Mathilde Godefroid
email: mathilde.an.godefroid@ulb.be

Philippe Dubois

Laetitia Hédouin

Thermal performance with depth: comparison of a mesophotic scleractinian and an antipatharian species subjected to internal waves in Mo'orea, French Polynesia

Keywords: Global warming, Thermal acclimatization, Mesophotic, Scleractinian, Antipatharian, Performance curve, Environmental variability

Local thermal environment has a strong influence on the physiology of marine ectotherms. This is particularly relevant for tropical organisms living close to their thermal optimum, well exemplified by the increasing frequency of bleaching occurrence in shallow-water corals. Mesophotic Coral Ecosystems (MCEs) were suggested as potential oases, especially when they are submitted to internal waves inducing short-term cooling events. Indeed, probability of bleaching occurrence in scleractinians was reported to decrease with depth in Mo'orea as temperature variability increases. However, ecophysiological data are currently lacking to understand the cause of lower susceptibility/increased plasticity of deeper corals. A growing interest has been devoted the last decade to MCEs, but our understanding of the physiological performance of benthic organisms living in this environment remains relatively unexplored. To tackle that question, we first compared the metabolic responses (dark respiration, net photosynthesis and photosynthetic efficiency) of the depth-generalist scleractinian Pachyseris speciosa from two heterogeneous thermal environment (25 and 85m depths) to acute heat stress to determine if the local thermal environment could predict coral response to warming. Then, we tested the thermal performance of two sympatric species (the scleractinian P. speciosa and the antipatharian Stichopathes sp.) to determine if there are inter-species differences in performances in species experiencing identical levels of temperature variability, at mesophotic depths (85m). Results revealed broader thermal performances in the mesophotic P. speciosa compared to mid-depth ones, and constrained performances in the mesophotic antipatharian compared to the scleractinian species. We hypothesize that the high fluctuations in temperature due to internal waves in deeper areas contribute to the broader thermal performances of mesophotic P. speciosa. However, the constrained performances of the mesophotic antipatharian compared to P. speciosa suggests that other processes than the symbiosis with zooxanthellae also influence thermal performances of these mesophotic organisms. Our results supported that Stichopathes sp. lives close to its thermal optimum, suggesting a (relatively) cold thermal specialist strategy. In this context, composition of MCEs in the future is unlikely to shift to antipatharian-dominated landscape and will remain coral-dominated landscape.

Introduction

Janzen (1967) hypothesized that the physiological tolerance of each organism should be large enough to encompass the entire gradient of conditions experienced throughout its life. This hypothesis influenced theoretical models in suggesting that organisms from more variable climate should be under selection for greater thermal acclimation ability than those from more stable climate ("The climate variability hypothesis"; [START_REF] Angilletta | Thermal adaptation: A theoretical and empirical synthesis[END_REF][START_REF] Ghalambor | Are mountain passes higher in the tropics? Janzen's hypothesis revisited[END_REF][START_REF] Gunderson | Plasticity in thermal tolerance has limited potential to buffer ectotherms from global warming[END_REF][START_REF] Tewksbury | Putting the Heat on Tropical Animals[END_REF]. Evidence in favour with this hypothesis has been accumulating, with studies indicating that tropical organisms have narrower thermal window of performance (Thermal breadth, Tbr) than temperate species, because their seasonal thermal range is smaller [START_REF] Deutsch | Impacts of climate warming on terrestrial ectotherms across latitude[END_REF][START_REF] Gaston | Macrophysiology: A Conceptual Reunification[END_REF][START_REF] Ghalambor | Are mountain passes higher in the tropics? Janzen's hypothesis revisited[END_REF][START_REF] Huey | Why tropical forest lizards are vulnerable to climate warming[END_REF][START_REF] Sunday | Thermal tolerance and the global redistribution of animals[END_REF][START_REF] Tewksbury | Putting the Heat on Tropical Animals[END_REF].

However, contrasting evidence were also obtained and suggested to reflect the influence of local/regional thermal variation in shaping thermal acclimation [START_REF] Angilletta | Thermal adaptation: A theoretical and empirical synthesis[END_REF][START_REF] Seebacher | Physiological plasticity increases resilience of ectothermic animals to climate change[END_REF]Shah et al., 2017aShah et al., , 2017b)). For example, experimental studies in the marine environment showed differences in heat stress tolerance across reef sites characterized by contrasting levels of thermal variability (i.e. fore-reef vs back-reef, inshore vs offshore reef ; [START_REF] Barshis | High-frequency temperature variability mirrors fixed differences in thermal limits of the massive coral Porites lobata (Dana, 1846)[END_REF][START_REF] Kenkel | Evidence for a host role in thermotolerance divergence between populations of the mustard hill coral ( Porites astreoides ) from different reef environments[END_REF][START_REF] Kenkel | Heritable differences in fitness-related traits among populations of the mustard hill coral, Porites astreoides[END_REF][START_REF] Oliver | Do fluctuating temperature environments elevate coral thermal tolerance?[END_REF].

