Desislav Andreev
email: desislav.andreev@gmail.co

Simona Petrakieva
email: petrakievas-te@tu-sofia.bg

Ina Taralova
email: ina.taralova@ec-nantes.fr

Zongchao Qiao
email: qiaozongchao@163.com

Applying quantum machine learning approach for detecting chaotically generated fake usernames of accounts Security systems with heightened safety of information

Keywords: chaotic pseudo random number generator (PRNG), quantum machine learning clustering, k-means, Silhouette I

The present paper is further development of our previous publication about revealing false usernames of accounts as a result of hackers' attacks. Although both papers use methods for machine learning analysis, the novelty in this publication consists in applying the quantum technique for making cluster analysis and a new chaotic generator producing fake usernames with unfixed length. The consequence of actions necessary for analyzing the names of users' accounts is following. First, a chaotic pseudo random number generator (PRNG) producing aperiodic time series is proposed. Following given rules, the latter are used to produce fake accounts usernames used as a data base to test the efficiency in avoiding malicious intentions. The generated names are with low, middle and high randomization. Second, these names feed as an input to the quantum machine learning algorithm, which divides them in different clusters. Next, the Quantum Silhouette algorithm for quality evaluation of the results from clusterization, is applied. In the end of the paper, the suggested new technique -chaotic generator and quantum clustering algorithm -is illustrated on the illustrative example including 100 000 usernames of accounts.

INTRODUCTION

Nowadays the information is the most esteemed and the most expensive thing in the world. That's why it has a key role in the relationships between the humans. For that reason the information has to be protected from unwanted hackers' attacks. Internet is enough aggressive environment and web systems are always attacked by malicious users. Users of the arbitrary electronic system have own private account, which is secured by username and password. But do they always take the necessary preventive measures for certain safeguard of their information against unwanted hackers' attacks? For this reason we apply some other additional methods for protection the users' accounts from illegal hackers' access. There exist a lot of methods for this protection, which are developed and sold as a software products in the market. For example:

ConsentIQ [START_REF] Consentcheq | GDPR Consent Management for Small Business[END_REF] is a solution which provides an easily implemented privacy regulation for any small business. It captures private data from EU citizens, which specifies its work with EU General Data Protection Regulation (GDPR) and the pending ePrivacy regulation \-sometimes called the 'cookie' law).

Dhound [START_REF] Ids Clobal Limited | Dhound Intrusion detection -security monitoring system[END_REF] is orienting on the protection against legitimate users with malicious intentions. It tracks and alerts successfully suspicious events, controls output traffic that allows to notify data owners quickly that somebody without any reason accessed the server or your server suddenly started working with unknown services in Internet. Dhound IDS does not require specific security knowledge to manage.

Other example about the efforts of the people for secure their information are measures made from the German Association for Data Protection [START_REF]DATENSCHUTZHANDBUCH DATA PROTECTION MANUAL[END_REF]. The latter advising medium-sized companies and corporations in all aspects of data security and protection of their personal data.

In the present paper we propose a new technique for detecting the false usernames of accounts. It combines the properties of the chaotic generator for generating the usernames and their false derivatives, which next divide in separately clusters using quantum machine learning clustering method.

Chaotic generators have been attracting increasing attention due to their excellent random properties: each different starting point gives rise to different output sequence. In the same time, they exhibit typical deterministic properties, such as respectively: identical initial conditions will give rise to identical output sequences [START_REF] El Assad | Design and Analyses of Efficient chaotic Generators for Crypto-Systems[END_REF][START_REF] Lozi | From chaos to randomness via geometric undersampling[END_REF].

Generally speaking, these generators can be successfully applied as Pseudo-Random Numbers Generators, and have excellent features, as for randomness, spectrum etc. [START_REF] Garasym | New nonlinear CPRNG based on tent and logistic maps[END_REF][START_REF] Garasym | Robust PRNG based on homogeneously distributed chaotic dynamics[END_REF].

However, many open problems arise, such as the required structure for an efficient chaotic generator (e.g. the chaotic map), the criteria to analyze the tuning parameters, the choice of the best coupling in order to satisfy the predefined criteria for strong chaoticity, etc.

Quantum machine learning technique is a unification between Quantum Physics (QP) and Machine Learning (ML) algorithms. It uses the ideas of the classical ML algorithms to analyze quantum systems [START_REF] Shaw | Quantum Machine Learning: An Overview[END_REF]. After applying Quantum machine learning technique the resulted clusters are examined through the Silhouette method [START_REF] Rousseeuw | Silhouettes: a Graphical Aid to the Interpretation and Validation of Cluster Analysis[END_REF]. So in briefly, in this paper, we do the following:

1) Generating the database of complex different names by Pseudo Random Chaotic Generator;

2) Clusterization by a quantum machine learning Technique;

3) Quality assessment of the resulting clusters by Silhouette method.

The present paper is organized as follows. In the next section II the problem to reveal false users' account is defined. It also describes the chaotic generator producing false names fed to the quantum machine learning algorithm. The main requirements for chaotic generator design are proposed in section III. In section IV the functionality of the new suggested quantum clustering technique for separating the names from chaotic generator, is described in details. This technique is applied in section V on the illustrative example including 100 000 names. The clustering results also are compared there with those obtained from the Silhouette method. The paper finishes with conclusion remarks about advantages and disadvantages of the new quantum machine learning clustering algorithm working with the false usernames, generated from the proposed pseudo random chaotic generator. II. PROBLEM STATEMENT Generated usernames will have the following features: n number of symbols (n > 6) where n is a combination of letters and alphanumerical symbols. UTF-8 coding has been used. The number of usernames is 100 as each of them will be in 1000 variations, i.e. totally 100 100 usernames in total will be clustered by a quantum machine learning algorithm.

In this particular application, some specific characteristics of the chaotic generator are required, so that it can be successfully used as a pseudo-random number generator with independent output sequences. Note that not all chaotic maps are suitable for this implementation. In particular, the chaotic generator is considered random, if it passes successfully the NIST tests for randomness, and if the generated output values are equidistributed, so they should all appear with the same probability. This is very important, because our laws for fake usernames generation are based on the assumption that all rules from 0 to 9 (taken as the first digit of the generated pseudorandom value) have the same chance to be drawn.

Quantum machine learning technique is expected to be faster for clustering than the related classical representation of k-means, Mini-batch k-means, agglomerative clustering, etc. [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF][START_REF] Pavan | An Automatic Clustering Technique for Optimal Clusters[END_REF].

III. METHODOLOGY FOR SYNTHESIS OF THE RANDOM CHAOTIC GENERATOR

In this paper we have used random law to generate the fake usernames from the real user data base containing a prespecified number of real usernames with not fixed length (6 symbols minimum). Note that in this particular application, it is not necessary to binarize the chaotic generator output as we deal with real values. Binarization is often a tricky point, since there is not a strict rule, and quantization can lead to loss of randomness etc.

The algorithm that we have defined for fake usernames generation is the following. According to the random generated value, different (specific) rules have been applied to alternate the symbols of each real username: remove, double, swap, add, shift, and combinations of the above. Additional test have selected symbols with higher frequency in the data base (e.g. vowels, etc.). As a result, 1000 fake usernames have been created for each real username, and served as input for the cluster analysis.

IV. BASIS OF THE QUANTUM MACHINE LEARNING TECHNIQUE

In The quantum machine learning technique is a new approach for clustering. It combines the Quantum Physics (QP) and Machine Learning (ML) in one algorithm. The originality of the new methodology consists in the fact that it uses classical ML algorithms applied in QP.

Before we start with the main concepts of the quantum representation transformation actions, we need to summarize the three concepts existing in the Quantum Mechanics that could be counted as prerequisites to the current research. In the first place this is the quantum parallelism [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF], which unites the superposition principle [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF] and the linearity of the quantum world in order to evaluate a single function simultaneously on arbitrarily many inputs. Immediately follows the quantum interference [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF], that makes possible the logical paths from a given execution to affect each other in a positive way. This is actually a welcomed, so-to-say, output, because the influence between the positive outputs could affirm one another and the negative would fall out at certain point. This selfrearrangement of the system is typical expectation of the machine learning, especially when it comes to evaluations of algorithms over big data [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF]. But there are also quantum states that are multi-particle, which cannot be described by the independent state of a single particle. Correlation between these states cannot be examined the classical way and therefore it is required to postulate the main source of the quantum information analysis and a powerful communication resourcethe quantum entanglement [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF].

The algorithms should be able to execute correctly the tasks for pattern classification, pattern recognition -to assign labels correctly to a given input set or finding a shape of patterns; pattern completion -adding missing values in the input dataset; associative memory -retrieving one of a number of stored memory vectors upon an input. Let's focus currently on the first one: If we take a look at the classification and clustering problems in the classical machine learning, it will be clear that the main concern is in finding the efficient calculations of the classical distances on a potential quantum computer. This is obviously required for the similarity measurement of two feature vectors. For the classical representation the Euclidian or the Hamming distances are used. For the quantum world there are few suggestions from [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF]. All of them are basing their similarity measure on the overlap or fidelity of two quantum states |a and |b , respectively. The swap test is proposed, where |a, b, 0 anc state with the two wavefunctions and one ancilla register -described in [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF], set to 0 is initially provided and fed to a Hadamard gate (Figure 1): for which the probability of measuring the ground state is given by:

     
the probability of 1/2 means that the two states do not overlap (they are orthogonal), while probability 1 indicates that they have maximum overlap.

