF] Walker | Controlling the narrative: Euphemistic language affects judgments of actions while avoiding perceptions of dishonesty[END_REF]. Nevertheless, the short-term benefits generated by euphemisms can be counterbalanced by long-term detrimental consequences such as loss of trust and image degradation.

2 Determining the intents of a specific euphemism's user is challenging but beyond the scope of our contribution.

It can be difficult to determine when an individual is making a deliberate attempt to disguise or misrepresent something, and when s/he just has a genuine difference of view about the best policy option to follow.

Words that work: The euphemistic names of astroturfing organizations

Astroturfing organizations can be described as fake grassroots movements. Astroturfing designates "a deceptive communication strategy initiated by a political actor and manufacturing public support to influence public policies" (Lits, 2021, p. 229). Environmental issues, including climate change, are often targeted by astroturfers [START_REF] Mcnutt | Astroturf, technology and the future of community mobilization: Implications for nonprofit theory[END_REF][START_REF] Hobbs | Turf wars: Using social media network analysis to examine the suspected astroturfing campaign for the Adani Carmichael Coal mine on Twitter[END_REF][START_REF] Lits | Detecting astroturf lobbying movements[END_REF]. For instance, the coal industry developed astroturf campaigns in the United States to defend their interests: among 72 interest groups identified as active on the shale gas debate, 12 groups were astroturfers [START_REF] Lits | Detecting astroturf lobbying movements[END_REF].

Astroturfing organizations typically use euphemistic names that disguise their real identities and/or agenda that might otherwise be considered harsh or unpleasant. Their names frequently convey a sense of in-group membership (by including words such as citizens, consumers) and trustworthiness (e.g., clean, sound) using well-selected euphemisms [START_REF] Lits | Detecting astroturf lobbying movements[END_REF]. For instance, several fossil-fuel industry front groups sponsor organizations with misleading euphemistic names such as "National Wetlands Coalition", "Global Climate Coalition", "Greening Earth Society", "Washington Consumers for Sound Fuel Policy" or "American Coalition for Clean Coal Energy". First impressions matter and these organizations seem explicitly dedicated to promoting environmental and climate change issues. They frequently target legitimate nonprofit and grassroots organizations with whom they can be confused. These euphemistic names conceal the true sponsors' interests and affect public opinion in directions that do not align with climate science [START_REF] Pfau | The influence of corporate front-group stealth campaigns[END_REF].

Naming astroturfing organizations is a strategic choice. Organization names can be one of the first and simplest steps to persuade a targeted audience. For example, the

Responsible Energy Citizen Coalition launched a campaign in 2012 to influence two

European Parliament reports regarding shale gas exploration [START_REF] Lits | Exploring astroturf lobbying in the EU: The case of Responsible Energy Citizen Coalition[END_REF]. This organization described itself as "an association of natural persons, representatives of self-governments and local authorities as well as social organizations" and was financially supported by three corporations with a significant interest in commercially developing shale gas [START_REF] Lits | Exploring astroturf lobbying in the EU: The case of Responsible Energy Citizen Coalition[END_REF].

According to the Elaboration-Likelihood Model of persuasion, there are central (effortful information processing) and peripheral (use of heuristics) routes of persuasion [START_REF] Petty | The elaboration likelihood model of persuasion[END_REF]. The model suggests the persuasive power of a message can be reinforced by exploiting heuristics about the qualities attributed to the messenger. The more similar the messenger is to the target audience (in-group effect) or the more trustworthy the messenger is considered to be, the more likely the message delivered will be accepted without critically evaluating its content [START_REF] Bolsen | The impact of message source on the effectiveness of communications about climate change[END_REF][START_REF] Brewer | Whose science do you believe? Explaining trust in sources of scientific information about the environment[END_REF].

The companies funding astroturfing organizations can embrace double standards. They may describe themselves as environmental stewards, while still supporting well-named astroturfing organizations working behind the scenes to promote climate disinformation or at least views that better align with their agenda [START_REF] Heald | Climate silence, moral disengagement, and self-efficacy: How Albert Bandura's theories inform our climate-change predicament[END_REF]. If the funding relationships are disclosed, these businesses can both displace responsibility and demonstrate plausible deniability. Moreover, using experimental data, [START_REF] Cho | Astroturfing global warming: It isn't always greener on the other side of the fence[END_REF] demonstrated that astroturfing organizations can remain persuasive even when their funding sources are disclosed. In short, euphemistic names exploit an immediate perceived emotional proximity to sway people's opinions or emotions to a particular side, frequently to deny or lessen climate change. 3 In the case of the shale gas industry, astroturfing organizations use words such as "economy", "creating jobs" and "growth" to describe the benefits of this industry [START_REF] Lits | Detecting astroturf lobbying movements[END_REF].

3 Astroturfing organizations are also successful at climate information because they have resources (e.g., money, power, competences) to design and manage influencing campaigns.

Policy implications and conclusion

Words are powerful instruments that influence thinking and behaviors related to climate change. They can numb people, lead them to be passive or can encourage them to take responsibility and appropriate actions. They offer a nudge that can generate first-order effect that will help or harm the pursued goals. Our analysis suggests several implications. [START_REF] Lakoff | Why it matters how we frame the environment[END_REF].

Second, although politicians and top bureaucrats are obviously aware of word power, we invite policymakers to take greater account of the power of words and recognize it as a central part of the policy cycle, especially in the climate change domain. For instance, many basic text books about the policy cycle omit the crucial step of developing a communication strategy. These decision makers should understand that their messages are received in a context where some groups have already built strong frames, well aligned with their interests.

We observe that, policy makers do not play a one-shot game where a single well-worded message will by itself result in desired changes. They are engaged in a long term construction of frames to counter or replace preexisting ones, where carefully selected words constitute elements that can activate the adequate frames.

Third, our recommendation should not be confused with a one-size-fits-all approach where an ideal word or expression is identified once and for all and applied indiscriminately.

We suggest tailoring and testing frames with target audiences (see e.g., [START_REF] Hung | Comparing the effects of climate change labelling on reactions of the Taiwanese public[END_REF]. Vocabulary is already used in various circles to influence and even manipulate and not necessarily with the best intentions [START_REF] Cho | Astroturfing global warming: It isn't always greener on the other side of the fence[END_REF]. Rather than allowing scientific, legal, or linguistic considerations to determine word choice, we suggest considering the psychological and behavioral reactions of policy audiences, notably the frequently overlooked automatic part (System 1). Words are selected for their objective content but also for their potential to activate unconscious automatic processes and emotions in people. This analysis can lead to tradeoffs between words that were not originally considered, e.g., by sacrificing scientific precision to make the policy more effective [START_REF] Reddy | Hypocrisy they name: "Colorado river flow shrinks from climate crisis, risking severe water shortages"-A note[END_REF] or making subtle variations to address audiences characteristics.

Another implication is to consider the 'supply' and 'demand' sides of euphemisms use.