Depth may also influence thermal variability, with shallow depths experiencing larger variations than subjacent mesophotic depths, which have long been considered as cooler and more stable thermally [START_REF] Bongaerts | Assessing the 'deep reef refugia' hypothesis: Focus on Caribbean reefs[END_REF][START_REF] Glynn | Coral reef bleaching: Facts, hypotheses and implications[END_REF]Turner et al., 2019). Therefore, organisms from mesophotic depths were hypothesized to have higher thermal sensitivities than shallow water ones [START_REF] Smith | Caribbean mesophotic coral ecosystems are unlikely climate change refugia[END_REF]. This has been supported by some studies (Frates et al., 2021;[START_REF] Smith | Caribbean mesophotic coral ecosystems are unlikely climate change refugia[END_REF] but contrasting evidence arose from physiological measurements and bleaching events [START_REF] Frade | Deep reefs of the Great Barrier Reef offer limited thermal refuge during mass coral bleaching[END_REF][START_REF] Gould | Upper-mesophotic and shallow reef corals exhibit similar thermal tolerance, sensitivity and optima[END_REF][START_REF] Rocha | Mesophotic coral ecosystems are threatened and ecologically distinct from shallow water reefs[END_REF]. Now, thermal stability of mesophotic environments may be compromised by cold water intrusions due to oscillations of water column isotherms, mixing, internal waves and tides [START_REF] Leichter | Variation beneath the surface: Quantifying complex thermal environments on coral reefs in the Caribbean, Bahamas and Florida[END_REF][START_REF] Leichter | The island-scale internal wave climate of Moorea, French Polynesia: internal waves on Mo'orea[END_REF][START_REF] Leichter | Regional synchrony of temperature variation and internal wave forcing along the Florida Keys reef tract: Regional synchrony of internal waves[END_REF][START_REF] Sheppard | Large temperature plunges recorded by data loggers at different depths on an Indian Ocean atoll: Comparison with satellite data and relevance to coral refuges[END_REF][START_REF] Wall | Largeamplitude internal waves benefit corals during thermal stress[END_REF][START_REF] Wolanski | Large amplitude, leaky, island-generated, internal waves around Palau, Micronesia[END_REF][START_REF] Wyatt | Heat accumulation on coral reefs mitigated by internal waves[END_REF]. Internal waves are created by the rise of cold and nutrient-rich waters being uplifted up the reef slope, inducing thermal fluctuations at the time scale of hours to days which often exceed the magnitude of temperature fluctuations in shallow waters [START_REF] Godefroid | Thermal stress responses of the antipatharian Stichopathes sp. From the mesophotic reef of Mo'orea, French Polynesia[END_REF][START_REF] Leichter | The island-scale internal wave climate of Moorea, French Polynesia: internal waves on Mo'orea[END_REF]. This cold water flushes to mesophotic depths may alleviate cumulated thermal stress during heat waves, resulting, for instance, in reduced scleractinian bleaching (Pérez-Rosalez et al., 2021;Storlazzy et al., 2020;[START_REF] Wall | Largeamplitude internal waves benefit corals during thermal stress[END_REF][START_REF] Wyatt | Heat accumulation on coral reefs mitigated by internal waves[END_REF] but, to our knowledge, it has never been addressed if such transient cold thermal variability resulted in increased thermal tolerance (wider Tbr) of mesophotic organisms.

Mesophotic Coral Ecosystems (MCEs) are characterized by the presence of light-dependent corals and associated communities, typically from around 30m depth and extending up to 150m in certain regions [START_REF] Hinderstein | Theme section on "Mesophotic Coral Ecosystems: Characterization, Ecology, and Management[END_REF][START_REF] Lesser | Ecology of mesophotic coral reefs[END_REF]. In tropical zones, scleractinian corals usually dominate in shallow waters (when conditions are appropriate), with some species extending their range to mesophotic depths. Reef community composition progressively shifts with depth and nonphotosynthetic organisms become dominant when light conditions do not meet the requirements for scleractinian photosynthesis (Pyle et al., 2019). Among these, antipatharians are particularly abundant at mesophotic depths, sometimes forming dense aggregations called Marine Animal Forests (MAFs, sensu Rossi et al., 2017), and even becoming dominant at depth in several regions. Their role as habitat providers and nursery areas then become fundamental to the maintenance of ecosystem functions [START_REF] Bo | A myzostomid endoparasitic in black corals[END_REF][START_REF] Bo | First record of a symbiotic relationship between a polyclad and a black coral with description of Anthoplana antipathellae gen. Et sp. Nov. (Acotylea, Notoplanidae)[END_REF][START_REF] De Clippele | The Diversity and Ecological Role of Non-scleractinian Corals (Antipatharia and Alcyonacea) on Scleractinian Cold-Water Coral Mounds[END_REF][START_REF] Terrana | Assessing trophic relationships between shallow-water black corals (Antipatharia) and their symbionts using stable isotopes[END_REF]. They differ from scleractinians because they are commonly referred to as azooxanthellate, with only a few specimens showing the presence of Symbiodiniaceae/Symbiodiniaceae-like cells [START_REF] Bo | First description of algal mutualistic endosymbiosis in a black coral (Anthozoa: Antipatharia)[END_REF][START_REF] Gress | Investigating densities of Symbiodiniaceae in two species of Antipatharians (black corals) from Madagascar[END_REF][START_REF] Wagner | Azooxanthellate? Most Hawaiian black corals contain Symbiodinium[END_REF].

They also differ from scleractinians by their proteinaceous (and not calcareous) skeleton [START_REF] Opresko | Antipatharia (Black Corals) for the New Zealand region: A field guide of commonly sampled New Zealand black corals including illustrations highlighting technical terms and black coral morphology[END_REF][START_REF] Wagner | The Biology and Ecology of Black Corals (Cnidaria: Anthozoa: Hexacorallia: Antipatharia)[END_REF].

To date, one study only has addressed the thermal sensitivity of a single antipatharian species and revealed that it lives close to its Topt (at least during the coldest months of the year) and has a rather narrow Tbr (tropical mesophotic Stichopathes sp., 15 days of exposure; [START_REF] Godefroid | Thermal stress responses of the antipatharian Stichopathes sp. From the mesophotic reef of Mo'orea, French Polynesia[END_REF]. This experiment was conducted in the island of Mo'orea in French Polynesia, which is known to be characterized by strong internal wave activity that impacts reef slopes at depths ≥30m around the entire island [START_REF] Leichter | The island-scale internal wave climate of Moorea, French Polynesia: internal waves on Mo'orea[END_REF]. Superimposed on seasonal temperature changes of approximately 3°C, internal waves create a pattern of high frequency temperature variation that is created by the arrival of cold and nutrient rich waters [START_REF] Leichter | The island-scale internal wave climate of Moorea, French Polynesia: internal waves on Mo'orea[END_REF]. The amplitude of these fluctuations increases with depth and are modulated across seasons, with markedly higher internal waves activity from Oct-May than from Jun-Sep [START_REF] Godefroid | Thermal stress responses of the antipatharian Stichopathes sp. From the mesophotic reef of Mo'orea, French Polynesia[END_REF][START_REF] Leichter | The island-scale internal wave climate of Moorea, French Polynesia: internal waves on Mo'orea[END_REF][START_REF] Pichon | French Polynesia[END_REF][START_REF] Wyatt | Heat accumulation on coral reefs mitigated by internal waves[END_REF]. Over a 40-days period during the cold season (Oct-Nov 2019), which coincides with the period of high internal wave activity, mean daily temperature fluctuations were twice as large at 90m than at 20m and the total temperature range over this period was 4.6°C at 90m vs 2.9°C at 20m [START_REF] Godefroid | Thermal stress responses of the antipatharian Stichopathes sp. From the mesophotic reef of Mo'orea, French Polynesia[END_REF].