V. APPLYING QUANTUM MACHINE LEARNING TECHNIQUE FOR REVEAL THE FALSE NAMES OF USERS' ACCOUNTS GENERATED BY CHAOTIC GENERATOR

Proceeding further we focus our experiment on the clustering algorithmsk-means to be precise, because it uses that distance measure we discussed above. It is also easy to be represented mathematically both in classical and quantum machine learning. The method is NP-hard, which means it is computationally difficult, which gives further stimulation to the quantum approach -this will be discussed later. Its overall classic complexity is O(kNI), where k is the number of clusters, N is the number of samples and I -the number of iterations. The algorithm is alternating constantly between two main steps -assign step and update step:

-Assign each observation x p to the cluster whose mean has the least squared Euclidean distance (nearest mean) and the observation is assigned to exactly one S(t): ). Having the observations in section IV above it would be expected, that the algorithm takes O(Nlog(ND)). Both times N is required at least once, because every vector is tested individually for the reassignment at each step [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF]. Let's construct the state:

  

where S is the current cluster for a set of N reference vectors { } of length P and input vector . The formulation of this equation results from [START_REF]DATENSCHUTZHANDBUCH DATA PROTECTION MANUAL[END_REF]. The distance can be efficiently calculated within error . Getting back to the swap test and apply it over the previous step, we construct the following:

   where 
This is repeated for each cluster until a desired confidence is reached, which is usually noted by the person, working with the system. This situation could be avoided by a usage of an algorithm that evaluates this confidence and marks the final iteration of the quantum k-means. Such method is the Silhouette method, which we already used in [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF] and [START_REF] Andreev | Reveal false names of accounts as a result of hackers' attacks[END_REF], and proved, that it the k-means assigns well the received data and has a high confidence. The formula is the following:

   
where a is the average distance of i to the points in its cluster and b -the minimal average distance of i to points in another cluster, e.g. b is the average dissimilarity between point i and the points in the closest cluster to its cluster. From the equation in Section IV and [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF] we conclude that the average distance is actually the average fidelity between the points in the cluster, in our case the average fidelity between the state of the i-th variable in the n-sized cluster, which brings us to:

     
Remembering that: the fidelity of two state is , where ρ and ρ′ are the density matrices of the i-th variable and any other one, respectively over all initial states [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF]. Further: s is the number of clusters, while c is the current one, n S is the number of elements in the cluster S, where i doesn't belong to them, e.g. in S c for the context of b. From the calculation of the Silhouette coefficient for all elements of would be:

  
The workability of the suggested in the previous two sections technique for reveal false users' names is illustrated on the example includes 100 names in 1000 variations, generated by the chaotic generator (see section III). Next, these names are clustered from quantum machine learning algorithm (see Section IV), which results' quality (confidence) is evaluated with the Quantum Silhouette method [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF].

The dataset consists of 140 000 samples. Our initial experiment goes through the whole process as discussed above in order to distinguish the number of clusters, where the confidence of the clustering algorithm is at its highest point. On Figure 2 (here below) we can observe that the confidence drops drastically with higher values of k -placed on the x-axis; the results from the Silhouette method are placed on the y-axis. As observed in Figure 4 we continue further with measuring the execution times -on the y-axis, for different portions of the dataset. The number of input samples is placed on the x-axis. This graph confirms what we observed previously on Figure 3, but does not correspond of our expectation for the complexity of the k-means as discussed in Section IV. We will discuss this briefly below, but before that we need to observe the final clusters -Figure 5: As it can be observed the three clusters of low, middle and high randomization have been clearly separated from the most plausible real usernames to the less probable ones that have to be verified with the highest priority. The cluster centroids are marked with the bigger grey circles and we can observe that their trend is to remain in the middle of the y-axis.

The quantum approach provides a computational complexity of , where I is the number of executions of the step. The Silhouette routine has at least complexity in the classical world, but for the QRAM execution we expect as already described in [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF]. The overall evaluation complexity would be . From the rules of asymptotic analysis we are removing the summation with N, but for the purposes of the paper we keep the multiplication by I. This explains the graph on Figure 4 and shows that the overall complexity is less than it would be if we used the classical approach.

VI. CONCLUSION

In the present paper a novel technique to reveal users' account names' validity is suggested. It uses the usernames generated by pseudo-random chaotic generator which are fed to the quantum machine learning algorithm for clustering of the names in different groups with respective probability about their truthiness. To confirm the efficiency of the proposed new technique the Silhouette method is used and the evaluation times are measured.

The chaotic pseudo-random number generator has shown very good efficiency, the binarization has been avoided thanks to the random rules attribution for each real value (the first significant digit). Moreover, the created database with fake usernames allowed to train successfully the quantum learning machine.

The results from the quantum machine learning algorithm suggested above show that this technique provides faster classification for revealing the real and the fake accounts than the other existing clustering methods. Some of more important areas for using this type of security systems are also discussed in [START_REF] Andreev | Reveal false names of accounts as a result of hackers' attacks[END_REF][START_REF] Andreev | A novel approach for protection of accounts' names against hackers combining cluster analysis and chaotic theory[END_REF]. The novelty of the present technique is that it uses quantum k-means and quantum Silhouette, which are faster than the classical approaches and the results from clustering are still reliable. The quantum approach for the Silhouette method from [START_REF] Desislav | Analysis of machine learning methods and algorithms in a quantum entanglement-based environment[END_REF] is applied. In summary, we can say that this technique can be used in important systems from the real world such as: electronic voting of government, parliament, finance institutions; e-banking; insurance; public health services; communications; continuous and discrete manufacturing etc.

Figure 1 .

 1 Figure 1. Swap test logical circuit presentation The following Hadamard transformation sets the ancilla into a superposition (|0 + |1 ). The SWAP-gate on a and b swaps the two states if the ancilla is in |1 . The second Hadamard gate results in state:

  the average distance) the means to be the centroids of the observations in the new clusters:    It stops when the assignments no longer change. The main idea is to partition N observations into k clusters. In the end the data space is partitioned in Voronoi cells. If the measurement space is D dimensional the time complexity of the algorithm would be O(D), where each step takes time O(

Figure 2 .Figure 3 .

 23 Figure 2. Results from the Silhouette evaluation for different number of clusters

Figure 4 .

 4 Figure 4. Evaluation time for different number of input samples

Figure 5 .

 5 Figure 5. Final Clusters gotten by Quantum KMeans

ACKNOWLEDGMENT

The authors of the paper are thankful to the project RILA 38652 UG for supporting this research project.

And also they express their special acknowledgements to Alexandre Boissel (from High School Externat Chavagnes, Nantes, France) and to Desislav Andreev (one of the authors of this paper) for creating the program codes. First one wrote Python code to generate the fake usernames, proposed as a free software. The second one applied machine learning approach (as a theory and as a program code on Python (QuTiP) and Quantum++ (C++)) for setting the usernames in different clusters and then evaluating their quality for multiple input entries.


Justyna Jońca

Andrey Ryzhikov

Myrtil L Kahn
email: myrtil.kahn@lcc-toulouse.fr

Katia Fajerwerg

Audrey Chapelle

Philippe Menini

Pierre Fau
email: pierre.fau@lcc-toulouse.fr

Self-Assembled Hollow SnO2 Octahedra for sub-ppm Gas Detection Sensors

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

FiguresFig: 1

 1 Figures

  Fig: 1: hollow SnO2 self-assembled octahedra Fig. 2: sub-ppm detection capability for SnO2 octahedra sensitive layer


Marie-Aimée Galeron

Olivier Radakovitch

Bruno Charrìère

Frederic Vaultier

John K Volkman

Thomas S Bianchi

Nicolas D Ward

Patricia M Medeiros

Henrique O Sawakuchi

Suzanne Tank

Bruno Charrière

Frédéric Vaultier

Philippe Kerhervé

Jean-François Rontani
email: jean-francois.rontani@mio.osupytheas.fr

Lipoxygenase-induced autoxidative degradation of terrestrial particulate organic matter in estuaries: A widespread process enhanced at high and low latitude

Keywords:

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Riverine particulate organic matter (POM), which consists in part of highly degraded residues from terrestrial higher plants, has long been considered to be refractory compared with marine-derived POM (de [START_REF] De Leeuw | A review of macromolecular organic compounds that comprise living organisms and their role in kerogen, coal, and petroleum formation[END_REF][START_REF] Wakeham | Degradation and preservation of organic matter in marine sediments[END_REF]). The assumption is not supported by the unexpected relatively low proportion of land-derived OM detected in marine sediments [START_REF] Hedges | Sedimentary organic matter preservation: an assessment and speculative synthesis[END_REF], suggesting extensive remineralization of this material at sea [START_REF] Hedges | What happens to terrestrial organic matter in the ocean?[END_REF]. In fact, the notion of OM being inherently refractory and/or labile in terrestrial and aquatic systems has been recently challenged, with the suggestion that the all this material is utilized by microbes under the ''right" environmental conditions [START_REF] Bianchi | The role of terrestrially derived organic carbon in the coastal ocean: a changing paradigm and the priming effect[END_REF][START_REF] Schmidt | Persistence of soil organic matter as an ecosystem property[END_REF]. Moreover, several studies have demonstrated that POM delivered by rivers is sensitive to microbial remineralization in some shelf areas [START_REF] Aller | Remineralization rates, recycling, and storage of carbon in Amazon shelf sediments[END_REF][START_REF] Aller | Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors[END_REF][START_REF] Mayer | Input of nutritionally rich organic matter from the Mississippi River to the Louisiana coastal zone[END_REF][START_REF] Van Dongen | Differential transport and degradation of bulk organic carbon and specific terrestrial biomarkers in the surface waters of a sub-arctic brackish bay mixing zone[END_REF][START_REF] Sampere | Sources of organic matter in surface sediments of the Louisiana Continental Margin: effects of primary depositional/transport pathways and a hurricane Event[END_REF][START_REF] Vonk | Molecular and radiocarbon constraints on sources and degradation of terrestrial organic carbon along the Kolyma paleoriver transect, East Siberian Sea[END_REF][START_REF] Bourgeois | Distribution and lability of land-derived organic matter in the surface sediments of the Rhône prodelta and the adjacent shelf (Mediterranean Sea, France): a multi proxy study[END_REF][START_REF] Karlsson | Carbon isotopes and lipid biomarker investigation of sources, transport and degradation of terrestrial organic matter in the Buor-Khaya Bay, SE Laptev Sea[END_REF]. More recently, it has been shown that processes such as biodegradation and autoxidation (free radical reaction of organic compounds with O 2 ) play a key role in the degradation of vascular plant-derived lipids discharged by the Mackenzie River to the Beaufort Sea (Rontani et al., 2014a).