The use of misleading euphemisms could be reduced by emphasizing the risks involved to the user such as potential legal actions, trust loss, and long-term negative consequences. Another option is to help people spot euphemism use and increase their awareness of the influence of these euphemisms on thinking and behaviors. Sophisticated methods are currently emerging to detect euphemisms, especially in online communication (Zhu et al., 2021). Developing media and environmental literacy notably in online environments, reinforcing skepticism defenses, critical thinking skills and appropriate online behaviors can prevent or reduce passive or automatic reactions to euphemism exposure and may contribute to proactive behaviors. Developing media and environmental literacy in school curricula can enhance understanding of environmental issues and prepare individuals to participate in environmental decision-making processes and reduce the likelihood that they will become victims of astroturfing attempts. Equipping people with ways to quickly detect whether a message comes from a dubious source (e.g., used frames, lack of corroborating sources, personal attacks rather than content-related arguments) could make them less prone to be influenced by misleading euphemisms, although recent evidence is not so encouraging [START_REF] Cho | Astroturfing global warming: It isn't always greener on the other side of the fence[END_REF].

For instance, when a seemingly reputable organization uses euphemistic arguments, it makes sense to develop in the audience the desire to take a deeper look at its funding, governance

arrangements and representatives to detect potential connections with sponsors that the organization may prefer to remain hidden. It is also important to acknowledge that people with strong political views of their own can make anti-climate change euphemisms seem more effective than they really are because they already hold those beliefs and avoid dissonance. It may also be useful to trace the trajectories of specific euphemisms, from their emergence to their growth and diffusion, including their lifecycle and disappearance given the well-known euphemism treadmill.

Figure 1 :

 1 Figure 1: Characterizing climate change euphemisms in a 2-dimensional space

First, researchers who

  want to influence policy need to give more attention to research communication, including developing skills required to effectively use social media such as tweets. Research communication could become a required or highly recommended course in a policy related degree. Climate scientists are typically not cognition or communication specialists and they often use, either purposefully or subconsciously, a model of human behaviour based on deliberate and unemotional reasoning where facts and scientific evidence are the most important elements of their argumentation

Table 2 .

 2 Types of euphemisms, their functions and examples (source:[START_REF] Burridge | Euphemism and language change: The sixth and seventh ages[END_REF]

	Euphemism type	Function	Example
	Protective	To shield and avoid offense	Offset vs. tax, especially for people who consider taxation as theft by the
			state
	Underhand	To mystify and misrepresent	Build soil organic matter, increase nutrient use efficiency vs. reduce
			greenhouse gases
			Nuisance flooding vs. sea-level rise
			Soil enhancer vs. manure
	Uplifting	To talk up and inflate	Chief officer climate change
			Nature-based solutions
	Provocative	To reveal and inspire	Increased density vs. overpopulation
	Cohesive

The Greek etymological origin of the word 'euphemism' means "speaking well of something". The motivation behind "speaking well' may be manifold.

Zeldin-O'Neill, S. 2019. "It's a crisis, not a change": The six Guardian language changes on climate matters. The Guardian, October 16. https://www.theguardian.com/environment/2019/oct/16/guardian-language-changesclimate-environment Zhu, W., Gong, H., Bansal, R., Weinberg, Z., Christin, N., Fanti, G., Bhat, S. 2021. Selfsupervised euphemism detection and identification for content moderation. Proceedings of the 42nd IEEE Symposium on Security and Privacy. https://arxiv.org/pdf/2103.16808.pdf


Nicolas Papon
email: nicolas.papon@univ-angers.fr

Brent R Copp
email: b.copp@auckland.ac.nz

Vincent Courdavault
email: vincent.courdavault@univ-tours.fr

Review -CORRECTED MANUSCRIPT R1 Marine Drugs: Biology, Pipelines, Current and Future Prospects for Production

Keywords: Marine drugs, pharmaceuticals, production, metabolic engineering, biotechnology

The marine environment is a huge reservoir of biodiversity and represents an excellent source of chemical compounds, some of which have large economical values. In the urgent quest for new pharmaceuticals, marine-based drug discovery has progressed significantly over the past several decades and we now benefit from a series of approved marine natural products (MNPs) to treat cancer and pain while an additional collection of promising leads are in clinical trials. However, the discovery and supply of MNPs had always been challenging given their low bioavailability and structural complexity. Their manufacture for pre-clinical and clinical development but also commercialization mainly relies upon marine source extraction and chemical synthesis, which are associated with high costs, unsustainability and severe environmental problems. In this review, we discuss how metabolic engineering now raises reasonable expectations for the implementation of microbial cell factories, which may provide a sustainable approach for MNP-based drug supply in the near future.

Introduction

As with land plants, fungi and bacteria, marine organisms represent an extremely large resource of valuable natural products for humans. The bioactive properties of many of these molecules find applications in a wide range of industrial sectors such as nutrition, biotechnologies, cosmetics, and health [START_REF] Sigwart | Unlocking the potential of marine biodiscovery[END_REF]. Among the large series of marine natural products (MNPs), those displaying a pharmaceutical potential are of great interest as part of our modern pharmacopeia [START_REF] Ghareeb | Insights about clinically approved and preclinically investigated marine natural products[END_REF]. Marine organisms continue to be a source of unique chemical entities with valuable biomedical potentialities [START_REF] Blunt | Marine natural products[END_REF]. This is illustrated nicely by the recent report on turbinmicin, a marine microbe sourced polyketide displaying potent antifungal activity, particularly towards pathogenic yeasts that represent a worrisome problem for human health in terms of virulence and drug resistance (Zhang et al., 2020;[START_REF] Zhang | A marine microbiome antifungal targets urgent-threat drug-resistant fungi[END_REF].

From an economic viewpoint, the world market of MNPs is considerable with a recent report estimating the market size for marine-derived pharmaceuticals will reach up to USD 10b in 2021 with an expected annual growth rate of more than 8% from 2020 to 2026 [START_REF] Daniotti | Marine Biotechnology: Challenges and Development Market Trends for the Enhancement of Biotic Resources in Industrial Pharmaceutical and Food Applications. A Statistical Analysis of Scientific Literature and Business Models[END_REF]ID: 5123389). As observed for the main anticancer drugs derived from plants, enormous costs of production lead to high cost therapeutics. For instance, the monthly cost for treating breast cancer or liposarcoma with eribulin now reaches USD 10,000 while that of brentuximab for the management of lymphoma exceeds USD 30,000 [START_REF] Howard | Pricing in the Market for Anticancer Drugs[END_REF][START_REF] Gordon | Trajectories of Injectable Cancer Drug Costs After Launch in the United States[END_REF].