This indicates that mesophotic depths in Mo'orea are more variable than shallow-water ones, at least during the cold season [START_REF] Godefroid | Thermal stress responses of the antipatharian Stichopathes sp. From the mesophotic reef of Mo'orea, French Polynesia[END_REF].

The aim of the present study was first to assess how the thermal physiology of a generalist coral living from mid-depth (25m) to mesophotic (85m) environment, characterised by distinct temperature regimes (warmer and less variable in shallower part of the reefs), varies along the depth gradient in Mo'orea. We established TPCs for respiration, photosynthetic efficiency (Fv/Fm) and net photosynthesis (Pnet) of these two populations of Pachyseris speciosa living in different environments, using a shortterm ramping method. We hypothesized that TPCs should differ across the depth gradient, with lower Topt and wider Tbr for mesophotic P. speciosa than for mid-depth ones. Secondly, we investigated the thermal performance of two sympatric species (the scleractinian Pachyseris speciosa and the antipatharian Stichopathes sp.) exposed to identical thermal regime (mesophotic zone, 85m). We established TPCs for respiration (both species), photosynthetic efficiency (Fv/Fm) and net photosynthesis (P. speciosa), their shapes and positions, to investigate the inter-specific variation of thermal strategy in mesophotic environment. Understanding thermal performance strategy and plasticity of key benthic organisms living in mesophotic areas will be an asset to better predict how climate change will shape the future of the reefs from shallow to mesophotic areas.

Materials and Methods

Coral collection and acclimation

Coral colonies were collected on the outer reef slope on the North shore of the island of Mo'orea (17°28'41.7''S, 149°51'09.9''W). The fore reefs on the North coast of Mo'orea are characterized by a gentle slope (40-60°) from the surface down to 70-80m depth, where it drops almost vertically (inclination of 60% or more) [START_REF] Pérez-Rosales | Mesophotic coral communities escape thermal coral bleaching in French Polynesia[END_REF]. Between 30 and 70m depth, the substrate is mainly covered by sedimentary deposits, known as the "sandy plain", which is virtually devoid of sessile benthos, except in some parts where there is an almost continuous monospecific cover of the substratum (up to 70-80%) by Pachyseris speciosa, over large areas [START_REF] Pichon | French Polynesia[END_REF]. P. speciosa is a keystone mesophotic species that is very common and widespread in the Society and Tuamotu Archipelagos [START_REF] Pichon | French Polynesia[END_REF].

On 26°C, salinity: 35.5). Light was controlled using AquaIllumination Hydra lamps (USA) in the blue spectrum (4-15 µmol photons m -2 s -1 ). The same day, eight colonies of P. speciosa were also collected at 25m depth (gentle slope inclination), on the same site than the mesophotic colonies (17°28'41.7''S, 149°51'09.9''W). They were transported to the CRIOBE and immediately fragmented (four fragments per colony) and placed in another sand-filtered aquarium with parameters of the corresponding site (Volume: 200L, Temperature: 28°C, salinity: 35.5, light: 200-250 µmol photons m -2 s -1 in the baseline spectrum). These depths (25 and 85m) were selected as 25m corresponds to the upper bathymetric range of P. speciosa in Mo'orea, extending at least up to 85m, corresponding to the upper bathymetric range of Stichopathes sp., while remaining accessible to technical divers. A complete characterization of the thermal environment of this site at different depths was reported in [START_REF] Godefroid | Thermal stress responses of the antipatharian Stichopathes sp. From the mesophotic reef of Mo'orea, French Polynesia[END_REF] The surface area of the P. speciosa fragments was measured using a high-quality 3D scanner (EinScan-SP Scanner, Shining 3D, China). The black and reflective properties of the skeleton prevented the use of the scanner for Stichopathes sp., so its surface area was calculated based on the surface formula of a cylinder. Surface areas of the fragments were 5.1 ± 0.3 cm 2 (n=21) for Stichopathes sp., 79.9 ± 3.9 cm 2 (n=24) and 71.2 ± 2.3 cm 2 (n=26; mean ± se) for Pachyseris speciosa 25 and 85m, respectively.

The fragments were left five days to recover and then temperature in both aquaria was gradually decreased/increased by 0.5°C/day to reach a final common temperature at 27°C (i.e. the acclimation temperature). 27°C was selected as it is the average temperature between both depths at the time of collection and it is within the annual range of temperature for both depths. When the target temperature of 27°C was reached, the fragments were left to acclimate to these new conditions for at least ten days before starting the ramp experiments. The experiment time schedule is depicted in Figure 1a.

Aquarium conditions

Once at the target acclimation temperature (27°C), the only parameter that differed between the aquaria was light, and all other parameters were manipulated identically. Both aquaria were opencircuit, with seawater pumped at 25m depth in the Bay in front of the facilities and sand and UV filtered. Temperature was manipulated using water chillers (one per aquarium, TECO TK1000, Italy)

and light was provided using AquaIllumination Hydra (one per aquaria, USA) following the natural diel cycle. From 05:30 to 10:00 am, light was ramped up to a maximum (around 200-250 µmol photons m - 2 s -1 , using the baseline spectrum provided by the AquaIllumination Hydra lamps, for 25m and 4-15 µmol photons m -2 s -1 , in the blue spectrum, for 85m), which was maintained until 2 pm, before ramping down to total darkness at 6 pm. Light intensity was selected based on in situ data from 20 and 90m collected using light loggers (DEFI2-L, JFE Advantech Co., Japan). As Stichopathes sp. depends on heterotrophic feeding, both aquaria were fed daily with a mix composed of live Artemia salina, copepods and phytoplankton.

The respirometry experiments were run in a separate system composed of an open-circuit buffer tank (150 L) where the temperature was manipulated, connected with a pump system to an experimental tank (55 L) where the respirometry chambers were installed for the respirometry analyses. The experimental tank was overflowing into the buffer tank and excess seawater in the buffer tank was removed from the system by overflow. The buffer tank and the experimental tank were equipped with circulation pumps to ensure a complete homogenisation of the temperature in both tanks. The high volume of seawater (200 L) in the system and the fact that temperature was only manipulated in the buffer tank allowed maintaining a very stable temperature in the experimental tank at all stages of the experiments. Moreover, the open-circuit system ensured that the accumulation of catabolism and stress molecules or mucus produced by the fragments during respirometry measurements would be eliminated between each measurement period, thus allowing good water quality throughout the experiments. Temperature in the buffer tank was controlled using one chiller (TECO TK2000, Italy) and three titanium heaters (Schego, Germany), each connected to a temperature controller (ITC-308, Inkbird, China). This allowed increasing water temperature quickly between the experimental stages.