Autoxidation is not spontaneous but autocatalytic; once started, it is self-propagating and self-accelerating [START_REF] Schaich | Lipid oxidation: theoretical aspects[END_REF]. The mechanisms of initiation have been debated for many years, but likely involve the homolytic cleavage of photochemically produced hydroperoxides in phytodetritus [START_REF] Girotti | Lipid hydroperoxide generation, turnover, and effector action in biological systems[END_REF][START_REF] Rontani | Photochemical oxidation and autoxidation of chlorophyll phytyl side chain in senescent phytoplanktonic cells: Potential sources of several acyclic isoprenoid compounds in the marine environment[END_REF]. The cleavage may be induced by heat, light, metal ions and lipoxygenases [START_REF] Schaich | Lipid oxidation: theoretical aspects[END_REF]. Autoxidation, largely ignored for the coastal zone, proceeds by a radical chain reaction and acts mainly on organic compounds possessing C@C bonds or CAH bonds whose bond energy is relatively low (e.g. allylic, tertiary, a to oxygen, etc.; [START_REF] Fossey | Free Radicals in Organic Chemistry[END_REF]. It can act not only on unsaturated lipids [e.g. sterols, unsaturated fatty acids (FAs), chlorophyll phytyl side chain, alkenes, tocopherols and alkenones; [START_REF] Rontani | Photo-and free radical-mediated oxidation of lipid components during the senescence of phototrophic organisms[END_REF], but also on amino acids [START_REF] Seko | Autoxidation of thiol-containing amino acid to its disulfide derivative that links two copper(II) centers: the important role of auxiliary ligand[END_REF], sugars [START_REF] Lawrence | Promotion by phosphate of Fe(III)-and Cu(II)-catalyzed autoxidation of fructose[END_REF] and polyphenols [START_REF] Hathway | Autoxidation of polyphenols. Part III. Autoxidation in neutral aqueous solution of flavans related to catechin[END_REF]. It can also affect biopolymers [START_REF] Schmid | Autoxidation of medium chain length polyhydroxyalkanoate[END_REF] and kerogen, inducing ring opening and chain cleavage.

To explain this induction, a mechanism involving homolytic cleavage of photochemically produced hydroperoxides, resulting from the senescence of higher plants on land, was proposed (Galeron et al., 2016c). Cleavage was attributed to some redoxactive metal ions released from suspended particulate matter (SPM) in the mixing zone of riverine water and marine water. Using new lipid tracers specific to the degradation of terrestrial higher plants at the mouth of the Rhône River (Galeron et al., 2016a,b;[START_REF] Rontani | EIMS fragmentation pathways and MRM quantification of 7a/b-hydroxy-dehydroabietic acid TMS derivatives[END_REF], we confirmed the role played by autoxidation in the degradation of terrestrial POM (TPOM) in estuaries [START_REF] Galeron | Impacts of coastal degradative processes: autoxidation a major player in the fate of terrestrial particulate organic matter in seawater[END_REF]. As in the Beaufort Sea (Rontani et al., 2014a), autoxidation rate and salinity level appeared to correlate well within the river plume in the Mediterranean Sea [START_REF] Galeron | Impacts of coastal degradative processes: autoxidation a major player in the fate of terrestrial particulate organic matter in seawater[END_REF]. However, it was not possible to detect a significant release of metal ions (able to catalyze hydroperoxide cleavage) during that study. Four important questions thus remained to be elucidated: (i) Which processes promote induction of autoxidation of TPOM in estuarine waters, (ii) why is autoxidation induced in TPOM in mixed waters (i.e. during an increase in salinity), (iii) why is autoxidation of TPOM enhanced in Arctic vs. temperate estuaries and (iv) what is the role of autoxidation in tropical zones? In order to answer these questions, the degradation of TPOM in several SPM and surface sediment samples collected from three contrasting key areas was monitored: (1) Arctic Ocean (Mackenzie River and Shelf), (2) tropical Atlantic Ocean (Amazon River and Shelf) and (3) Mediterranean Sea (Rhône River and Shelf).

The Arctic Ocean receives the largest riverine input of all the oceans, relative to its size. More specifically, its upper layer, which represents only 0.1% of the global ocean volume, receives 11% of the global riverine discharge [START_REF] Fichot | Pan-Arctic distributions of continental runoff in the Arctic Ocean[END_REF]. Moreover, since global warming has accelerated twice as rapidly in the northern Hemisphere [START_REF] Kug | Two distinct influences of Arctic warming on cold winters over North America and East Asia[END_REF] than other regions of the globe, the Arctic remains a key focal point for climate change research. With a huge watershed (1.8 Â 10 6 km 2 ) and annual water discharge of 330 km 3 /yr delivered mainly in the short summer period, the Mackenzie is the fourth largest Arctic river and the primary source of particulate matter (PM) in the Arctic Ocean (e.g. [START_REF] Hilton | Erosion of organic carbon in the Arctic as a geological carbon dioxide sink[END_REF]. Its total suspended matter load was estimated at 40-160 Mt/yr and its particulate organic carbon (POC) flux at 0.2-2.2 Mt/yr [START_REF] Macdonald | A sediment and organic carbon budget for the Canadian Beaufort Shelf[END_REF][START_REF] Doxaran | A 50% increase in the mass of terrestrial particles delivered by the Mackenzie River into the Beaufort Sea (Canadian Arctic Ocean) over the last 10 years[END_REF].

The Mediterranean is a semi-enclosed sea subject to both anthropogenic pressure and climate change. Since the damming of the Nile, the Rhône is the main supplier of fresh water (55 km 3 /yr), suspended matter (2-8 Mt/yr) and POC (0.2 Mt/yr) to the Mediterranean [START_REF] Sempéré | Carbon inputs of the Rhône River to the Mediterranean Sea: Biogeochemical implications[END_REF][START_REF] Sadaoui | Controls, budgets and variability of riverine sediment fluxes to the Gulf of Lions (NW Mediterranean Sea)[END_REF]. It 816 km long with a drainage area of 97,800 km 2 .

The Amazon is the largest river in the world, with a drainage basin area of 6.1 Â 10 6 km 2 [START_REF] Goulding | The Smithsonian Atlas of the Amazon[END_REF] covered by diverse vegetation, including tropical rainforest, inundated floodplain ('várzea') forest, floating grass and extensive grassland/ savannah [START_REF] Hedges | Compositions and fluxes of particulate organic material in the Amazon River[END_REF]. Its discharge has a strong seasonal variation, with a maximum of 7500 km 3 /yr in May-June and a minimum of 2500 km 3 /yr in October-November [START_REF] Lentz | Seasonal variations in the horizontal structure of the Amazon Plume inferred from historical hydrographic data[END_REF]. The total export of POC from the historic gauging station, Óbidos, to the ocean was estimated at 12.8-14.4 Mt/yr [START_REF] Moreira-Turcq | Seasonal variability in concentration, composition, age, and fluxes of particulate organic carbon exchanged between the floodplain and Amazon River[END_REF]. However the POC load has been reported to decrease by ca 50% between Óbidos and the river mouth, 850 km further downstream [START_REF] Ward | The compositional evolution of dissolved and particulate organic matter along the lower Amazon River-Óbidos to the ocean[END_REF].

Here we evaluate the mechanisms that induce autoxidation across a diverse suite of high to low latitude river-to-ocean gradients. Since it is well known that peroxidation of membrane lipids by lipoxygenases (LOXs) may play an important role in promoting oxidative damage during environmental stress [START_REF] Thompson | The role of free radicals in senescence and wounding[END_REF], we hypothesize that LOX activity is directly related to level of estuarine TPOM autoxidation and that the balance between photo-, bio-, and auto-oxidative TPOM breakdown depends on salinity, temperature, and light penetration.

Methods

2.1. Sampling of SPM and sediments from the Rhône, Mackenzie and Amazon rivers and shelfs SPM samples were collected in February 2012 along a transect following the salinity gradient in the Rhône plume (Fig. 1). Sampling and hydrographic conditions have been described by [START_REF] Galeron | Impacts of coastal degradative processes: autoxidation a major player in the fate of terrestrial particulate organic matter in seawater[END_REF]. High frequency monitoring of the nutrients and of the PM input from the Rhône to the Mediterranean has been carried out since 2010 in the framework of the national program MOOSE (Mediterranean Ocean Observing System for the Environment). Monitoring is undertaken at the Arles station (43°40 0 44 00 N, 4°37 0 16 00 E), 40 km upstream from the river mouth (Fig. 1). Inriver sampling included particle collection using a Teflon-coated high speed centrifuge (CEPA Z61). Sediment material (0-1 cm) was collected in August 2006 at stations R28 and R30 in the Rhône pro-delta (Fig. 1) using a box multitube corer. Sediments were immediately frozen at À20 °C on board, then freeze-dried and stored in the dark in the laboratory until analysis.

SPM samples from the Mackenzie River were collected in Tsiigehtchic (67°27 0 7 00 N, 133°45 0 32 00 W) in June 2011 (Fig. 1), using the same protocol as that used for the Rhône plume SPM collection [START_REF] Galeron | Impacts of coastal degradative processes: autoxidation a major player in the fate of terrestrial particulate organic matter in seawater[END_REF]. Sediment material was collected at station M434 near to the river mouth (70°10 0 12 00 N, 133°35 0 24 00 W) as part of the ArcticNet and IPY-CFL system studies on board the CCGS Amundsen in 2008. A surface sample (ca. 0-1 cm) was collected from a box core, freeze dried and stored (< 4 °C) prior to analysis.

Particles from the Amazon were collected in the Solimões River (near the city of Manacapuru, 03°19 0 29 00 N, 60°32 0 58 00 W) in June 2005 and March 2006, and in the Madeira River (near its mouth where it reaches the Amazon; 03°27 0 24 00 N, 58°47 0 57 00 W) in June 2005 (Fig. 1). Surface sediments were collected along the tidally influenced reaches of the lower Amazon near the city of Almeirim (1°34 0 428 00 S, 52°40 0 79 00 W) and in the two main north/south channels at the river mouth near the city of Macapá (00°05 0 240 00 S, 50°37 0 212 00 W and 00°09 0 249 00 S, 51°03 0 120 00 W, respectively).

Sediment samples from the lower Amazon were collected near the river margin using a Van Veen grab. Samples were stored in 50 ml sterilized centrifuge tubes and frozen at À20 °C prior to analysis.

In the tropical Atlantic, particle samples were collected in 2011 and 2012 at the surface (ca. 2 m) using a gentle impeller pumping (modified Rule 1800 submersible pump) through 10 m of Tygon tubing to the ship's deck. Immediately after collection, 2-5 l of plume water were filtered through 0.7 mm Whatman GF/F filters (pre-combusted at 450 °C for 5 h), carefully wrapped using precombusted Al foil, and kept frozen at À20 °C until analysis. Sediments were collected using a multi-corer with 9.8 cm (i.d.) core tubes from depths of 4000-4600 m (Table 3). Immediately after recovery, cores were transferred to a temperature-controlled cold van (ca. 2 °C), where sediments were sectioned in 1 cm intervals, and the top layer was transferred to pre-combusted Al foil, carefully wrapped and kept frozen at À20 °C until analysis.