From a structural point of view and with few exceptions, MNPs of pharmaceutical importance derive from the so-called secondary metabolism of marine organisms. This means that their biosynthetic routes are diverse in terms of length, starting precursor and characterization level. MNPs also display complex structures that potentially render their total chemical syntheses de novo impractical. As a consequence, production strategies relying on marine macroorganism harvesting or culturing coupled with extraction procedures remain sometimes the only alternative for the production of these compounds. In this regard, while their supply can be often implemented with laboratory scale cultures for bacterial, fungal or microalgal sources, this is much more complex to consider for invertebrate sources where secondary metabolites usually naturally accumulate in only very small quantities. Most of the time, the concentrations of the promising MNPs in marine invertebrates may be sufficient for their chemical characterization and preliminary activity evaluation, however, they are far from providing a sustainable supply for pre-clinical and clinical trials, requiring quantities in a gram range, and even less in the perspective of a future commercialization where annual demands usually reach several kilograms [START_REF] Gomes | Marine Invertebrate Metabolites with Anticancer Activities: Solutions to the "Supply Problem[END_REF]. For ecological issues, it is inconceivable to think that the supply of these MNPs with promising properties could only depend on wild harvest of marine invertebrates [START_REF] Heffernan | Why a landmark treaty to stop ocean biopiracy could stymie research[END_REF].

Importantly, similar production limits have been observed for plant-derived pharmaceuticals in past decades [START_REF] Courdavault | Metabolic engineering for plant natural products biosynthesis: new procedures, concrete achievements and remaining limits[END_REF]. This has promoted the stepwise emergence of alternative strategies of supply based on metabolic engineering approaches that began to appear alongside the emergence of the synthetic biology era in the early 2000's [START_REF] Kotopka | Synthetic biology strategies toward heterologous phytochemical production[END_REF][START_REF] Nielsen | Cell factory engineering for improved production of natural products[END_REF]. This type of strategy relies on the heterologous expression of biosynthetic genes from the source organism in a recipient bacterium and/or yeast to allow the de novo synthesis of the expected drugs by the transformed microbe. Likewise, in line with the pioneering success of the antimalarial semi-synthetic artemisinin [START_REF] Paddon | High-level semi-synthetic production of the potent antimalarial artemisinin[END_REF], genetically engineered microorganisms have emerged as suitable cell factories for the production of plant-derived drugs. In this context, we discuss here how all the background gained in recent years concerning the biotechnological production of plant drugs using engineered microbes will undoubtedly pave the way for innovative MNP manufacturing alternatives in the near future.

Trends in marine drugs: sources, chemical classes, biological activities and current pipeline

Over five decades of sustained research, more than 30,000 compounds of marine origin have been identified within a diverse palette of biological sources, including both prokaryotic (eubacteria, cyanobacteria) and eukaryotic organisms (fungi, dinoflagellates, algae, sponges, cnidarians, bryozoans, mollusks, ascidians, and echinoderms) [START_REF] Alves | From Marine Origin to Therapeutics: The Antitumor Potential of Marine Algae-Derived Compounds[END_REF]Caroll et al., 2021). Sponges (porifera) and cnidarians (mainly corals and anemones) for invertebrates as well as marine fungi and actinomycetes for microorganisms have always been the prominent MNP sources in term of quantity and structural diversity [START_REF] Hu | Statistical research on the bioactivity of new marine natural products discovered during the 28 years from 1985 to 2012[END_REF]. With the exception of some peptides (e.g. conotoxin), fatty acids (e.g. omega-3-acid ethyl esters) and polysaccharides (e.g. fucoidan, carrageenans, alginates, ulvans, chitins, and chitosans), all others classes of biologically active MNPs arise from the so-called "specialized metabolism" (or secondary metabolism ) pathways which means that most of the interesting MNPs are biosynthesized through clade-specific metabolic pathways. The major classes of marine secondary metabolites include alkaloids, terpenoids, polyketides, acetogenins, non-ribosomal peptides, and glycosides [START_REF] Lu | Application of marine natural products in drug research[END_REF]. Figure 1 summarises examples of the structural diversity of MNPs that include approved therapeutics, clinical candidates and bioactive structures that are the focus on ongoing studies. Although these chemical classes are also primarily found in the portofolio of metabolites produced by terrestrial natural sources (mainly plants, fungi and bacteria), recent advances shed light on shared but also on differential features between terrestrial and marine secondary metabolites. While displaying a global lower solubility, MNP scaffolds are substantially larger, containing more long chains and large rings with fewer oxygen but more nitrogen atoms compared to terrestrial natural products. Another distinguishing general trait of MNPs is that a fair proportion of compounds contain bromine and to a lesser extent chlorine, associated with the high concentration of halides in seawaters and the presence of particular halogenation enzymes (e.g. vanadium bromoperoxidase) in a broad range of marine organisms [START_REF] Shang | Cheminformatic Insight into the Differences between Terrestrial and Marine Originated Natural Products[END_REF]Voser et al., 2022). About a quarter of the MNPs isolated so far have shown interesting biological activity including antiproliferative, antimicrobial, anti-inflammatory, anti-coagulation, analgesic, neuroprotective, and cardioprotective properties [START_REF] Hu | Statistical research on the bioactivity of new marine natural products discovered during the 28 years from 1985 to 2012[END_REF]. To date, fourteen MNP-based drugs are clinically approved by various worldwide health organizations and are currently used to cure/treat some of the main medical pathologies including cancer, cardiovascular diseases, pain, and viral infections [START_REF] Dyshlovoy | Marine Compounds and Cancer: Updates 2020[END_REF]. A timeline of their discovery is depicted in Figure 2 and their main characteristics are compiled in Table 1. Beyond these proven pharmaceutical successes, we can anticipate that the number of approved MNP-derived drugs will keep increasing since 33 marine-derived pharmaceutical compounds are currently in clinical trials (5 in phase III, 12 in phase II, and 16 phase I, Table 2). In addition, it is highly likely that MNP research will continue to boom insofar as the recent developments in analytical technology, spectroscopy, machine learning, and high-throughput screening of activities all support the discovery of more than 1,000-1,500 new structures from marine sources annually [START_REF] Ghareeb | Insights about clinically approved and preclinically investigated marine natural products[END_REF][START_REF] Wilson | Creating and screening natural product libraries[END_REF], Carroll et al., 2021).

As these biomolecules are sourced from nature, bioprospecting researchers in this field need to be aware of the moral and legal obligations on them to ensure equitable benefit sharing with the country from which the biological/genetic material was sourced. In October 2014, parties to the Convention on Biological Diversity (CBD) adopted the supplementary agreement (known as the Nagoya Protocol) which provides a legal framework regarding access and fair and equitable sharing of benefits arising from the utilization of genetic resources. One of the founding aims of the Nagoya Protocol is to put an end to biopiracy, with researchers being required to establish access and benefit-sharing (ABS) agreements before being granted access to a country's national jurisdiction. In a marine context, this jurisdiction covers territorial seas and exclusive economic zones (EEZ), out to a 200 nautical mile boundary (https://www.cbd.int/abs/). Thus it is a legal imperative for any bioprospecting in these environments that, in addition to the usual collection permits, ABS agreements are put in place. To date, these closeto-shore and EEZ marine environments are the locations of the overwhelming majority of bioprospecting efforts. There are however ongoing deep-sea bioprospecting efforts, searching the water column and sediment for novel biomolecules. Exploration in ocean territory beyond any national jurisdiction boundaries is explicitly not covered by the Nagoya Protocol and so the concepts of access and benefit-sharing are somewhat vague. The closest regulatory framework is the United Nations Convention on the Law of the Sea (UNCLOS), which contains references to the sustainable use of marine living resources. There is a need for the development of sharing agreements for products arising from open ocean research, beyond bilateral/multilateral agreements, to perhaps extend so that benefits are shared by nations.