A bubbling system was installed in the experimental tank to increase seawater aeration.

Thermal ramping assays

Ramp experiments were performed to determine the TPCs of respiration (Rdark; Stichopathes sp. and P. speciosa), photosynthesis (Pnet) and photochemical efficiency (Fv/Fm) (P. speciosa only as Stichopathes sp. does not host zooxanthellae). Each ramp experiment was divided in two legs, both starting at the acclimation temperature: a hot ramp (27, 28, 29, 30, 31, 32°C) and a cold ramp (27, 26, 25, 24, 23°C). Four experiments were performed in the dark to assess the TPCs for Rdark and Fv/Fm. One fragment from eight Pachyseris speciosa 25m colonies, eight Pachyseris speciosa 85m colonies and eight Stichopathes sp. colonies was used for the hot ramp and another (from the same colony) for the cold ramp. This way, a fragment was only used in a single hot ramp experiment (and was not reused)

and another fragment from the same colony for the cold ramp. In total, eight independent replicates (colonies) per species/population were tested for each leg of the ramp experiment, with paired (from the same colony) replicates between cold and hot ramps.

Ramp experiments were performed similarly under light to assess the TPCs for Pnet. As light intensity decreases with depth, distinct ramps were performed for P. speciosa 25m and 85m, at the respective light intensities used for the acclimation period. One hot and one cold ramp were performed on fragments from 5 colonies for P. speciosa 25m and the same on fragments from 5 colonies for P. speciosa 85m. So, a total of 5 independent replicates (colonies) per population were tested for each leg of the ramp experiment, with paired (from the same colony) replicates between cold and hot ramps.

Each ramp experiment proceeded similarly, with six cycles of a hot ramp (on day 1), followed by a cold ramp (on day 2) performed on another fragment from the same colonies. For example, in the case of the hot ramp experiments in the dark, six coral fragments were moved from their aquarium at about 6 am to one of the eight respirometry chambers that were held in the experimental tank at 27°C (Fig. 1b). After 1 hour of acclimation in either dark or light conditions (i.e. light intensity corresponded to the maximum light intensity used in the acclimation tank), the chambers were closed and oxygen consumption was measured for 40 min to estimate Rdark or Pnet. Then, the chambers were opened in the experimental tank (allowing water exchange between the water in the chambers and from the experimental tank) and Fv/Fm was measured in triplicate for each fragment (only in the dark incubations) before increasing the temperature to the next target temperature (+1°C), for one hour (i.e. 30-min temperature increase and 30-min of acclimation to the new temperature). Then, the chambers were sealed again to start a new 40-min measurement period to estimate Rdark or Pnet. The same procedure was repeated until the full range of experimental temperatures had been tested (up to 32°C), after which temperature was decreased back to 27°C in one step (i.e. the acclimation temperature) and new measurements of oxygen consumption were taken after 2.5 (around 7 pm) and 12 hours (the next morning) to evaluate the recovery capacity of the fragments (Fig. 1b). Between the end of the ramp experiment and the 2.5 and 12h "recovery" measurements, chambers were opened and water exchanged with the water of the experimental tank. For the cold ramp experiments, the same procedure (acclimation, measurement of Rdark or Pnet, then Fv/Fm) was followed at 27, 26, 25, 24 and 23°C but the recovery capacity was not assessed at the end. These blanks allowed measuring the background respiration (i.e. the part of the respiration attributed to the seawater microbes). At the end of the measurements, the surface area of the fragments was measured as previously described. Oxygen saturation measurements were normalized based on the volume of seawater in the experimental chambers (𝑉), calculated from the density of seawater [START_REF] Millero | The density of seawater as a function of salinity (5 to 70 g kg -1 ) and temperature (273.15 to 363.15 K)[END_REF][START_REF] Millero | International one-atmosphere equation of state of seawater. Deep Sea Research Part A[END_REF]. Oxygen saturation was then converted to oxygen concentration and the rate of oxygen consumption (𝑉𝑂 $ ) was calculated from the slope of its linear regression %[𝑂 $ ] ( ) -[𝑂 $ ] ( + , with time (𝑡): Oxygen consumption rate of the fragments was corrected according to the consumption rate of the seawater-only chamber and normalized using the surface area (𝑆𝐴) of the fragments, and was expressed in micromole of oxygen per square centimetre per hour.

𝑉𝑂 $ = /0%[𝑂 $ ] ( ) -[𝑂 $ ] ( + ,
At the end of each metabolic rate measurements, all respirometry chambers were opened in the experimental tank and the performance of the Symbiodiniacea hosted by fragments of P. speciosa was quantified by measuring their photochemical efficiency (Fv/Fm), in triplicate per fragment. These measurements were repeated at each temperature of all ramp experiments performed in the dark using a pulse-amplitude modulation fluorimeter (Diving-PAM, Walz, Germany). Dark-acclimated Fv/Fm should decrease if the symbionts become thermally stressed [START_REF] Aichelman | Adaptive signatures in thermal performance of the temperate coral Astrangia poculata[END_REF][START_REF] Fitt | Coral bleaching: Interpretation of thermal tolerance limits and thermal thresholds in tropical corals[END_REF][START_REF] Thornhill | Correspondence between cold tolerance and temperate biogeography in a Western Atlantic Symbiodinium (Dinophyta) lineage[END_REF]Warner et al., 1996). Pgross, the total amount of oxygen produced by the fragments, was calculated by adding Rdark to Pnet.

Statistical analyses

All data were analysed using the statistical software R version 1.3.959 (R Core Team, 2020).

Non-linear least-square regression models were fitted to the variables Rdark for Stichopathes sp. and Fv/Fm for P.speciosa, using a symmetrical Gaussian function (as recommended by Angilletta, 2006, and widely used since, see e.g. [START_REF] Rodolfo-Metalpa | Thermally tolerant corals have limited capacity to acclimatize to future warming[END_REF], Jurriaans et al., 2019[START_REF] Wyatt | Heat accumulation on coral reefs mitigated by internal waves[END_REF] with the R package nlstools (Baty et al., 2015):

𝑃 = 𝑃𝑓 C8D * exp H-0.5 3 LML NOP L QR < $ S ( 1
)
where P is the physiological response, Pfmax is the maximum value of that response, Topt is the temperature at which the response value is maximal (i.e. the mean of the Gaussian function) and Tbr is the breadth of the response curve (i.e. the standard deviation of the Gaussian function). Significant differences between the parameters Pfmax, Topt and Tbr were defined as non-overlapping 95% confidence intervals.