Incubation of SPM at different salinity values

In order to determine the degradation kinetics of the Rhône SPM upon reaching seawater, we incubated SPM from the river at different salinity values. Rhône water was collected on November 18, 2014, during a flood (water flow 3700 m 3 /s, SPM 217.1 mg/ l, POC 119.01 lM) and seawater (salinity 38 g/kg) during the MOR-TIMER sampling campaign of November 12, 2014 [START_REF] Galeron | Impacts of coastal degradative processes: autoxidation a major player in the fate of terrestrial particulate organic matter in seawater[END_REF]. Dilution was carried out with 1 l glass flasks using Rhône water and filtered 0.7 mm seawater in order to reach salinity of 0 (Rhône water only), 15 and 25 g/kg. No extra SPM was added to the dilutions since the collected Rhône water displayed a sufficiently high level of SPM for incubation. Flasks were kept at room temperature in the dark, and mixed and oxygenated every other day. Filtering for analysis occurred after 102 days, for each salinity level. For each incubation flask, 3 Â 250 ml incubated water (triplicate) were filtered on pre-combusted GF/F filters (Whatman). Filters were immediately frozen at À20 °C until analysis.

Treatment of SPM and sediment samples

After thawing, filters were reduced with NaBH 4 and saponified. NaBH 4 reduction of hydroperoxides to alcohols is essential for estimating the importance of photooxidative and autoxidative degradation in natural samples [START_REF] Marchand | Characterisation of photo-oxidation and autoxidation products of phytoplanktonic monounsaturated fatty acids in marine particulate matter and recent sediments[END_REF]. Without this treatment, these labile compounds could be thermally cleaved during alkaline hydrolysis or gas chromatography (GC) analysis and thereby be overlooked during conventional organic geochemical studies. All manipulations were carried out using foil-covered vessels in order to exclude photochemical artifacts. It is well known that metal ions can promote autoxidation during hot saponification [START_REF] Pokorny | Major factors affecting the autoxidation of lipids[END_REF]. Prior reduction of hydroperoxides with NaBH 4 allowed us to avoid such autoxidation artifacts during the alkaline hydrolysis.

Filters were placed in MeOH (20 ml) and hydroperoxides were reduced to the alcohols with excess NaBH 4 (70 mg; 30 min at 20 °C). During this treatment, ketones are also reduced to alcohols and the possibility of some ester cleavage cannot be totally excluded. Saponification was carried out on reduced samples. After NaBH 4 reduction, 20 ml water and 2.8 g KOH were added and the mixture directly saponified by refluxing for 2 h. After cooling, the flask contents were acidified with HCl (pH 1) and extracted (3Â) with dichloromethane (DCM). The combined DCM extracts were concentrated to give the total lipid extract (TLE). After solvent evaporation, the residue was taken up in 300 ml pyridine/N,O-bis( trimethylsilyl)trifluoroacetamide (BSTFA; Supelco; 2:1, v:v) and silylated for 1 h at 50 °C to convert OH-containing compounds to trimethylsilyl ether or ester derivatives. After solvent removal under a stream of N 2 , the derivatized residue was taken up in 100 ll BSTFA (to avoid desilylation of FAs) and an amount of solvent (EtOAc) depending on the mass of the TLE.

GC-EI tandem mass spectrometry (GC-EIMS-MS)

GC-EIMS-MS was performed in multiple reaction monitoring (MRM) mode using an Agilent 7890A/7000A tandem quadrupole gas chromatograph system (Agilent Technologies, Parc Technopolis -ZA Courtaboeuf, Les Ulis, France). Operating conditions are described by [START_REF] Rontani | EIMS Fragmentation and MRM quantification of TMS derivatives of cucurbic acid and its 6,7-stereoisomers[END_REF]. Technologies, Parc Technopolis -ZA Courtaboeuf, Les Ulis, France). Operating conditions are described by [START_REF] Rontani | Identification of di-and triterpenoid lipid tracers confirms the significant role of autoxidation in the degradation of terrestrial vascular plant material in the Canadian Arctic[END_REF].

Standards

Quantification of lipids and their oxidation products was carried out using GC-EIMS-MS or GC-QTOF with the following external standards. FAs, sterols, amyrins, betulin and jasmonic acid were from Sigma-Aldrich. NaBH 4 reduction of commercial jasmonic acid (containing a small proportion of iso-jasmonic acid) afforded isomeric cucurbic acids. Oxidation products of monounsaturated FAs, sitosterol, amyrins and betulin were produced according to previously described procedures [START_REF] Marchand | Characterisation of photo-oxidation and autoxidation products of phytoplanktonic monounsaturated fatty acids in marine particulate matter and recent sediments[END_REF]Rontani et al., 2014a;Galeron et al., 2016a,b). A standard of threo 7,10-dihydroxyoctadec-8(E)-enoic acid containing 10% of threo 7,10-dihydroxyhexadec-8(E)-enoic acid produced by Pseudomonas aeruginosa PR3 [START_REF] Suh | Production of 7,10dihydroxy-8(E)-octadecenoic acid from olive oil by Pseudomonas aeruginosa PR3[END_REF] was obtained from H.R. Kim (School of Food Science and Biotechnology, Kyungpook National University, Daegu, South Korea).

Estimation of autoxidative, photooxidative and LOX degradation

Sitosterol and more specific triterpenes (betulin, aand bamyrins) were used to estimate the oxidation state of higher plant material. The proportions of photooxidation and autoxidation of sitosterol were estimated from 24-ethylcholest-4-en-3b,6 a/bdiol and 24-ethylcholesta-3b5,6b-triol concentration, respectively, using equations proposed by [START_REF] Christodoulou | Use of lipids and their degradation products as biomarkers for carbon cycling in the northwestern Mediterranean Sea[END_REF] and [START_REF] Rontani | The fate of marine lipids: biotic vs. abiotic degradation of particulate sterols and alkenones in the Northwestern Mediterranean Sea[END_REF]. Autoxidation state of aand b-amyrins was estimated thanks to the proportion of 11-oxo-a-amyrin and 11oxo-b-amyrin, respectively (Galeron et al., 2016b). In the case of betulin, estimation of autoxidation was carried out with lupan-20-one-3b,28-diol (Galeron et al., 2016a). The part played by autoxidation in the degradation of palmitoleic acid was estimated thanks to the proportion of its specific Z-oxidation products [START_REF] Frankel | Lipid Oxidation[END_REF] and of the water temperature according to the approach described by [START_REF] Marchand | Characterisation of photo-oxidation and autoxidation products of phytoplanktonic monounsaturated fatty acids in marine particulate matter and recent sediments[END_REF]. After subtraction of the amounts of oxidation products of autoxidative origin, it remained to determine the relative parts played by photooxidative and enzymatic processes in the degradation. Taking into account the production of equal amounts of 9-E and 10-E oxidation products during the photooxidation of the D 9 monounsaturated FA [START_REF] Frankel | Lipid Oxidation[END_REF] and their specific allylic rearrangement to 11-E and 8-E isomers, respectively [START_REF] Porter | Mechanisms of free radical oxidation of unsaturated lipids[END_REF], the part played by photooxidative degradation was estimated to be 2Â (9-E + 11-E). Concerning 10S-DOX degradation, this was obtained from the difference between (10-E + 8-E) and (9-E + 11-E) oxidation products, to which was added the amount of threo 7,10-dihydroxyhexadec-8(E)-enoic acid formed.

d 13 C analysis

Compound specific carbon isotope (d 13 C) analysis was performed on the TLE using a HP7890B gas chromatograph coupled 1518, 213.1635, 329.1968, 343.2125, 225.1670, 315.2171, 327.1807 chromatograms showing presence of oxidation products of palmitoleic acid in SPM sample collected in February 2012 at station R7 (43°15 0 55 00 N, 4°58 0 6 00 E) from the Rhône River plume. (Abbreviated names: e.g. 10-hydroxyC 16:1D8E FA = 10-hydroxyhexadec-8 (E)-enoic acid).

to an Isoprime Vision stable isotope ratio mass spectrometer via a GC-5 combustion interface at 870 °C. The GC instrument was equipped with a BPX5 column (30 m  0.25 mm  0.10 mm film thickness) and a cool on-column injector, with He as carrier gas (1 ml/min). Samples were injected at 60 °C and the oven temperature was ramped to 130 °C at 20 °C/min, then to 300 °C (held 30 min) at 4 °C/min. Samples were analyzed in duplicate and the d 13 C values were corrected for instrument deviation using the Indiana University B4 mixture and for the BSTFA derivatizing agent [START_REF] Jones | Determination of d 13 C values of sedimentary straight chain and cyclic alcohols by gas chromatography/isotope ratio mass spectrometry[END_REF].

Results

Re-examination of SPM samples from Rhône estuary

In order to identify the processes causing the induction of autoxidation in coastal waters, TLEs from different SPM samples previously collected [START_REF] Galeron | Impacts of coastal degradative processes: autoxidation a major player in the fate of terrestrial particulate organic matter in seawater[END_REF] from the Rhône and along a transect in its estuary (Fig. 1) were re-examined. These samples were subjected to NaBH 4 reduction (allowing analysis of labile hydroperoxides in the form of the corresponding alcohols) and subsequent alkaline hydrolysis (allowing hydrolysis of esterified lipids). Unusual profiles of C 16:1x7 (palmitoleic) acid oxidation products dominated by 10-hydroxyhexadec-8(E)-enoic and 8hydroxyhexadec-9(E)-enoic acid were observed at stations R1 and R7 close to the river mouth (Fig. 2). Indeed, singlet oxygenmediated photooxidation of D 9 monounsaturated FAs such as palmitoleic acid produces (after NaBH 4 reduction) equal proportions of isomeric 9-and 10-hydroxyacids with an allylic E double bond [START_REF] Frankel | Lipid oxidation[END_REF], which can subsequently undergo highly stereoselective radical allylic rearrangement to 11-E and 8-E hydroxyacids, respectively [START_REF] Porter | Mechanisms of free radical oxidation of unsaturated lipids[END_REF]. In contrast, autoxidation (free radical induced oxidation) produces a mixture of 9-E, 10-E, 11-E, 11-Z, 8-E and 8-Z hydroxyacids also exhibiting equal proportions of the major 9-E and 10-E isomers [START_REF] Frankel | Lipid Oxidation[END_REF]. The strong predominance of the 10-E isomer (and its allylic rearrangement product 8-E) relative to the 9-E isomer (and its allylic rearrangement product 11-E) observed at the stations closer to the river mouth (Fig. 2) is thus particularly interesting. It should be noted that diastereoisomers of 7,10-dihydroxyhexadec-8(E)enoic acid could be also detected at these stations (Fig. 2).