How approved MNP-based pharmaceuticals are manufactured?

The currently approved MNP-derived pharmaceuticals cover diverse chemical classes encompassing nucleosides, peptides, fatty acids, macrolides, depsipeptides, antibody-drug conjugates, hydroxamic acids, and alkaloids. As a consequence, their production process and cost differ sharply from one compound to another. In this section, we summarize how pharmaceuticals companies currently ensure the supply of these important medicines at an industrial scale.

The process for production of the lipid-regulating omega-3-acid ethyl esters (Lovaza ® and Vascepa ® ) (Figure 2) is among the simplest and cheapest since triglyceride biosynthesis inhibitors are made from fish body oils that are purified and esterified [START_REF] Cholewski | A Comprehensive Review of Chemistry, Sources and Bioavailability of Omega-3 Fatty Acids[END_REF]. The anticancer drugs cytarabine (Cytosar-U ® ) and nelarabine (Arranon ® /Atriance ® ), and the antiviral compound vidarabine (Vira-A ® ), are analogs of the marine nucleosides spongothymidine and spongouridine (Figure 1), which were originally isolated from the Caribbean sea sponge Tectitethya crypta (Figure 2). Due to their fairly simple chemical structures, these DNA/RNA polymerase inhibitors are currently manufactured by classical chemical synthesis [START_REF] Shelton | Metabolism, Biochemical Actions, and Chemical Synthesis of Anticancer Nucleosides, Nucleotides, and Base Analogs[END_REF]. The powerful analgesic agent ziconotide (Prialt ® ) (Figure 2) is ω-contoxin MVIIA (Figure 1), a hydrophilic peptide originally isolated from the venom of the Pacific fish-hunting marine mollusk Conus magus (Figure 2). This N-type calcium channel blocker [START_REF] Gao | Structure of human Cav2.2 channel blocked by the painkiller ziconotide[END_REF] is produced by solid phase peptide synthesis [START_REF] Schroeder | Therapeutic potential of conopeptides[END_REF]. The anticancer agent eribulin mesylate (Halaven ® ) (Figure 2) corresponds to a simplified analogue of the marine polyether macrolide, halichondrin B (Figure 1), which was originally isolated from the marine sponge Halichondria okadai (Figure 2). This microtubule-targeted anticancer drug is nowadays considered as the most complex synthetic drug and above all a perfect demonstration of how far the limits of total synthesis of natural products have been pushed on an industrial scale [START_REF] Bauer | Story of Eribulin Mesylate: Development of the Longest Drug Synthesis[END_REF].

Another relatively new and efficient class of marine drug-derived pharmaceuticals corresponds to antibody-drug conjugates (ADC), combining a cytotoxic agent of marine origin with a selective monoclonal antibody. Four examples are currently approved to treat specific types of cancer including brentuximab vedotin (Adcetris ® ), enfortumab vedotin (PADCEV TM ), polatuzumab vedotin (Polivy TM ), and belantamab mafodotin (Blenrep TM ) (Figure 2). These MNP-derived ADC commonly consist of (i) a monoclonal antibody specific to surface antigens present on particular tumor cell types, (ii) a cleavable or non-cleavable linker, (iii) and a microtubule disrupting MNP derivative (either monomethyl auristatin E, MMAE, or monomethyl auristatin F, MMAF) [START_REF] Joubert | Antibody-Drug Conjugates: The Last Decade[END_REF]. Monoclonal antibodies are classically produced by mammalian (Chinese hamster ovary) cells while linkers are prepared with commercially available chemicals. MMAE and MMAF are synthetic analogs of the highly cytotoxic dolastatin 10 (Figure 1) that was originally isolated from the Indian Ocean sea hare Dolabella auricularia (Figure 2). To date, MMAE and MMAF are manufactured by classical chemical synthesis [START_REF] Zhou | A practical approach to asymmetric synthesis of dolastatin 10[END_REF]. The anticancer MNPs trabectedin (Yondelis ® ) (Figure 1) and its derivative lurbinectedin (Zepzelca TM ) are tetrahydroisoquinoline alkaloids -trabectedin is ecteinascidin 743 while lurbinectin is a modified analog of ET 736, both natural products having been originally identified in the Caribbean sea squirt Ecteinascidia turbinata (Figure 2). Although recent advancements provide new scalable total synthesis protocols, the production of these two complex alkaloids still comprises semisynthetic steps performed on the precursor cyanosafracin B produced by fermentation of the bacterium Pseudomonas fluorescens [START_REF] Cuevas | Development of Yondelis (trabectedin, ET-743). A semisynthetic process solves the supply problem[END_REF][START_REF] He | A Scalable Total Synthesis of the Antitumor Agents Et-743 and Lurbinectedin[END_REF]. Finally, Plitidepsin (Aplidin ® ) (Figure 1) is a cyclic depsipeptide anticancer drug isolated from the marine tunicate Aplidium albicans (Figure 2). Since its approval, this eEF1A2 inhibitor is manufactured by total chemical synthesis [START_REF] Alonso-Álvarez | Plitidepsin: design, development, and potential place in therapy[END_REF].

Anticipating the development of efficient production processes for future approved MNPs

More than 30 MNP-derived pharmaceutical are currently in clinical trials (Table 2), about a hundred are in preclinical development each year, and the discovery of new compounds with interesting biomedical potentialities and novel mechanisms of action continues with little sign of slowing down [START_REF] Mayer | The odyssey of marine pharmaceuticals: a current pipeline perspective[END_REF][START_REF] Newman | Drugs and Drug Candidates from Marine Sources: An Assessment of the Current "State of Play[END_REF][START_REF] Mayer | Marine Pharmacology in 2016-2017: Marine Compounds with Antibacterial, Antidiabetic, Antifungal, Anti-Inflammatory, Antiprotozoal, Antituberculosis and Antiviral Activities; Affecting the Immune and Nervous Systems, and Other Miscellaneous Mechanisms of Action[END_REF][START_REF] Daniotti | Marine Biotechnology: Challenges and Development Market Trends for the Enhancement of Biotic Resources in Industrial Pharmaceutical and Food Applications. A Statistical Analysis of Scientific Literature and Business Models[END_REF]. As a consequence, whether for preclinical and clinical development or marketing, it seems important to be able to anticipate efficient processes of sustainable supply for promising MNP-derived drugs. In this respect, all the background accumulated in the past decades that have allowed implementing such industrial production strategies for many different classes of natural products from highly diversified sources may combine to benefit the supply of new marine drugs.

As established with eribulin mesylate (Halaven ® ), advances in chemical synthesis continuously stretch the boundaries of our capacities of manufacturing pharmaceuticals with increasingly complex structures [START_REF] Bauer | Story of Eribulin Mesylate: Development of the Longest Drug Synthesis[END_REF]. We can thus expect that this powerful process of supply may continue to stand out as a prominent strategy to produce future approved MNP-derived drugs at an industrial scale.