Simple linear regression models (𝑦 = 𝑎𝑥 + 𝑏) were fitted to the variables Rdark, Pnet and Pgross for P.

speciosa, with temperature as independent variable (n=10). Comparison of slopes were performed using the R package lsmeans (Russel, 2016).

As Rdark data were not modelled identically for Stichopathes sp. (Gaussian curve) and P. speciosa (Linear regression), thermal sensitivity was compared by calculating the temperature coefficient Q10 (i.e. the proportional change of respiration in response to a temperature increase of 10°C) using the van't Hoff equation:

𝑄 Z[ = H 𝑅 $ 𝑅 Z S ^Z[ L _ ML + `
where R1 and R2 are the rates of oxygen consumption corresponding to low temperature (T1) and high temperature (T2), respectively. Q10 was calculated in the range 23-32°C for P. speciosa and 23-28°C for Stichopathes sp. (in the range of linear increase with temperature). Q10 values were first calculated for each individual colony between consecutive temperatures and these were then averaged per population. Q10 is typically around 2-3 for physiological processes; a lower Q10 indicates that respiration rates were less sensitive to increasing temperatures, whereas a higher Q10 indicates higher thermal sensitivity.

Analysis of residuals was carried out for all regression analysis. No trend in residuals was evidenced.

Recovery capacity for each variable and P. speciosa population was evaluated using two-factor mixed model ANOVA, with population and time as fixed cross factors and fragment as a random repeated subject (Doncaster & Davey, 2007). When the interaction was significant, a post-hoc Tukey's test was performed using the appropriate mean square error. Rdark for Stichopathes sp. was analysed separately using one-factor ANOVA with Time as fixed factor, as the surface values of the two studied species were highly different due to their distinct morphologies (voluminous and flat vs. thin and filiform), which strongly influenced the Rdark value after normalisation.

Results

Metabolic responses with temperature

Dark respiration (Rdark) increased linearly with temperature for P. speciosa living at 25m (mid-depth colonies) and 85m (mesophotic colonies) (Figure 2a, b; Linear regression, p<0.001 for both species, Table S1). However, the rate of increase (i.e. the slope of the regression) is significantly higher for the mid-depth than for the mesophotic colonies (Comparison of slopes, p<0.05, Table S2). For Stichopathes sp., Rdark data can be fitted by a nonlinear Gaussian relationship with temperature (Figure 2c). Topt was calculated at 29.6°C, corresponding to a maximum Rdark of 0.2 µmol O2 cm -2 h -1 (Table S3). Q10 was calculated in the linear part of the regression (before Topt for Stichopathes sp.) to allow comparing the three groups (P. speciosa 25m, 85m and Stichopathes sp.). Q10 did not differ between depths and stayed around 2-3 for P. speciosa , but was much higher and variable for Stichopathes sp. (45.68 ± 61.96, mean ± sd n=6) (Table 1).

Absolute values of Net Photosynthesis (Pnet) were positive at all temperatures for P. speciosa 25m but were negative for the mesophotic fragments of P. speciosa (Figure 2d, e). At both depth, Pnet decreased linearly with temperature (Linear regression, p<0.001 for P. speciosa 25m and p<0.005 for P. speciosa 85m; Table S1) but the decrease was significantly more important for the mid-depth fragments (Comparison of slopes, p<0.05; Table S2).

The compensatory effect of Pnet on Rdark was assessed by calculating gross photosynthesis. Pgross tended to decrease linearly with temperature for mid-depth fragments and increased significantly for mesophotic ones (Linear regression, p>0.05 for P. speciosa 25m and p<0.001 for P. speciosa 85m; Table S1). Comparison of slopes revealed that depth had significant influence on Pgross (p<0.001), but temperature did not (p>0.05; Table S2).

Photosynthetic efficiency (Fv/Fm) can be fitted by a nonlinear Gaussian relationship with temperature, for fragments of P. speciosa from both depths (Figure 2f, g). The Topt was 26.2°C for the mid-depth fragments and 27.8°C for the mesophotic ones (corresponding to maximum yields of 0.68 for both depths). However, confidence intervals overlapped for all the estimated parameters (Topt, Tbr, Fv/Fm max)

for P. speciosa 25 and 85m, indicating no significant differences between the two populations for these parameters (Table S3). S3.

Recovery capacity after the ramping

Recovery capacity was assessed by comparing measures at acclimation temperature before ramping and 2.5 and 12h after ramping and getting back to the acclimation temperature. Rdark of P.speciosa significantly differed according to depth (Mixed model ANOVA, pdepth<0.001), time (ptime<0.001) and the interaction of these factors (pdepth * time=0.015) Values of Rdark were significantly higher for the middepth fragments of P. speciosa than for the mesophotic ones, at acclimation temperature (27°C) and all recovery stages (pTukey<0.001; Figure 3a, b; Table S5). In addition, 2.5 hours after being brought back to acclimation temperature, fragments of P. speciosa from all depths had a significantly higher respiration rate than before the ramping (pTukey=0.048 for P. speciosa 25m and pTukey=0.028 for P. speciosa 85m), but a respiration rate not significantly different from that before the ramping was reached after 12 hours for both populations (pTukey ≥0.112). For Stichopathes sp., there were no significant differences between Rdark pre-ramping and post-ramping recovery measurements (ptime=0.075) (Figure 3c, Table S4).

Absolute values of Pnet were significantly higher for the mid-depth colonies of P. speciosa than for the mesophotic ones (Mixed model ANOVA, pdepth<0.001; Table S5). The Pnet of mesophotic P. speciosa is so low (around zero) than no effect can be observed. No significant differences were highlighted between Pnet values pre-ramping and post-ramping recovery nor the interaction between time and depth (pTime=0.144, pTime*depth=0.462) (Figure 3d, e, Table S5).

Photosynthetic efficiencies (Fv/Fm) of fragments of P. speciosa did not differ according to depth (Mixed model ANOVA, pdepth= 0.370) but were significantly reduced after 2.5 and 12 hours of recovery postramping (pTime<0.001; pTukey ≤0.013); the two latter did not differ (pTukey >0.083; Figure 3f, g; Table S5). (Tables S4,S5).