To obtain information about the origin of palmitoleic acid and its oxidation products, the stable carbon isotopic (d 13 C) signatures of the main lipids were measured in SPM samples from R1 and R7 (Table 1). This monounsaturated FA is usually a minor component of higher plants [START_REF] Bridson | Lipid fraction in forest litter: early stages of decomposition[END_REF], in these samples it was therefore more likely derived from bacteria and/or phytoplankton. Based on its depleted d 13 C signatures (À30.9‰ and À27.8‰ at R1 and R7, respectively), we can likely assume that the contribution from marine phytoplankton (d 13 C values generally À19‰ to À24‰; [START_REF] Tolosa | Carbon sources in suspended particles and surface sediments from the Beaufort Sea revealed by molecular lipid biomarkers and compound-specific isotope analysis[END_REF] to this acid was negligible. Moreover, the relatively enriched d 13 C signature of 24-methylenecholesterol (a wellknown diatom marker; [START_REF] Volkman | Sterols in microorganisms[END_REF] at R7 (À21.6‰) (Table 1) attests to the lack of freshwater phytoplankton [which typically exhibits more depleted d 13 C values (À34‰ to À36‰; [START_REF] Tolosa | Carbon sources in suspended particles and surface sediments from the Beaufort Sea revealed by molecular lipid biomarkers and compound-specific isotope analysis[END_REF]] in these samples. Thus, the main source of palmitoleic acid was likely riverine bacteria growing on C 3 terrestrial plant material.

The preferred substrates for LOXs in plants are C 18:2x6 (linoleic)

and C 18:3x3 (a-linolenic) FAs (Siedow, 1991). a-Linolenic acid is converted to 7-iso-jasmonic and jasmonic acids by a series of enzymatic reactions in the chloroplast [START_REF] Kazan | Jasmonate signaling: toward an integrated view[END_REF]; Fig. 3). To determine if LOX activity increased in higher plant detritus at stations closer to the Rhône mouth, we quantified, using GC-MS-MS in MRM mode, isomeric cucurbic acids resulting from the reduction of 7-iso-jasmonic and jasmonic acids during the treatment (Fig. 3). The transitions employed were based on specific fragmentations of isomeric cucurbic acid trimethylsilyl ether derivatives recently elucidated by [START_REF] Rontani | EIMS Fragmentation and MRM quantification of TMS derivatives of cucurbic acid and its 6,7-stereoisomers[END_REF]. Interestingly, the values of the ratio Rcucurbic acids/parent a-linolenic acid appeared to be more than two orders of magnitude higher at station R1 than in the Rhône River (Table 2).

Incubation of SPM from Rhône River at different salinity values

Past work indicates that LOX activity increases significantly in higher plants cultivated in a NaCl-supplemented medium [START_REF] Rodriguez-Rosales | Changes induced by NaCl in lipid content and composition, lipoxygenase, plasma membrane H + -ATPase and antioxidant enzyme activities of tomato (Lycopersicon esculentum Mill) calli[END_REF][START_REF] Mittova | Salt stress induces up-regulation of an efficient chloroplast antioxidant system in the salt-tolerant wild tomato species Lycopersicon pennellii but not in the cultivated species[END_REF][START_REF] Zhang | Effect of soil salinity on physiological characteristics of functional leaves of cotton plants[END_REF]. The effect of this higher LOX activity can be either deleterious (increasing formation of FA hydroperoxides and free radicals inducing membrane deterioration) or beneficial (increasing formation of jasmonic acid, which participates in the onset of defense reactions to biotic and abiotic stress) to the cells [START_REF] Rodriguez-Rosales | Changes induced by NaCl in lipid content and composition, lipoxygenase, plasma membrane H + -ATPase and antioxidant enzyme activities of tomato (Lycopersicon esculentum Mill) calli[END_REF]. To determine if LOXs were activated in higher plant debris carried by rivers during the mixing of riverine and marine waters, SPM from the Rhône was incubated at room temperature under darkness at different salinity values. Concentrations of isomeric cucurbic acids (NaBH 4 -reduced 7-iso-jasmonic and jasmonic acids) after incubation for 102 days increased significantly with increasing post incubation salinity (2.8 ± 0.4 ng/g dry wt at S = 0 g/kg, 4.3 ± 0.7 ng/g dry wt at S = 15 g/kg and 14.5 ± 5. 0 ng/g dry wt at S = 25 g/kg) (Table 3).

Examination of SPM from the Amazon

In SPM samples from the Solimões and Madeira rivers, in the central Amazon River basin, we observed relatively low sitosterol photooxidation, averaging 11.8 ± 2.2%, with photooxidation products of phytol, oleic and palmitoleic acids below limits of detection. Autoxidation was, on the other hand, much higher than for the Rhône River [START_REF] Galeron | Impacts of coastal degradative processes: autoxidation a major player in the fate of terrestrial particulate organic matter in seawater[END_REF], with conversion averaging 55. 0 ± 1.6% for sitosterol, 73.8 ± 7.0% for a-amyrin and 56.8 ± 15.5% for b-amyrin. A high proportion of sitostanol (ranging from 40 to 78% of the parent sterol for the different stations) was also detected in these samples.

Comparison of efficiency of autoxidative degradation in vascular plant debris of Arctic, temperate and tropical coastal zones

To compare the efficiency of autoxidation in vascular plant debris discharged by temperate and tropical rivers with that previously measured in Arctic sediments [START_REF] Rontani | Identification of di-and triterpenoid lipid tracers confirms the significant role of autoxidation in the degradation of terrestrial vascular plant material in the Canadian Arctic[END_REF], oxidation products of specific tracers of vascular plants (a-and b-amyrins and betulin) were quantified in surface sediment (0-1 cm) and SPM samples from the Amazon River, fan and shelf and surface sediment (0-1 cm) from the Rhône Shelf. The extent of autoxidation of vascular plant debris appeared to be considerably higher in the Amazon Shelf than in the Rhône Shelf (Table 4). Interestingly, diastereoisomeric 7,10-dihydroxyhexadec-8(E)-enoic acids could be detected in sediments from M434 and in SPM from A8 (Fig. 4). tion, are thermally cleaved to the corresponding ketones during GC injection (Galeron et al., 2016b). Thus, they were quantified in the form of 11-oxoamyrins in SPM samples from the Rhône, Mackenzie and Amazon rivers and their hydroperoxide nature was confirmed in subsamples after reduction with LiAlD 4 . The results clearly showed the presence of a higher proportion of 11hydroperoxyamyrins in SPM from the Mackenzie and Amazon rivers than in SPM from the Rhône River (Table 5).

Quantification of isomeric cucurbic acids in SPM from Amazon and Mackenzie rivers

Isomeric cucurbic acids resulting from NaBH 4 reduction of 7iso-jasmonic and jasmonic acids (products of linolenic acid degradation by lipoxygenases; [START_REF] Kazan | Jasmonate signaling: toward an integrated view[END_REF], were also quantified in SPM from the Amazon and Mackenzie rivers (Fig. 5). The results showed considerably higher values of this ratio in the Amazon and Mackenzie rivers than in the Rhône estuary and shelf (Table 2).

Discussion

4.1. Rhône River and Mediterranean shelf water 4.1.1. Origin of induction of autoxidation processes in POM in coastal water d 13 C analyses revealed that palmitoleic acid at R1 and R7 likely derived from riverine bacteria that appeared to grow on C 3 terrestrial plant material (Table 1). The similar d 13 C values of C 18:1x7 (vaccenic) and branched pentadecanoic acids (FAs specific to bacteria; [START_REF] Sicre | Characterization of seawater samples using chemometric methods applied to biomarker fatty acids[END_REF] measured by [START_REF] Bourgeois | Distribution and lability of land-derived organic matter in the surface sediments of the Rhône prodelta and the adjacent shelf (Mediterranean Sea, France): a multi proxy study[END_REF] in sediments from the same deltaic region of the Rhône River provide further evidence for the presence of FAs derived from bacteria utilizing vascular plant sources.

The predominance of 10-E and 8-E hydroxyacids among the oxidation products of palmitoleic acid was thus attributed to the involvement of a specific bacterial enzymatic process -and more precisely a LOX-like activity. LOXs are non-heme iron dioxygenases responsible for a wide range of functions (e.g. regulation of host defense, stress response, inflammation and development) in eukaryotes [START_REF] Garreta | Structure and interaction with phospholipids of a prokaryotic lipoxygenase from Pseudomonas aeruginosa[END_REF]. They are also found in some prokaryotes, where their possible biological role remains unclear. A 10S-DOX-like lipoxygenase was notably isolated from Pseudomonas aeruginosa 42A2 [START_REF] Guerrero | Oxidation of oleic acid to (E)-10-hydroperoxy-8-octadecenoic and (E)-10-hydroxy-8-octadecenoic acids by Pseudomonas sp. 42A2[END_REF][START_REF] Busquets | Isolation and characterization of a lipoxygenase from Pseudomonas[END_REF]. Interestingly, the activity of this enzyme is highest in the case of FAs containing a double bond at D 9 . In fact, this 10S-DOX-like lipoxygenase converts palmitoleic acid to 10(S)hydroperoxyhexadec-8(E)-enoic acid, which may then undergo stereoselective allylic rearrangement [START_REF] Porter | Mechanisms of free radical oxidation of unsaturated lipids[END_REF] to 8hydroperoxyhexadec-9(E)-enoic acid (Fig. 6). The involvement of such enzymes could thus explain the unusual dominance of isomers 10-E and 8-E among palmitoleic acid oxidation products at stations closer to the Rhône mouth (Fig. 2). This assumption is supported by the detection of threo 7,10-dihydroxyhexadec-8(E)-enoic acid in these samples (Fig. 2). Indeed, the 10(S)hydroperoxyhexadec-8(E)-enoic acid produced by P. aeruginosa 42A2 is then converted to 7(S),10(S)-dihydroxyhexadec-8(E)enoic acid by way of a diol synthase (Fig. 6; [START_REF] Gardner | All (S) stereoconfiguration of 7,10-dihydroxy-8(E)octadecenoic acid from bioconversion of oleic acid by Pseudomonas aeruginosa[END_REF]. The presence of erythro isomers (Fig. 2) likely resulted from the involvement of hydroperoxide isomerase, well known to produce these enantiomers [START_REF] Jernerén | Reaction mechanism of 5,8-linoleate diol synthase, 10R-dioxygenase and 8,11hydroperoxide isomerase of Aspergillus clavatus[END_REF]. The combined bacterial (10S)-DOX and diol synthase activities, which may be expressed extracellularly [START_REF] Kim | 10(S)-Hydroxy-8(E)-octadecenoic acid, an intermediate in the conversion of oleic acid to 7,10-dihydroxy-8(E)octadecenoic acid[END_REF], may be associated with the modification of the membrane lipids or could contribute to detoxification of FAs in the bacterial environment, promoting colonization and growth [START_REF] Martínez | Biochemical characterization of the oxygenation of unsaturated fatty acids by the dioxygenase and hydroperoxide isomerase of Pseudomonas aeruginosa 42A2[END_REF].