However, we have to admit that this production process is still poorly environment-friendly and very costly insofar the methods require a lot of different reagents and deliver overall low yields. Hence, especially in the case of new MNPs of great potential but displaying structures far too complex for a profitable production by chemical synthesis, alternative biotechnological approaches could be considered for which we summarize some of these key features hereafter.

For instance, concerning MNPs originally biosynthesized by a single cultivable marine microbe such as a bacterium or a fungus, axenic macroscale cultures (in a bioreactor) could be a relevant strategy (Quian et al, 2020). This type of production process has proved its worth for the industrial scale supply of many different antibacterial (erythromycin, vancomycin, streptomycin, penicillins, cephalosporins), antifungal (griseofulvin, amphotericin B) antiparasitic (avermectins, fumagillin), and immunosuppressant (tacrolimus, cyclosporine, mycophenolic acid) natural products from a broad palette of terrestrial bacterial and fungal sources [START_REF] Panter | Synergizing the potential of bacterial genomics and metabolomics to find novel antibiotics[END_REF]. In line with this statement, Zhao and colleagues recently identified a highly oxidized type II polyketide from the marine actinomycetes Micromonospora sp., turbinmicin, which displays antifungal activity by inhibiting the vesicle-mediated trafficking pathway of many life-threatening fungi [START_REF] Zhang | A marine microbiome antifungal targets urgent-threat drug-resistant fungi[END_REF]Zhang et al., 2020). This promising new antifungal marine drug demonstrated mouse-model efficacy toward multiresistant yeasts and molds. We can thus anticipate that, in case of clinical success, axenic macrocultures would represent the most efficient process strategy to reach an industrial scale production of turbinmicin.

It is also important to remember that, in some specific cases, complementary semi-synthesis steps can be developed to complete the scaffold of some complex microbial secondary metabolites, as illustrated for the anticancer MNPs trabectedin (Yondelis ® ) and lurbinectedin (Zepzelca TM ) that are semisynthetic derivatives of cyanosafracin B (produced by axenic cultures of P. fluorescens) [START_REF] Cuevas | Development of Yondelis (trabectedin, ET-743). A semisynthetic process solves the supply problem[END_REF]. A similar approach is currently achieved for the industrial production of the antifungals caspofungin (Cancidas®), micafungin (Mycamine®), and anidulafungin (Ecalta®) that are obtained following additional semisynthstic steps on the precursor pneumocandin B0 (produced by axenic cultures of the ascomycetes mold Glarea lozoyensis) [START_REF] Balkovec | Discovery and development of first in class antifungal caspofungin (CANCIDAS®)--a case study[END_REF]. It is highly likely that such semisynthetic processes would continue to be a procedure of choice for the industrial production of many different classes of clinicaly used secondary metabolites, as observed for some plant-derived anticancer drugs [START_REF] Courdavault | Towards the Microbial Production of Plant-Derived Anticancer Drugs[END_REF].

By contrast, production processes for complex MNPs that are originally biosynthesized by marine invertebrates are more difficult to implement. As currently perfomed for several drugs or their precursors that are extracted from terrestrial medicinal plants, random sampling of the source material from sessile marine organisms directly from the natural environment is not acceptable. Aside from ecological impact considerations, the overall naturally-available bulk mass would usually be insufficient to supply an industrial demand. Most importantly, such a practice is known to direcly endanger biodiversity [START_REF] Heffernan | Why a landmark treaty to stop ocean biopiracy could stymie research[END_REF]. By analogy to what has been achieved in the context of the monoculture-based production of some plant drugs such as anticancer vinca-alkaloids (vincristine, vinblastine, vinorelbine) from the Madagascar periwinkle Catharanthus roseus or the antimalarial artemisinin from the sweet wormwood Artemisia annua, aquaculture of medicine-producing marine invertebrates has progressively been considered since the 1980's [START_REF] Springer | Aquaculture and the production of pharmaceuticals and nutraceuticals[END_REF]. This trend is a logical extension of current commercial aquaculture practices, including for instance corals and shells (for tourism souvenir gifts), sponges (personal hygiene and cleaning), seaweeds (production of proteins, lipids and carbohydrates for food and cosmetics) and shellfish farming (mainly oysters for food or mother-ofpearls, clams, nutshells and mussels) [START_REF] Naylor | A 20-year retrospective review of global aquaculture[END_REF]. To date, there are two main aquaculture methods including mariculture (also termed "in situ" aquaculture) and captive breeding (also referred to as "ex situ" aquaculture) and both types of aquaculture display advantages and drawbacks from a MNP production perspective [START_REF] Sheridan | Diseases in coral aquaculture: causes, implications and preventions[END_REF][START_REF] García-Poza | The Evolution Road of Seaweed Aquaculture: Cultivation Technologies and the Industry 4.0[END_REF].

On the one hand, in situ techniques represent a low cost and simple methodology that relies entirely on the natural environment to supply essential light and nutrients for marine species growth.

Importantly, it is nowadays well established in the literature that the production of a plethora of bioactive secondary metabolites initially identified in sponges, cnidarians, bryozoans, mollusks, ascidians, and echinoderms does not exclusively rely on the intrinsic biosynthetic pathways evolved by these marine metazoan species, but rather derives from associated micro-organisms. [START_REF] Gomes | Marine Invertebrate Metabolites with Anticancer Activities: Solutions to the "Supply Problem[END_REF]. In other words, there is evidence that culturing marine species in their natural habitats through in situ aquaculture promotes holobiont (this means for instance a marine invertebrate associated to different microorganisms) homeostasis and this equilibrium requires the correct orchestration of the different metabolic fluxes necessary for the production and accumulation of specific MNPs [START_REF] Sheridan | Diseases in coral aquaculture: causes, implications and preventions[END_REF]McCauley et al., 2020). However, in practice, monitoring growth conditions is fairly limited and the developing marine invertebrates are usually threatened by environmental stresses (predation, eutrophication, thermal anomalies) [START_REF] Sheridan | Diseases in coral aquaculture: causes, implications and preventions[END_REF]. As a consequence, and in the perspective of production of a specific MNP, both the biotic or abiotic stresses could have potential uncontrolled effects on the regulation of MNP biosynthetic pathways. Most importantly, the annual production of a specific metabolite by in situ aquaculture may be impeded by climatic variations or even natural disasters leading to a significant reduction in bulk mass necessary for extraction.

In contrast, since a proximate source of natural seawater is not mandatory, ex situ practices have also been considered in the framework of future industrial production of valuable MNPs. Captive breeding, although more expensive to implement, prevents occurrence of perturbating abiotic and biotic factors, but also allows the monitoring of optimal developmental conditions that are considerably species specific. All these previously mentioned aspects have thus favored an improved control of MNP production. However, with the exception of corals, cultivation of whole marine organisms ex situ has been globally less successful compared to mariculture. In addition, it is important to keep in mind that while ex situ aquaculture allows the fine tuning of growth conditions, this practice may change the holobiont composition and causes either the disruption of important microorganisms when cultured in their natural sites or promotes uncontrolled development of undesired microorganisms. As a consequence, although farming has been undertaken for preliminary studies of several compound classes including the halichondrins, ecteinascidins and mycalamides, experimentation aimed at producing valuable MNPs by captive breeding have never been reported hitherto [START_REF] Sheridan | Diseases in coral aquaculture: causes, implications and preventions[END_REF][START_REF] Gomes | Marine Invertebrate Metabolites with Anticancer Activities: Solutions to the "Supply Problem[END_REF].