Discussion

Ongoing ocean warming is increasingly damaging coral reefs and evidence of mass mortality events are multiplying. Researchers have suggested that depth may provide refuge from events occurring at shallower depth, as heat accumulation decreases with increasing depth (Turner et al., 2019). Yet, evidence from experimental studies, oceanographic thermal models and bleaching events are mixed [START_REF] Frade | Deep reefs of the Great Barrier Reef offer limited thermal refuge during mass coral bleaching[END_REF]Frates et al., 2021;[START_REF] Gould | Upper-mesophotic and shallow reef corals exhibit similar thermal tolerance, sensitivity and optima[END_REF][START_REF] Pérez-Rosales | Mesophotic coral communities escape thermal coral bleaching in French Polynesia[END_REF][START_REF] Rocha | Mesophotic coral ecosystems are threatened and ecologically distinct from shallow water reefs[END_REF][START_REF] Smith | Caribbean mesophotic coral ecosystems are unlikely climate change refugia[END_REF][START_REF] Venegas | The Rarity of Depth Refugia from Coral Bleaching Heat Stress in the Western and Central Pacific Islands[END_REF][START_REF] Wall | Largeamplitude internal waves benefit corals during thermal stress[END_REF]. Differences in sensitivity across depths can be attributed to differing environmental conditions and/or species physiological capacities [START_REF] Page | Seeking Resistance in Coral Reef Ecosystems: The Interplay of Biophysical Factors and Bleaching Resistance under a Changing Climate: The Interplay of a Reef's Biophysical Factors Can Mitigate the Coral Bleaching Response[END_REF]. While abiotic differences across depths have been addressed multiple times, intrinsic metabolic differences have not yet received much attention [START_REF] Ferrier-Pagès | Symbiotic stony and soft corals: Is their host-algae relationship really mutualistic at lower mesophotic reefs?[END_REF].

Pachyseris speciosa 25m vs. 85m

In this study, we assessed the thermal performance of the photosynthetic scleractinian Pachyseris speciosa from distinct depths (25 and 85m). Given the cooler mean temperatures and highly fluctuating thermal conditions (owing to internal waves) at mesophotic depths in Mo'orea [START_REF] Leichter | The island-scale internal wave climate of Moorea, French Polynesia: internal waves on Mo'orea[END_REF][START_REF] Godefroid | Thermal stress responses of the antipatharian Stichopathes sp. From the mesophotic reef of Mo'orea, French Polynesia[END_REF], we expected that mesophotic colonies of P. speciosa would have lower Topt and wider Tbr compared to mid-depth colonies. Topt was not reached by specimens from both depths for respiration and photosynthesis: performances increased linearly with temperature, so we could not determine Topt nor Tbr for P. speciosa. This is likely related to the experimental protocol used: longer exposure time or further temperature increase (above 32°C) could allow achieving Topt. Despite this, the effect on performances (i.e. the slope or the rate of activation, Ea) was higher on mid-depth than on mesophotic colonies, suggesting that mesophotic colonies have a higher capacity to encompass large range of temperatures and will be less affected by temperature regimes higher than

Topt, compared to their mid-depth congeners.

Symbiont density, that was not assessed in the present study, might be one of the factors influencing depth-specific thermal performance in terms of respiration. Photoacclimatation is a well-known process in corals, allowing to maintain efficient performance in distinct light intensity regimes. In low light environment as in mesophotic depths, contrasting results were reported, showing in some cases an increase, decrease or stable symbiont density [START_REF] Fitt | Seasonal patterns of tissue biomass and densities of symbiotic dinoflagellates in reef corals and relation to coral bleaching[END_REF][START_REF] Leletkin | Photosynthesis and respiration of the zooxanthellae in hermatypic corals habitated on different depths of the Gulf of Filat[END_REF][START_REF] Mass | The spectral quality of light is a key driver of photosynthesis and photoadaptation in Stylophora pistillata colonies from different depths in the Red Sea[END_REF]Padilla-Gamino et al., 2019;[START_REF] Smith | Acclimatization of symbiotic corals to mesophotic light environments through wavelength transformation by fluorescent protein pigments[END_REF]. Our data revealed a higher photosynthetic rates (Pnet) in mid-depth than in mesophotic fragments, which may be due to higher symbiont density and/or chlorophyll pigments. Differences in thermal performance across depths could also be related to the composition in Symbiodiniaceae, but previous research in Mo'orea showed that Symbiodiniaceae and bacterial communities did not differ across depths (6-120m) for P. speciosa (Rouzé H., unpublished results).

Photoacclimatation is not the main or single driver modulating the capacity of corals to thrive under fluctuating environmental conditions. Other parameters, such as changes in colony morphology or in the contribution of heterotrophy, through feeding, are also known to differ across depths [START_REF] Kahng | Light, Temperature, Photosynthesis, Heterotrophy, and the Lower Depth Limits of Mesophotic Coral Ecosystems[END_REF]. In the present study, we did not characterize the heterotrophic contribution of corals along the depth gradient. So, we cannot exclude that the heterotrophic energy input through feeding, that

is known in some cases to represent an important contribution in the mesophotic corals [START_REF] Kahng | Light, Temperature, Photosynthesis, Heterotrophy, and the Lower Depth Limits of Mesophotic Coral Ecosystems[END_REF], will affect thermal performance. Further research will be required to determine the role of photo-physiological capacities and heterotrophy into the performance of corals from distinct depths.