Table 2

Proportions of isomeric cucurbic acids and their parent linolenic acid in SPM samples from Rhône River and shelf and Amazon and Mackenzie rivers.

Sample Location

Rcucurbic acids/parent alinolenic acid a SPM Rhône Shelf (station R1) (43°18 0 57 00 N, 5°1 0 49 00 E)

3.1 ± 2.2 Â 10 À3 SPM Rhône River (43°40 0 44 00 N, 4°37 0 16 00 E)

3.8 ± 1.7 Â 10 À5 SPM Mackenzie River (67°27 0 7 00 N, 133°45 0 32 00 W) 0.14 ± 0.13 SPM Amazon River (03°27 0 24 00 N, 58°47 0 57 00 W) 0.20 ± 0.17 a Average of triplicates.

Table 3

Production of isomeric cucurbic acids (from NaBH 4 reduction of jasmonic acids) after incubation of SPM from Rhône River at different salinity values for 102 days.

Post incubation salinity (g/kg)

7-iso-cucurbic acid (ng/g dry wt)

6-epi-7-iso-cucurbic acid (ng/g dry wt) a 0 0.12 ± 0.07 a 2.64 ± 0.30 15 0.30 ± 0.10 a 4.00 ± 0.70 25 0.79 ± 0.13 a 13.8 ± 4.9

a Average of triplicates.

Table 4

Autoxidation of vascular plant lipid tracers (betulin and amyrins) in surface sediment (0-1 cm) and SPM samples from Amazon River, fan and shelf and in surface sediment (0-1 cm) from Rhône shelf (nd, not detected).

Sample Location Betulin autoxidation (%)

a-Amyrin autoxidation (%)

b-Amyrin autoxidation (%)

Amazon

Surface sediment (Almeirim) (01°34 0 428 00 S, 52°40 0 79 00 W) 26.9 16.9 23.2 Surface sediment (Macapá south channel) (00°09 0 249 00 S, 51°03 0 120 00 W) nd 20.0 18.0 Surface sediment (Macapá north channel) (00°05 0 240 00 S, 50°37 0 212 00 W) nd 15.9 17.9 SPM (station A10) (01°20 0 26 00 S, 49°22 The relative parts played by autoxidation, photooxidation and LOX activities in the degradation of this acid could be estimated for the Rhône River and along the coastal transect investigated. The results showed a lack of LOX reactions in the Rhône River itself, with a greater contribution of LOX reactions to degradation at stations closer to the river mouth and a dominance of autoxidation at the more distant marine stations (Fig. 7). The increasing proportions of reduction products of 7-iso-jasmonic and jasmonic acids (isomeric cucurbic acids) observed between the Rhône River and the station R1 (Table 2) showed that LOX activity increased at stations closest to the Rhône Mouth, not only in bacteria. but also in higher plant material.

Induction of autoxidation in POM in estuarine waters

The concentrations of isomeric cucurbic acids (resulting from NaBH 4 reduction of 7-iso-jasmonic and jasmonic acids), after incubation for 102 days, increased significantly with increasing post incubation salinity (Table 3), attesting to the key role played by salinity in the induction of LOXs in riverine SPM. Indeed, 7-iso-jasmonic and jasmonic acids are specifically produced during LOX oxidation of a-linolenic acid in higher plants [START_REF] Kazan | Jasmonate signaling: toward an integrated view[END_REF]. Thus, the increase in LOX reactions observed at R1 and R7 (Fig. 7) can be attributed to the strong change in salinity between the Rhône (S 0 g/kg) and these off-shore stations (S 33 g/ kg and 38 g/kg at R1 and R7, respectively).

Interestingly, it is generally considered that LOXs play a central role in promoting oxidative injury in plants during environmental stress [START_REF] Bhattacharjee | Membrane lipid peroxidation and its conflict of interest: the two faces of oxidative stress[END_REF]. Indeed, because the Fe present in LOXs is usually in the inactive ferrous Fe 2+ state, an activation step is required for these enzymes to enter the catalytic cycle. This activation involves the reaction of the ferrous enzyme with FA hydroperoxides, producing an active ferric (Fe 3+ ) enzyme and an alkoxyl radical [START_REF] Ivanov | Dual role of oxygen during lipoxygenase reactions[END_REF]Fig. 7). This generation of radicals in the course of the LOX catalytic cycle may also act like a catalyst in autoxidation [START_REF] Fuchs | Iron release from the active site of lipoxygenase[END_REF]. Moreover, when LOX activity becomes very high, increasing amounts of free radicals may damage the active site of LOXs and release Fe 2+ ions [START_REF] Sato | Hydroxyl radical production by H 2 O 2 plus Cu, Zn-superoxide dismutase reflects the activity of free copper released from the oxidatively damaged enzyme[END_REF][START_REF] Fuchs | Iron release from the active site of lipoxygenase[END_REF], which may very effi- 1670, 315.2171, 327.1807, 487.3090 chromatograms showing presence of 7,10-dihydroxyhexadec-8(E)-enoic acid in: (A) surface sediment from the Mackenzie River plume (Station M434) (70°10 0 12 00 N, 133°35 0 24 00 W) and (B) in SPM from station A8 (02°35 0 30 00 S, 49°38 0 55 00 W) in the Amazon Estuary. (Abbreviated name: 7,10-dihydroxyC 16:1D8E FA = 7,10-dihydroxyhexadec-8(E)-enoic acid).

ciently catalyze the reduction of hydroperoxides to alkoxyl radicals [START_REF] Schaich | Lipid oxidation: theoretical aspects[END_REF] and thereby induce free radical oxidation chains.

To explain the induction of autoxidation of TPOM observed in estuaries, we therefore propose the scenario in Fig. 8. Higher plant debris carried by rivers contains various amounts of hydroperoxides produced by photo-or autoxidation during the senescence of higher plants on land as well as inactive LOXs. When this material is delivered from rivers to coastal waters, the increase in salinity induces LOX activation, allowing additional production of FA hydroperoxides and alkoxyl radicals. The increase in the concentration of these free radicals can result to the following processes: (i) autoxidation of the components of higher plant debris and (ii) degradation of LOXs, resulting in the release of Fe 2+ ions, which strongly enhance alkoxyl radical production and thus autoxidation. Accordingly, while it is well known that estuaries are reactive zones for the processing of OM (e.g. [START_REF] Bianchi | Biogeochemistry of Estuaries[END_REF], we contend that the role of autoxidation in these systems needs to be further examined as a potentially important process affecting the fate of TPOM.

Arctic estuarine water

Enhancement of TPOM autoxidation

The important role played by autoxidation in the degradation of TPOM in coastal waters, initially observed in the Mackenzie Estuary (Canadian Arctic) (Rontani et al., 2014a), was subsequently confirmed in the temperate zone of the Rhône Estuary [START_REF] Galeron | Impacts of coastal degradative processes: autoxidation a major player in the fate of terrestrial particulate organic matter in seawater[END_REF]. The results demonstrated that the level of autoxidation in Arctic coastal waters was extraordinarily high. For example, autoxidation of 24-ethylcholest-5-en-3b-ol (sitosterol) can reach 98% in SPM from surface waters of the Beaufort Sea shelf (Rontani et al., 2014a), compared with only 10% of similar material in the Mediterranean Sea [START_REF] Galeron | Impacts of coastal degradative processes: autoxidation a major player in the fate of terrestrial particulate organic matter in seawater[END_REF]. Moreover, autoxidation of specific biomarkers of vascular plants (a-and b-amyrins and betulin) reached 99.8, 99.7 and 93.5%, respectively in surface sediments from different regions of the Canadian Arctic [START_REF] Rontani | Identification of di-and triterpenoid lipid tracers confirms the significant role of autoxidation in the degradation of terrestrial vascular plant material in the Canadian Arctic[END_REF], while they appeared to be considerably lower in similar material collected from the Rhône Shelf (Table 4). In order to explain the high level of autoxidation in the Arctic, we compared several parameters (efficiency of photooxidation in terrestrial higher plant debris, hydroperoxide concentration and LOX activity in river SPM) in the Mackenzie and Rhône rivers.

Synergy between TPOM photo-and autoxidation

Work has shown exceptional efficiency of type II (i.e. involving singlet oxygen) photosensitized oxidation in Arctic phytoplankton in summer [START_REF] Rontani | Intense photooxidative degradation of planktonic and bacterial lipids in sinking particles collected with sediment traps across the Canadian Beaufort Shelf (Arctic Ocean)[END_REF]. In order to explain this unexpected efficiency, we recently carried out in vitro incubation of the diatom Chaetoceros neogracilis RCC2022 at different temperature and irradiance values. Interestingly, the results allowed us to show that type II photosensitized oxidation in senescent phytoplanktonic cells is strongly favored at low temperature and low irradiance [START_REF] Amiraux | Paradoxical effects of temperature and solar irradiance on the photodegradation state of killed phytoplankton[END_REF]. This apparent paradox has been attributed to: (i) the relative preservation of the sensitizer (chlorophyll) at low irradiance, which permits a longer production time for singlet oxygen, and (ii) the slower diffusion rate of singlet oxygen through the cell membranes at low temperature [START_REF] Ehrenberg | Kinetics and yield of singlet oxygen photosensitized by hypericin in organic and biological media[END_REF], thereby favoring the intracellular involvement of type II photosensitized reactions. Although never measured, enhanced Table 5 Proportion of hydroperoxyl amyrins (relative to residual parent compound) in SPM from Rhône, Mackenzie and Amazon rivers.