On the collective basis of these issues, there is an urgent need to implement alternative supply paths for future medically important classes of MNPs to prevent recurrent shortages and to limit the impact of their production on nature. In such a perspective, we discuss in the coming section, how the use of engineered microbes could progressively stand out as a promising production strategy to overcome potential sourcing limitations of complex MNPs belonging to a broad palette of secondary metabolite classes, as recently emerged for plant-derived pharmaceuticals [START_REF] Kotopka | Synthetic biology strategies toward heterologous phytochemical production[END_REF][START_REF] Nielsen | Cell factory engineering for improved production of natural products[END_REF][START_REF] Courdavault | Metabolic engineering for plant natural products biosynthesis: new procedures, concrete achievements and remaining limits[END_REF].

Marine drug production: a metabolic engineering perspective

The pioneering landmark production of the antimalarial artemisinin in yeast successfully achieved in the 2000's has inspired many subsequent developments in the field. Indeed, a boom in microbial engineering procedures for the production of different types of complex plant natural products (PNPs) has been observed in the past decades, particularly among the major families of polyphenols, terpenes, taxanes, indole alkaloids, opioids, cannabinoids, and tropane alkaloids [START_REF] Courdavault | Metabolic engineering for plant natural products biosynthesis: new procedures, concrete achievements and remaining limits[END_REF]. On this basis, now is the time to reflect on these outstanding acheivements to guide future research in this domain to open new windows for sustainable production processes of complex pharmaceutical MNPs (Figure 3).

A rapid overview of the literature illustrates that the capacity to develop optimized microbial cell factories heterologously producing PNPs has accelerated thanks to very recent technological advancements. These consist in (i) a marked acceleration in the elucidation of the biosynthetic pathways of secondary metabolites [START_REF] Lau | Six enzymes from mayapple that complete the biosynthetic pathway to the etoposide aglycone[END_REF][START_REF] Medema | Computational genomic identification and functional reconstitution of plant natural product biosynthetic pathways[END_REF][START_REF] Caputi | Missing enzymes in the biosynthesis of the anticancer drug vinblastine in Madagascar periwinkle[END_REF][START_REF] Nett | Discovery and engineering of colchicine alkaloid biosynthesis[END_REF][START_REF] De Bernonville | Identifying Missing Biosynthesis Enzymes of Plant Natural Products[END_REF], (ii) an improved rationalization of the reconstruction stages of the biosynthetic pathway in the host microbe [START_REF] Luo | Complete biosynthesis of cannabinoids and their unnatural analogues in yeast[END_REF]Srinivasan andSmolke, 2020, Guirimand et al., 2021 ;[START_REF] Srinivasan | Engineering cellular metabolite transport for biosynthesis of computationally predicted tropane alkaloid derivatives in yeast[END_REF], (iii) the identification of new routes for optimizing the microbial production to reach industrial titers [START_REF] Pyne | A yeast platform for highlevel synthesis of tetrahydroisoquinoline alkaloids[END_REF]Zhang et al., 2020;[START_REF] Carqueijeiro | Beyond the semisynthetic artemisinin: metabolic engineering of plant-derived anti-cancer drugs[END_REF].

As an obvious starting point, reconstructing biosynthetic pathways in heterologous hosts to create a MNP cell factory requires a comprehensive knowledge of the enzymatic reactions involved in the biosynthesis of the targeted compound in the native host organisms (Figure 3A). In bacteria and fungi, this step is facilitated by the genomic clustering of genes encoding enzymes from the biosynthetic pathways of most the secondary metabolites [START_REF] Keller | Fungal secondary metabolism: regulation, function and drug discovery[END_REF][START_REF] Chevrette | Evolutionary dynamics of natural product biosynthesis in bacteria[END_REF][START_REF] Lee | Systems and synthetic biology to elucidate secondary metabolite biosynthetic gene clusters encoded in Streptomyces genomes[END_REF][START_REF] Zhang | Genetic platforms for heterologous expression of microbial natural products[END_REF]. However, in the context of a valuable MNP produced by an holobiont such as for instance a marine invertebrate associated to various microorganisms, the research is more challenging since the elucidation of the complete biosynthetic pathway requires the identification of highly specific enzymes encoded by genes present in multiple species and belonging to distinct superfamilies containing several tens to hundreds of members such as reductases, oxidases, methyltransferases, and cytochrome P450s among others.