Another factor that is likely to influence the thermotolerance of P. speciosa from different depths is the adaptation and/or acclimatization of the corals to the local temperature regime. A large body of research on scleractinian corals showed that thermal history, in particular temperature variability, affects coral physiological tolerance under heat stress [START_REF] Barshis | High-frequency temperature variability mirrors fixed differences in thermal limits of the massive coral Porites lobata (Dana, 1846)[END_REF][START_REF] Castillo | Influence of thermal history on the response of Montastraea annularis to short-term temperature exposure[END_REF][START_REF] Guest | Contrasting Patterns of Coral Bleaching Susceptibility in 2010 Suggest an Adaptive Response to Thermal Stress[END_REF][START_REF] Kenkel | Evidence for a host role in thermotolerance divergence between populations of the mustard hill coral ( Porites astreoides ) from different reef environments[END_REF][START_REF] Kenkel | Heritable differences in fitness-related traits among populations of the mustard hill coral, Porites astreoides[END_REF][START_REF] Oliver | Do fluctuating temperature environments elevate coral thermal tolerance?[END_REF][START_REF] Safaie | High frequency temperature variability reduces the risk of coral bleaching[END_REF]. However, research that have experimentally compared thermal sensitivity of coral conspecifics from distinct depths are relatively scarce. These studies either found no differences in thermal sensitivity across depth (instantaneous acute heat stress, 5-10m vs 30-35m; [START_REF] Gould | Upper-mesophotic and shallow reef corals exhibit similar thermal tolerance, sensitivity and optima[END_REF] or lower tolerance to acute heat stress in mesophotic colonies (ramping of 1°C/day; Frates et al., 2021). Our study is the first to present thermal performance of corals from lower mesophotic zone (45m at the deepest in previous studies). We unravelled distinct thermal performance of mesophotic colonies compared to mid-depth ones, with a broader thermal performance of mesophotic corals exposed to heat stress. Our results therefore suggest that high temperature variability (owing to internal waves activity) at mesophotic depths in French Polynesia likely contributes to the observed broader thermal performance of colonies from those depths.

Q10 results did not differ between depths, which does not support the higher thermotolerance of mesophotic colonies previously discussed based on the comparison of slopes between depths. This is counterintuitive as Q10 derives from the slope, but this is due to the different methods of calculation.

Indeed, Q10 was first calculated on each individual subjected to the different temperatures and all Q10

were then averaged (mean ± sd), while slopes of the linear regressions were calculated based on all data together (and not individual colonies first). Q10 value therefore provides an indication of the variability of responses between depths, which was higher in mesophotic than in mid-depth colonies (Table 1). This higher variability in mesophotic colonies could be explained by the more fluctuating environmental conditions at these depths, and thus potentially a greater diversity in response strategies to these fluctuating conditions, compared to the mid-depth colonies that are exposed and have acclimated/adapted to more stable environmental conditions.

Comparing thermal performance of corals living at distinct depths has a real ecological interest if both shallow and deep zones are impacted by heat stress (e.g. marine heatwaves). MCEs are often viewed as areas of environmental stability, being spared by heat stress, which creates good conditions for the development and survival of corals compared to their shallow-water counterparts. However, MCEs may also be affected by heat stress and depending on the thermotolerance of corals living in these areas, the consequences of heat stress affecting mesophotic reefs may be totally different. Although long-term temperature data at 85m were not available in French Polynesia, data were available from 3 to 50m and included the 2019 mass bleaching event around Mo'orea. These data confirmed ours in that temperature is colder and more variable in deeper areas of the reefs and they also revealed that the 2019 marine heatwave did not spare mesophotic depth, with a clear rise in temperature in both shallow and mesophotic areas (at least up to 50m; Fig. S1). This confirmed that mesophotic corals (at 50m) are impacted by heat stress and we could hypothesize that such heat stress was also present up to 85m. The suspected presence of heat stress at 85m, combined with the broader thermal performance of corals at 85m, support that mesophotic zone may serve as resistant oases [START_REF] Guest | A framework for identifying and characterising coral reef "oases" against a backdrop of degradation[END_REF] that are able to tolerate thermal anomaly without losing coral cover due to specific traits of the corals living there, as P. speciosa. We suggest that this is likely because mesophotic colonies, exposed to more variable seawater temperatures, developed broader thermal performance to withstand warming than their mid-depth congeners. This supports the assumption that mesophotic environments exposed to large thermal fluctuations, such as caused by internal waves, will be an important asset in the future from a management perspective, as they are most likely to be less susceptible to global warming than their shallower counterparts. We acknowledge that this observation of broader thermal performance of mesophotic corals was performed on a single species using acute heat stress. More endpoints at longer time scale and other depth-generalist species should be tested to confirm that pattern at a larger scale. In addition, even if bleaching was not observed per se, the increase in metabolic activity can be interpreted as an increase in sensitivity of the process [START_REF] Silbiger | Comparative thermal performance of the reef-building coral Orbicella franksi at its latitudinal range limits[END_REF], that could potentially lead to bleaching if exposure was extended over a longer period.

In contrast with previous parameters, the effect of temperature on photochemical efficiency (dark acclimated Fv/Fm) did not differ significantly between mid-depth and mesophotic colonies of P.

speciosa. The relationship Fv/Fm according to temperature was described by a Gaussian function, with a maximum (Topt) at 26.2 and 27.8°C (at 25 and 85m, respectively) beyond which it started decreasing.

Moreover, 12 hours post-recovery, photochemical efficiency of the Symbiodiniaceae was not fully recovered, at any depths. It is well known that, in darkness, oxygen concentration declines significantly in coral tissues due to chlororespiration, leading to hypoxic conditions that results in a decline in Fv/Fm of the symbionts during the night (Shashar et al., 1993;Jones et al., 2001). However, the addition of exogenous oxygen to hypoxic coral tissues (held in darkness for several hours), as was continuously provided during the ramping, was showed to be sufficient to increase Fv/Fm back to normal values (Jones et al., 2001). So, we suggest that this decline in Fv/Fm around Topt is unlikely to be an experimental artefact. Rather, we suggest that it shows the rapid thermal acclimation capacity of the symbionts. Indeed, Topt (maximum Fv/Fm) was measured around 27°C at both depth, which corresponds to the acclimation temperature. This suggests that an acclimation of 10 days at the common temperature of 27°C was sufficient for the Symbiodiniaceae to optimize their photochemical efficiency.