11a-Hydroperoxy-a-amyrin 11a-Hydroperoxy-b-amyrin Rhône (43°40 0 44 00 N, 4°37 0 16 00 W; Feb. 2012) 20.7 ± 6.2 a,b,c 6.3 ± 1.8 a,b,c Rhône (43°40 0 44 00 N, 4°37 0 16 00 W; March 2014) 7.5 ± 0.3 b,c 6.8 ± 0.4 b,c Rhône (43°40 0 44 00 N, 4°37 0 16 00 W; Nov. 2014) 20.4 ± 0.2 b,c 9.4 ± 1.3 b,c Mackenzie (67°27 0 7 00 N, 133°45 0 32 00 W; June 2011) 70.9 ± 21.6 a,c 25.2 ± 6.8 a,c Amazon (03°19 0 29 00 N, 60°32 0 58 00 W; June 2005) 75.9 ± 1.7 a,d 70.8 ± 6.8 a,d Amazon (03°19 0 29 00 N, 60°32 0 58 00 W; June 2006) 72.0 ± 1.2 a,d 71.4 ± 0.4 a,d Amazon (03°27 0 24 00 N, 58°47 0 57 00 W; June 2005) 74.8 ± 1.0 a,d 67.7 ± 9. photooxidation of the components of senescent higher plants in the Arctic thus seems likely. This is supported by a comparison between the photooxidation state of sitosterol (arising mainly from terrestrial vascular plants in the Mackenzie and Rhône rivers; [START_REF] Tolosa | Carbon sources in suspended particles and surface sediments from the Beaufort Sea revealed by molecular lipid biomarkers and compound-specific isotope analysis[END_REF][START_REF] Galeron | Seasonal survey of the composition and degradation state of particulate organic matter in the Rhône River using lipid tracers[END_REF], which showed a greater extent of photooxidation in the Mackenzie River (48 ± 2%, n = 3; Rontani et al., 2014a) compared with the Rhône River (10 ± 1%, n = 30; [START_REF] Galeron | Seasonal survey of the composition and degradation state of particulate organic matter in the Rhône River using lipid tracers[END_REF]. It should be noted that in senescent cells photochemically produced hydroperoxides may undergo one electron reduction, which exacerbates peroxidative damage [START_REF] Girotti | New trends in photobiology[END_REF]. These processes result in the formation of alkoxyl radicals, which either directly or indirectly [START_REF] Gardner | Oxygen radical chemistry of polyunsaturated fatty acids[END_REF]) may initiate rounds of free radical peroxidation (autoxidation) by H abstraction.

Intense autoxidation may thus be associated in some cases with strong photooxidation in senescent leaves of higher plants (Galeron et al., 2016a).

Hydroperoxide induced LOX activation

Due to the strongest photooxidation of higher plants in the Arctic, we predicted that SPM of the Mackenzie River should contain a higher proportion of hydroperoxides than the Rhône River. In order to confirm this hypothesis, some hydroperoxides resulting from the oxidation of components of higher plants (a-and b-amyrins; Galeron et al., 2016b) in SPM samples from the Rhône and Mackenzie rivers were quantified (Table 5). As predicted, the proportion of hydroperoxides (relative to the residual parent compound) was highest in SPM from the Mackenzie River. These results are particularly interesting due to the high specificity of these compounds (unambiguous tracers of higher plants; [START_REF] Otto | A comparison of plant and microbial biomarkers in grassland soils from the Prairie Ecozone of Canada[END_REF][START_REF] Volkman | Lipid markers for marine organic matter[END_REF]. They demonstrate that higher plant debris in the Mackenzie River contain a high proportion of hydroperoxides.

The molar ratio Rcucurbic acids (resulting from the reduction of 7-iso-jasmonic and jasmonic acids during the treatment; Fig. 3 parent a-linolenic acid was more than three orders of magnitude higher in the Mackenzie River (0.14 ± 0.13) than in the Rhône River (3.8 ± 1.7 Â 10 À5 ; Table 2), which confirms greater activity of LOXs in the Mackenzie River particles containing a higher concentration of hydroperoxides (Table 5). This high LOX activation in the Arctic zone is also supported by the detection of threo 7,10dihydroxyhexadec-8(E)-enoic acids (arising from 10S-DOX oxidation of palmitoleic acid and subsequent diol synthase activity, Fig. 6) in sediments from station M434 (Fig. 4a).

In Arctic zones (Fig. 9), due to enhanced photooxidation occurring during the senescence of terrestrial higher plants, the detritus from these organisms which is transferred to the rivers, contains a high proportion of hydroperoxides. These compounds strongly contribute to the activation of LOXs (particularly during the increase in salinity in estuarine waters). The resulting high LOX activity strongly enhances alkoxyl radical production (Fig. 8) and thereby autoxidation. In contrast, in temperate zones such as the Rhône River (Fig. 9) area, photooxidation in terrestrial higher plant detritus is more limited (Rontani et al., 2014b), probably due to the relatively high temperatures favoring migration of singlet oxygen outside the membranes [START_REF] Ehrenberg | Kinetics and yield of singlet oxygen photosensitized by hypericin in organic and biological media[END_REF]. Consequently, higher plant debris in rivers contains only a moderate proportion of hydroperoxides. Under these conditions, activation of LOXs in estuarine waters is more limited and the induction of autoxidation less significant than in the Arctic. This hypothesis is supported by the strong difference in autoxidative reactivity previously observed during the incubation of SPM from the Rhône and Mackenzie rivers in seawater at room temperature. Indeed, the proportion of sitosterol autoxidation products increased from 2.6 ± 0.4% to 10.6 ± 0. 7% after incubation of Rhône SPM for 49 days (Galeron et al., 2016c) and from 16.1 ± 3.2% to 51.4 ± 1.6% after incubation of Mackenzie SPM for 15 days (Rontani et al., 2014a).

Tropical coastal waters

Role of autoxidation and photooxidation

While the effect of both photo-and autoxidation seems less important in the Rhône than in the Mackenzie estuarine regions, it is reasonable to speculate that tropical areas should have even lower photooxidation and autoxidation rates, assuming a decreasing impact trend is linear. Examination of SPM samples from the Solimões and Madeira rivers, located in the central Amazon River basin, confirmed the low efficiency of photooxidative processes in this zone. Photooxidation is generally low in Amazon waters due to the high sediment load and low light penetration, accounting for 1% of CO 2 emission from the basin [START_REF] Amon | Photochemical and microbial consumption of dissolved organic carbon and dissolved oxygen in the Amazon River system[END_REF][START_REF] Remington | Effects of DOM photochemistry on bacterial metabolism and CO 2 evasion during falling water in a humic and a whitewater river in the Brazilian Amazon[END_REF]. As sediments begin to settle along the lower Amazon River and nearshore plume, the potential for photooxidation then increases [START_REF] Medeiros | Fate of the Amazon River dissolved organic matter in the tropical Atlantic Ocean[END_REF][START_REF] Seidel | Molecular-level changes of dissolved organic matter along the Amazon river-to-ocean continuum[END_REF]. The low photooxidation state of sitosterol in SPM from the river shows that this process is also limited on land in senescent terrestrial higher plants. As discussed above, such results might seem paradoxical in these highly irradiated regions (solar irradiance ranging from 100 to 250 W/m 2 in the Amazon Basin; [START_REF] Pinker | Interannual variability of solar irradiance over the Amazon Basin including the 1982-83 El Niño Year[END_REF], but recent findings have shown that both high temperature and high irradiance increase the diffusion rate of singlet oxygen outside cell membranes and quickly consume the photosensitizer [START_REF] Amiraux | Paradoxical effects of temperature and solar irradiance on the photodegradation state of killed phytoplankton[END_REF]. The conjunction of the short lifespan of the photosensitizer, coupled with the high diffusion rate of singlet oxygen, results in the involvement of a weakly damaging photooxidative process. In contrast, autoxidation of vascular plant material appears to be strongly enhanced in the Amazon River (Section 3.3). This may be attributed to the high tropical temperature (avg. in the Amazon Basin, 26 °C) well known to favor homolytic cleavage of hydroperoxides [START_REF] Chaiyasit | Role of physical structures in bulk oils on lipid oxidation[END_REF] and thus initiation of free radical-induced oxidation [START_REF] Schaich | Lipid oxidation: theoretical aspects[END_REF].

The high proportion (40-70%) of sitostanol (relative to its parent sitosterol) contrasts with values generally considered typical of healthy phytoplanktonic cells (5-10%; [START_REF] Wakeham | Compositions and transport of lipid biomarkers through the water column and surficial sediments of the EqPac Ocean[END_REF]. Such high values are generally attributed to the involvement of intense bacterial degradation [START_REF] Wakeham | Compositions and transport of lipid biomarkers through the water column and surficial sediments of the EqPac Ocean[END_REF], as previously observed in this river [START_REF] Amon | Photochemical and microbial consumption of dissolved organic carbon and dissolved oxygen in the Amazon River system[END_REF][START_REF] Ward | Degradation of terrestrially derived macromolecules in the Amazon River[END_REF][START_REF] Ward | The reactivity of plant-derived organic matter and the potential importance of priming effects in the lower Amazon River[END_REF]. Whereas photooxidation clearly appears to be the driving degradative force in riverine SPM from the Mackenzie River (Rontani et al., 2014a), autoxidation and biodegradation are the major processes at play in the tropical Amazon region.

In order to estimate the involvement of LOX reactions, we also measured the molar ratio Rcucurbic acids/parent a-linolenic acid in this material. The values (0.20 ± 0.17) are relatively close (Table 2) to those for the Mackenzie (0.14 ± 0.13), but still more than three orders of magnitude higher than in the Rhône (3.8 ± 1. 7 Â 10 À5 ), confirming the highest LOX activation in Amazonian waters. These results allow us to draw a more precise degradation scenario for estuarine areas, whether tropical, temperate, or polar (Fig. 9). In tropical zones, photooxidation does not play a major role in the degradation of riverine SPM, for reasons discussed by [START_REF] Amiraux | Paradoxical effects of temperature and solar irradiance on the photodegradation state of killed phytoplankton[END_REF]. However, the higher temperature seems to favor autoxidation, which is clearly the main driver in SPM degradation within the Amazon River, along with biodegradation [START_REF] Amon | Photochemical and microbial consumption of dissolved organic carbon and dissolved oxygen in the Amazon River system[END_REF][START_REF] Ward | Degradation of terrestrially derived macromolecules in the Amazon River[END_REF]. This intense autoxidation produces a large amount of hydroperoxides, favoring in turn important LOX activation in this river, which is able to induce intense autoxidation of TPOM in seawater (Fig. 8).