Fortunately, the arrival of massive omics resources and tools could accelerate such investigations [START_REF] Hautbergue | From genomics to metabolomics, moving toward an integrated strategy for the discovery of fungal secondary metabolites[END_REF][START_REF] De Bernonville | Identifying Missing Biosynthesis Enzymes of Plant Natural Products[END_REF][START_REF] Ramzi | Streamlining Natural Products Biomanufacturing With Omics and Machine Learning Driven Microbial Engineering[END_REF] (Figure 3B). In that sense, an emerging and powerful strategy relies on identifying a first set of candidate genes through metabolomic and transcriptomic analysis. More specifically, various metabolomics approaches can now lead to the accurate detection of plausible reaction intermediates while recent advancements in molecular networking provide new efficient opportunities in the processing of the acquired metabolomics data [START_REF] Ramos | Natural products targeting strategies involving molecular networking: different manners, one goal[END_REF][START_REF] Stuart | Metabolomic tools used in marine natural product drug discovery[END_REF]. Once compounds are connected and a reaction sequence has been postulated in silico (reflecting for instance a metabolic branch), bioinspired organic chemistry and literature data may then help in identifying a type of enzymatic activity required for each step. At this stage, the availability of the holobiont metagenome and massive transcriptomics data are of primary importance since searching the specific functional conserved domains may lead to define first sets of candidate genes. At this step, identifying the respective genes encoding specific enzymes involved in each catalytic reaction of the investigated pathways requires further prioritization procedures (Figure 3C). The various possibilities include first the screen for homology to orthologous gene sequences of already characterized biosynthesis steps in other models. A BLAST search for protein sequences from multiple species coupled with reconstruction of phylogenies statistically fitted on evolutionary models represent a basic approach to this aim [START_REF] Farrow | Cytochrome P450 and O-methyltransferase catalyze the final steps in the biosynthesis of the anti-addictive alkaloid ibogaine from Tabernanthe iboga[END_REF]. Another possibility consists in prioritizing candidate genes by searching physical gene clusters. Some specific branches of the studied biosynthetic pathway, notably those achieved by microflora species associated to the holobiont, can indeed involve enzymes encoded by genes physically clustered in the same genomic region [START_REF] Keller | Fungal secondary metabolism: regulation, function and drug discovery[END_REF][START_REF] Chevrette | Evolutionary dynamics of natural product biosynthesis in bacteria[END_REF]. Once again in this context, the knowledge of the metagenome sequence from the investigated holobiont is mandatory and the search for gene cluster is particularly efficient with more complete genome sequences composed of very long DNA sequence assemblies (McCauley et al., 2020 ;[START_REF] Richardson | A roadmap for metagenomic enzyme discovery[END_REF]. It is interesting to mention that such a global strategy has already begun to bear fruit since it has allowed deciphering of the biosynthetic origin of the anticancer drug trabectedin [START_REF] Rath | Meta-omic characterization of the marine invertebrate microbial consortium that produces the chemotherapeutic natural product ET-743[END_REF][START_REF] Schofield | Identification and analysis of the bacterial endosymbiont specialized for production of the chemotherapeutic natural product ET-743[END_REF]. Above all, the most powerful and global approach that have recently emerged to rationally prioritize gene sequences in metabolic engineering relies on gene co-expression analysis (Dugé de [START_REF] De Bernonville | Identifying Missing Biosynthesis Enzymes of Plant Natural Products[END_REF][START_REF] Ramzi | Streamlining Natural Products Biomanufacturing With Omics and Machine Learning Driven Microbial Engineering[END_REF]. Briefly, these computational strategies are based on the hypothesis that metabolites accumulated specifically in an organism or a holobiont, even in a tissue-specific manner, should be produced through a chain of enzymes displaying a similar spatio-temporal expression pattern (Ambrosino et al., 2019 ;Paul et al., 2019). Several major approaches can now be used for browsing such co-expression data and guiding identification of biosynthetic enzymes. This includes the statistical comparison of contrasted samples (e.g. MNP accumulating sample versus a non-accumulating sample) to identify differentially expressed genes (DEG), the construction of gene co-expression networks (GCNs) to provide a more exhaustive view of co-expression relationships among genes (Liesecke et al., 2018), and on the use of unsupervised machine learning (UML) that assists in grouping similarly expressed genes into a specific expression cluster (Prihoda et al., 2021). All these concepts have been extensively reviewed elsewhere (Dugé de [START_REF] De Bernonville | Identifying Missing Biosynthesis Enzymes of Plant Natural Products[END_REF][START_REF] Ramzi | Streamlining Natural Products Biomanufacturing With Omics and Machine Learning Driven Microbial Engineering[END_REF].

While new computational approaches now facilitate the identification of candidate biosynthetic genes by browsing both genomic and transcriptomic data, a laborious but essential step consists of validating the correct activity of each predicted genes/enzymes (Figure 3D). To this aim, biochemical characterization of recombinant proteins heterologously expressed in bacteria or yeasts still represents a basic but reliable approach. These functional assays are coupled with metabolomic techniques to detect product formation. Importantly, this approach is sometimes impeded by unknown, unpurifiable, or commercially unavailable reaction substrates. In this specific case, this requires the use of bulk mass extraction from the marine source. It is noteworthy that the validation of multiple genes from the same metabolic branch can be achieved following biosynthetic precursor feeding of the host microbe (expressing bacteria or fungi) co-overexpressing simultaneously several candidate genes. In this respect, tremendous advancement has been recently achieved since scalable processes for the expression of large numbers of full-length biosynthetic genes in a same vector are now accessible [START_REF] Clevenger | A scalable platform to identify fungal secondary metabolites and their gene clusters[END_REF][START_REF] Harvey | HEx: A heterologous expression platform for the discovery of fungal natural products[END_REF]).

Once the complete set of biosynthetic genes identified are biochemically validated, engaging the reconstruction of the pathway in a microbial chassis becomes possible (Figure 3E). A key factor of success to this aim is the choice of an appropriate heterologous host. Indeed, the high number of enzymatic steps, complex species, tissue, differential subcellular localization of intermediates or enzymes, and interconnected regulation networks in the MNP-producing holobiont could dramatically challenge the whole pathway reconstruction in the microbial host. Furthermore, the choice of carbon sources for feeding and the ease of final product purification must be considered to ensure the sustainability of the production process. On this basis, two main microbial models are usually preferred including the bacterium Escherichia coli and the baker's yeast Saccharomyces cerevisiae [START_REF] Carqueijeiro | Beyond the semisynthetic artemisinin: metabolic engineering of plant-derived anti-cancer drugs[END_REF][START_REF] Guirimand | Innovative Tools and Strategies for Optimizing Yeast Cell Factories[END_REF]. The specific requirement of post-translational protein modifications and the presence of membrane-bound proteins condition the selection of the host organism towards the use of a prokaryotic (E. coli) or eukaryotic chassis (S. cerevisiae). Fortunately, extensive synthetic biology toolboxs are available for both microbial models such as genomic integrative constructs, including CRISPR-Cas9 technologies, and auto-replicative plasmids [START_REF] Cravens | Synthetic biology strategies for microbial biosynthesis of plant natural products[END_REF]. These versatile genetic tools now allow rapid heterologous pathway prototyping. The bacterium E. coli displays advantages in metabolic engineering such as a well-understood native metabolism and fast doubling time. However, this microbial model shows significant physiological differences with eukaryotic cells including the lack of subcellular compartmentation and protein post-translational modification mechanisms, which can alter the metabolic flux of the heterologous pathway and reduce product titer.

By contrast, the yeast S. cerevisiae displays a canonical eukaryotic cell architecture allowing the expression of membrane-bound proteins, as usually observed in biosynthetic pathways (e.g. cytochrome P450). However, there is still very few information on the heterologous expression of enzymes from marine invertebrates, in particular relating to post-translational modifications such as protein glycosylation [START_REF] Zhu | Structures and functions of invertebrate glycosylation[END_REF]. An important and recent advancement in this field is the development of minimal synthetic genome organisms that only include documented genetic elements (Richardson et al., 2017 ;[START_REF] Fredens | Total synthesis of Escherichia coli with a recoded genome[END_REF][START_REF] Glass | Minimal Cells-Real and Imagined[END_REF]Lu et al., 2019 ;[START_REF] Lloyd | COBRAme: A computational framework for genome-scale models of metabolism and gene expression[END_REF].

These microbial chassis have the advantage to significantly decrease the risk of metabolic flux shifting.

While recent breakthroughs now facilitate the development of a prototypical microbial strain producing the µg-low mg range per liter, optimizing MNP synthesis in the microbial host up to industrial scale represents a last but considerable step (Figure 3F). While global approaches usually provide the lead for the main backbone compounds, production of the most effective bioactive molecule will require multiple iterative refinements using the Design-Built-Test-Learn cycle (Figure 3G). The mining of the synthesis pathways of natural biological products is indeed a primary application of system biology, however, the latter has also a great potential in the transformation and optimization of the metabolic network of microbial cell factories. In this regard, genome-scale metabolic network model, metabolic network optimization based on omics technologies now sustains optimization strategies for the efficient synthesis of active natural products in microbial cell factories (Srinivasan and Smolke, 2019). Whereas optimization of media and culture conditions as well as implementation of bioreactors can increase the production titers of the desired MNP, recent advances provide complementary solutions and strategies for improving production yields in the prototypical engineered microbes (Pyne et al., 2019 ;Zhang et al., 2020;[START_REF] Carqueijeiro | Beyond the semisynthetic artemisinin: metabolic engineering of plant-derived anti-cancer drugs[END_REF] (Figure 3E). For instance, it is now well documented that optimizing the microbe native metabolism, notably by increasing production of enzyme co-factors, is a first efficient way to revise metabolic fluxes in the microbial host. In addition, it was found that the timely induction of transgene expression and/or product capture during yeast culture can lead to an improvement of host tolerance to the produced drug. Interestingly, low heterologous gene expression can now be solved by increasing gene copy number using the CRISPR/Cas9 technology [START_REF] Kulagina | Enhanced bioproduction of anticancer precursor vindoline by yeast cell factories[END_REF].