Pachyseris speciosa vs. Stichopathes sp. from the same depth (85m)

At mesophotic depths, scleractinian corals co-exist with other cnidarians, including antipatharians. The present study revealed distinct metabolic performances under thermal stress between P. speciosa and Stichopathes sp. collected from the same site at 85m depth. Indeed, Topt for dark respiration was reached for Stichopathes sp. but not for P. speciosa and the higher sensitivity of Stichopathes sp. was also supported by its high Q10 value, much out of the typical range of 2-3 for thermochemical enzymatic reactions. Does that mean that Stichopathes sp. from Mo'orea is a (relatively) cold thermal specialist and that its bathymetric range (from 60m and deeper) is constrained by temperature? Indeed, a previous study of the same species showed that a 15d thermal exposure to 1°C above Topt induced deleterious effects [START_REF] Godefroid | Thermal stress responses of the antipatharian Stichopathes sp. From the mesophotic reef of Mo'orea, French Polynesia[END_REF]. Reanalysing the data of [START_REF] Godefroid | Thermal stress responses of the antipatharian Stichopathes sp. From the mesophotic reef of Mo'orea, French Polynesia[END_REF], using the same Gaussian regression as in the present study, yielded a Topt of 28.3°C and a Tbr of 2.15°C. In the present ramping experiment, Topt was higher (29.6°C) and Tbr was wider (4.26°C) than in [START_REF] Godefroid | Thermal stress responses of the antipatharian Stichopathes sp. From the mesophotic reef of Mo'orea, French Polynesia[END_REF], which is likely a consequence of seasonal acclimation, as the sampling was performed during different sampling periods (February 2021 vs. October 2019 in [START_REF] Godefroid | Thermal stress responses of the antipatharian Stichopathes sp. From the mesophotic reef of Mo'orea, French Polynesia[END_REF], which differed in temperature (Figure S1). This mismatch could also be partly explained by the acute nature of the stress performed, preventing acclimation to occur. Indeed, studies have previously showed that acute ramping methodologies tend to overestimate Topt in comparison with medium/long-term methods [START_REF] Schulte | Thermal Performance Curves, Phenotypic Plasticity, and the Time Scales of Temperature Exposure[END_REF][START_REF] Sinclair | Can we predict ectotherm responses to climate change using thermal performance curves and body temperatures?[END_REF]). Yet, differences in TPC metrics between the two experimental approaches used (medium-term exposure in [START_REF] Godefroid | Thermal stress responses of the antipatharian Stichopathes sp. From the mesophotic reef of Mo'orea, French Polynesia[END_REF] vs acute ramping in the present study) were not significant (overlapping confidence intervals for each parameters, Tbr, Topt and Pfmax). This indicates that the method used has limited influence on the results, supporting the use of acute ramping methodologies for antipatharians (especially in a comparative approach).

Conclusion

No consensus currently exists as if Mesophotic Coral Ecosystems may or not provide resistant and/or resilient coral oases to the increasing frequency of heat stress events. MCEs in Mo'orea were suggested to play a role as short-term resistant oases from bleaching [START_REF] Pérez-Rosales | Mesophotic coral communities escape thermal coral bleaching in French Polynesia[END_REF]. Our study contributed to this statement by providing the first metabolic evidence of a broader thermal performance of the depth-generalist scleractinian P. speciosa at mesophotic depths compared to middepth, which suggests the capacity of that species to adopt a thermal strategy depending on local temperature regime. This supports that mesophotic environments exposed to large thermal fluctuations are most likely to be more tolerant under global warming. Surprisingly, our work also shed light on the constrained thermal performance of antipatharian compared to scleractinian, living in a similar site at mesophotic depth (85m). This has important implication for predicting the future of mesophotic reefs, as it unravelled that other biological processes than the Symbiodiniaceae symbiosis may modulate the thermal performance of benthic organisms in that particular zone of the reefs.

Regarding the lack of plasticity of antipatharian compared to scleractinian in response to heat stress, it is unlikely to observe a shift of dominance from scleractinian to antipatharian in future mesophotic reefs.

CRediT author statement

Figure 1 .

 1 Figure 1. (a) Time schedule of the experiments with coral collection and fragmentation, post-

Figure 2 .

 2 Figure 2. Thermal performance curves for (a-c) Dark Respiration (Rdark); (d, e) Net Photosynthesis (Pnet);

Figure 3 .

 3 Figure 3. Recovery capacity of (a,d,f) P. speciosa 25m, (b,e,g) 85 m and (c) Stichopathes sp. Metabolic

  February 11th, 2021, the team of technical divers from Under The Pole expeditions

	(https://www.underthepole.com/) collected eight colonies of the antipatharian Stichopathes sp. and
	eight colonies of the scleractinian P. speciosa on a vertical slope at 85m depth. For the taxonomic
	determination of Stichopathes sp., preliminary analyses targeted the nuclear Internal Transcribed
	Spacers of the ribosomal DNA (ITS1) and suggested this Stichopathes sp. to be part of clade C as
	described in Bo et al. (2012). All colonies were immediately transported to the "Centre de Recherches
	Insulaires et Observatoire de l'Environnement" (CRIOBE), fragmented on the same day (two fragments
	per colony for Stichopathes sp. and four fragments per colony for P. speciosa) and placed in an open-
	circuit and sand-filtered aquarium with parameters of the collection site (Volume: 200L, Temperature:

Table 1 .

 1 Q10 (mean ± sd) value for oxygen consumption rate with temperature, calculated for P.

	speciosa 25m, 85m and for Stichopathes sp. (n=6-8)
		P. speciosa 25m	P. speciosa 85m	Stichopathes sp.
	Q10	2.26 ± 0.42	3.29 ± 1.36	45.68 ± 61.96

Godefroid Mathilde :

 Mathilde Conceptualization, Methodology, Formal Analysis, Investigation, Writing: Original Draft, Review and Editing; Philippe Dubois: Conceptualization, Methodology, Writing: Review and Editing; Under The Pole Consortium: Resources; Laetitia Hédouin: Conceptualization, Methodology, Writing: Review and Editing.

Acknowledgements

M. Godefroid is holder of a Belgian FRIA grant (FRS-FNRS, number 1.E.066.19F). Ph. Dubois is a Research Director of the National Fund for Scientific Research (FRS-FNRS; Belgium). L. Hédouin is a CNRS Research Fellow (France). This work was supported by the FNRS projet COBICO (T0084.18), the ANR DEEPHOPE (ANRAAPG 2017 grant no. 168722), the Délégation à la Recherche DEEPCORAL, the CNRS DEEPREEF, the Agence Française pour la Biodiversité POLYAFB and the IFRECOR. We sincerely thank G. Iwankow for his help in the collection of the colonies, A. Mercière and Y. Lacube for their advices with the setup of the aquarium system. Service d'Observation CORAIL (SO Corail) from CRIOBE kindly provided the temperature data (Figure S1). We thank the different reviewers for their significant contribution in improving the final version of the manuscript. Under The Pole Consortium: G. Bardout, A. Ferucci, F. Gazzola, G. Lagarrigue, J. Leblond, E. Marivint, N. Mollon, N. Paulme, E. Périé-Bardout, S. Pujolle, G. Siu. The authors have no conflicts of interest to declare.