In order to test this assumption, we examined: (i) different surface sediment samples from the mouth of Amazon and in its north-west shelf (under the influence of the North Brazil Current; [START_REF] Sun | Origin and processing of terrestrial organic carbon in the Amazon system: lignin phenols in river, shelf and fan sediments[END_REF] and (ii) SPM samples from its plume (Fig. 1). The aand b-Amyrins and betulin (unambiguous tracers of angiosperms; [START_REF] Otto | A comparison of plant and microbial biomarkers in grassland soils from the Prairie Ecozone of Canada[END_REF] showed a clear increase in autoxidation state from the Amazon River to the sea (Table 4). It is interesting to note that the high extent of autoxidation observed in the Amazon shelf sediment samples (Table 4) was comparable with that recently observed in Arctic sediments [START_REF] Rontani | Identification of di-and triterpenoid lipid tracers confirms the significant role of autoxidation in the degradation of terrestrial vascular plant material in the Canadian Arctic[END_REF]. The net increase in autoxidation state (Table 4) in SPM samples from A10 (S 0 g/kg) to 8 (S 29.6 g/kg) confirmed the key role played by the increase in salinity in the induction of autoxidation. The participation of LOXs in this induction is supported by the presence of threo 7,10-dihydroxyhexadec-8(E)-enoic acids derived from initial attack on palmitoleic acid by 10S-DOX-like lipoxygenase [START_REF] Guerrero | Oxidation of oleic acid to (E)-10-hydroperoxy-8-octadecenoic and (E)-10-hydroxy-8-octadecenoic acids by Pseudomonas sp. 42A2[END_REF][START_REF] Busquets | Isolation and characterization of a lipoxygenase from Pseudomonas[END_REF] at station A8 (Fig. 4b).

Conclusions

Although autoxidation of TPOM has long been overlooked in coastal studies, recent findings demonstrating its importance have rekindled scientific interest in abiotic degradation across salinity gradients. Through the analysis of SPM samples from different rivers and estuaries at high and low latitude, we were able to confirm that autoxidation is favored in estuarine waters, and that its induction seems linked to LOX activation, which increases with salinitya finding that supports previous lab-based studies confirming an increase in LOX activity in a NaCl-supplemented medium. The LOX catalytic cycle itself, through the generation of radicals, may also induce autoxidative damage. The release of Fe 2+ , when the LOX activity is high enough that the radicals generated cause damage at the active site of LOX itself, could be another mechanism that induces autoxidation in estuarine waters.

The differences between estuaries, whether they are at high or low latitude, can be explained by the relative importance of each transformation process within the wider OM decay cycle across steep estuarine gradients. The low temperature and irradiance at high latitude favor photooxidative damage and in turn, hydroperoxide production. This high hydroperoxide content drives LOX activity and autoxidative damage in estuarine waters. At low latitude, photooxidation is less effective, but higher temperature and irradiance favor riverine autoxidation, which, through the production of hydroperoxides, also induces high LOX activation in estuarine waters. In temperate zones, riverine photooxidative and autoxidative damage, and hence hydroperoxide production, are limited, causing the moderate increase observed in autoxidation and LOX activation in mixed waters.

The interactions between biotic and abiotic degradation processes appear to be critical in the overall carbon cycle in coastal systems, but further work is needed. It is interesting to note that autoxidation is not limited to lipids, carbohydrates and amino acids; it can also affect biopolymers [START_REF] Schmid | Autoxidation of medium chain length polyhydroxyalkanoate[END_REF], lignin [START_REF] Waggoner | Formation of black carbon-like and acyclic aliphatic compounds by hydroxyl radical initiated degradation of lignin[END_REF] and kerogen [START_REF] Fookes | A chemical investigation of shale oil ageing[END_REF], inducing ring opening and chain cleavage, which may then enhance mineralization of these generally considered recalcitrant substrates via priming [START_REF] Bianchi | The role of terrestrially derived organic carbon in the coastal ocean: a changing paradigm and the priming effect[END_REF]. Thus, the ramifications of autoxidation/photoxidation processes appear to be quite important across a broader spectrum of important compounds found in the coastal zone and thus clearly warrant more attention.

Although the involvement of other radical production mechanisms cannot be totally excluded at this time, LOXs seem to play a key role in the induction of autoxidation in estuaries. Further studies will be needed to better understand the origins and induction mechanisms of these widespread enzymes across the aquatic continuum [START_REF] Ward | Where carbon goes when water flows: carbon cycling across the aquatic continuum[END_REF], from vascular plants to the streams, rivers, estuaries, and into the marine realm. Investigations of more systems across different latitudes, with different plant communities, light regimes and over multiple seasons are needed to better constrain the important biotic and abiotic drivers of these processes.

Fig. 1 .

 1 Fig. 1. Summary map showing sampling locations: (A) Rhône River and shelf, (B) Amazon River and shelf and (C) Mackenzie River and shelf.

mFig. 2 .

 2 Fig.2. Partial m/z 199.1518, 213.1635, 329.1968, 343.2125, 225.1670, 315.2171, 327.1807 chromatograms showing presence of oxidation products of palmitoleic acid in SPM sample collected in February 2012 at station R7 (43°15 0 55 00 N, 4°58 0 6 00 E) from the Rhône River plume. (Abbreviated names: e.g. 10-hydroxyC 16:1D8E FA = 10-hydroxyhexadec-8 (E)-enoic acid).

3. 5 .Fig. 3 .

 53 Fig. 3. Degradation of linolenic acid by 13-lipoxygenase.

Fig. 4 .

 4 Fig. 4. Partial m/z 225.1670, 315.2171, 327.1807, 487.3090 chromatograms showing presence of 7,10-dihydroxyhexadec-8(E)-enoic acid in: (A) surface sediment from the Mackenzie River plume (Station M434) (70°10 0 12 00 N, 133°35 0 24 00 W) and (B) in SPM from station A8 (02°35 0 30 00 S, 49°38 0 55 00 W) in the Amazon Estuary. (Abbreviated name: 7,10-dihydroxyC 16:1D8E FA = 7,10-dihydroxyhexadec-8(E)-enoic acid).

Fig. 5 .

 5 Fig.5. MRM chromatograms (m/z 266 ? 134 and m/z 335 ? 93) showing proportions of isomeric cucurbic acids and their parent linolenic acid in SPM samples collected from station Tsiigehtchic (67°27 0 7 00 N, 133°45 0 33 00 W) in the Mackenzie River.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Main products resulting from oxidation of palmitoleic acid by 10S-DOX-like lipoxygenase.

Fig. 8 .

 8 Fig.8. Proposed pathways for lipoxygenase-induced autoxidative degradation of terrestrial higher plant material discharged by rivers to seawater.

  Photooxidation

Fig. 9 .

 9 Fig. 9. Degradation of terrestrial higher plant material discharged by rivers in Arctic, temperate and tropical zones.

Table 1 d

 1 13 C signature (‰) of FAs and sterols at stations R1 (43°18 0 57 00 N, 5°1 0 49 00 E) and R7 (43°15 0 55 00 N, 4°58 0 6 00 E) in the Rhône River plume.

	Compounds	Station R1	Station R7
	C 14:0 FA	À28.6 ± 1.0 a	-
	Branched C 15:0 FAs	-b	-
	C 16:1D9 FA	À30.9 ± 0.4	À27.8 ± 1.1
	C 16:0 FA	À28.0 ± 0.2	À30.4 ± 0.2
	C 18:1D9 FA	À29.5 ± 0.7	À28.6 ± 1.1
	C 18:1D11 FA	-	-
	C 18:0 FA	À25.6 ± 0.4	À29.2 ± 1.6
	Cholesterol	À20.6 ± 0.5	À23.6 ± 0.6
	24-Methylenecholesterol	-	À21.6 ± 1.4
	Sitosterol	-	À27.1 ± 2.2

a Analytical error. b Not measured (below detection limit).

10-dihydroxyC 16:1Δ8E FA Threo 7,10-dihydroxyC 16:1Δ8E FA Erythro 7,10-dihydroxyC 16:1Δ8E FA Erythro 7,10-dihydroxyC 16:1Δ8E FA

	A
	Threo 7,
										m/z 225.1670
										m/z 327.1807
										m/z 417.2308
										m/z 315.2171
	22.4	22.8	23.2	23.6	24.0	24.4	24.8	25.2	25.6	26.0	26.4
					Retention time (min)
	B
										m/z 225.1670
										m/z 327.1807
										m/z 417.2308
										m/z 315.2171
	22.4	22.8	23.2	23.6	24.0	24.4	24.8	25.2	25.6	26.0	26.4
					Retention time (min)

Acknowledgments

The work was supported by the LEFE-CYBER (Les Enveloppes Fluides et l'Environnement) national program, as part of the MOR-TIMER (Matière ORganique Terrestre rejetée par les fleuves et les rIvières en MER) research program. Thanks are due to the FEDER 1166-39417 for the funding of the apparatus employed. The work is also a contribution to the Labex OT-Med (through ANR-11-LABX-0061) funded by the French Government ''Investissements d'Avenir" program of the French National Research Agency (ANR) through the A⁄MIDEX project (ANR-11-IDEX-0001-02). Additional data were provided by ''MOOSE" (Mediterranean Oceanic Observing System for the Environment) with the support of the ''Agence de l'Eau Rhône-Méditerranée-Corse". Thanks are due to J. Bouchez for the donation of the SPM Amazon samples. Lower Amazon River samples were collected by Sawakuchi and Ward as part of FAPESP Grant #08/58089-9 and National Science Foundation Grant #1256724 (PI, J. Richey). We acknowledge GBMF-MMI Grants 2293 and 2928 and NSF-OCE-0934095 for the financial support provided to Amazon plume collections, as well as the Brazilian government (Ministério da Marinha). The Jon and Beverly Thompson Chair in Geological Sciences supported time for T.S.B. on this project. We are also grateful to S.T. Belt, G. Massé, A. Rochon and the officers and crew of the CCGS Amundsen for help with obtaining box core sediment material from the Beaufort Sea. Special thanks go to A. Delmont and M. Fornier for help collecting samples. Finally, we thank the two anonymous reviewers for their useful and constructive comments.

Associate Editor-A. Pearson