Finally, recent advancements also firmly demonstrate that the identification and expression of more active orthologues is indispensable to reach high production levels [START_REF] Srinivasan | Biosynthesis of medicinal tropane alkaloids in yeast[END_REF].

Taken together, all these optimizations may be required to prevent undesired accumulation of metabolic intermediates and to reroute critical native precursors into the desired MNP biosynthetic pathways without overly impacting cellular fitness. This is an essential prerequisite to follow to reach an industrial scale production.

Concluding remarks

As recently successfully achieved for some bacterial, fungal and PNP-derived drugs [START_REF] Lee | Systems and synthetic biology to elucidate secondary metabolite biosynthetic gene clusters encoded in Streptomyces genomes[END_REF], the prospect of exploiting the potential offered by microbial metabolic engineering for developing alternative supply processes for promising MNP-derived pharmaceuticals is now within our grasp (Figure 4). While no MNPs have been produced by metabolic engineering to date, it will be important, in the near future, to carry out the first proof-of-concept studies in this field, notably by attempting to implement pioneering prototypical microbial strains in which biosynthetic pathways of several lead MNP are reconstructed. However, to this aim, it is highly likely that one will have to face some new difficulties of using systems biology methods to synthesize MNPs compared to PNPs. For MNPs that are biosynthesized (at least in part for a complex pathway) by marine invertebrates, the implementation would be requiring extensive research into the specialized metabolic routes in marine macroorganisms.

In addition, there is no reported experience on the heterologous expression of marine invertebrate enzymes in bacterial or yeast platforms. Indeed, while the reconstruction of a whole PNP biosynthetic pathway in microbial chassis is feasible, what about the subcellular targeting and activity of MNP enzymes, the metabolic fluxes, the toxicity of MNP intermediates or final products in these recipient cells? It behooves the research community to pursue production of a MNP in this way, particularly one with a complex scaffold, which currently suffers from a high cost of production, and for which the biosynthetic pathways are fully elucidated (Table 3). It is anticipated that such implementations will benefit from the recent advancements made concerning the production of PNP in microbes, notably for MNPs belonging to chemical classes for which benchmark and successful examples were achieved for PNPs [START_REF] Carqueijeiro | Beyond the semisynthetic artemisinin: metabolic engineering of plant-derived anti-cancer drugs[END_REF]. For instance and in such a perspective, producing trabectedin by microbial metabolic engineering could be supported by the recent development of a yeast platform for the synthesis of tetrahydroisoquinoline alkaloids at a gram per liter scale (Pynes et al., 2020). These technological developments seem ripe for future advancement in marine drugs and raise unprecedented expectations for the implementation of engineered cell factories, which may provide in a near future a sustainable approach for MNP pharmaceutical supply.

Figure 1 .

 1 Figure 1. The structures of bioactive MNPs that are either themselves or the inspiration for approved therapeutics and clinical candidates.

Figure 2 .

 2 Figure 2. Timeline depicting the successive approval of the currently used MNP-derived pharmaceuticals.Some prominent examples of marine source organisms are indicated. For instance, the anticancer drugs cytarabine and nelarabine, as the antiviral compound vidarabine are analogs of a couple of marine nucleosides, spongothymidine and spongouridine, which were originally isolated from the Caribbean sea sponge Tectitethya crypta. The painkiller ziconotide is an ω-conopeptide from the venom of the Pacific fish-hunting marine mollusk Conus magus. The anticancer agent eribulin mesylate corresponds to a simplified analogue of the marine polyether macrolide, halichondrin B, which was originally isolated from the sea sponge Halichondria okadai. Antibody-drug conjugate (ADC) combining a cytotoxic agent of marine origin with a selective monoclonal antibody such as brentuximab vedotin, enfortumab vedotin, polatuzumab vedotin, and belantamab mafodotin. These ADC's notably include in their structures a microtubule disrupting MNP derivative, i.e. either monomethyl auristatin E or monomethyl auristatin F that are synthetic analogs of the highly cytotoxic dolastatin 10 that was historically isolated from the Indian Ocean sea hare Dolabella auricularia. The anticancer MNPs trabectedin and its derivative lurbinectedin are tetrahydroisoquinoline alkaloids originally identified in the Caribbean sea squirt Ecteinascidia turbinata. Plitidepsin is a cyclic depsipeptide anticancer drug historically isolated from the marine tunicate Aplidium albicans.

Figure 3 .

 3 Figure 3. A benchmark procedure to implement MNP production in microbial cell factories. A. Theorical pathway reconstruction. Powerful metabolomics profiling approaches lead to the accurate detection of plausible reaction intermediates while recent advancements in molecular networking provide new efficient opportunities in the processing of the acquired metabolomics data. B. Identifying a first set of candidate genes. Once compounds are connected and a reaction sequence has been postulated in silico, bioinspired organic chemistry and literature data may then help in identifying a type of enzymatic activity required for each step. At this stage, the availability of the holobiont metagenome and massive transcriptomics data are of primary importance since searching the specific functional conserved domains may lead to define first sets of candidate genes. C. Prioritization procedures. The various possibilities include the screen for homology to orthologous gene sequences of already characterized biosynthesis steps in other models or the search of physical gene clusters. The most powerful and global approach that have recently emerged to rationally prioritize gene sequences in metabolic engineering relies on gene coexpression analysis. D. Validating the correct activity of each predicted genes/enzymes. To this aim, biochemical characterization of recombinant proteins heterologously expressed in bacteria or yeasts still represents a basic but reliable approach. E. Pathway reconstruction in a microbial chassis. A key factor of success to this aim is the choice of an appropriate heterologous host (a bacterium or a yeast for instance). In such a perspective, extensive synthetic biology toolboxs are available for both microbial models such as genomic integrative constructs, including CRISPR-Cas9 technologies, and auto-replicative plasmids. F. MNP synthesis in the microbial host. Main strategies include optimization of media and culture conditions but also implementation of bioreactors, screening of enzyme variant with improved activities, adaptation of the host metabolism, and optimization of enzyme targeting. G. Optimizing MNP synthesis with multiple iterative refinements based on the Design-Built-Test-Learn cycle.

Figure 4 .

 4 Figure 4. Current (A) and future (B) application fields of metabolic engineering and microbial cell factories.