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

Rémi Amiraux

Catherine Lalande

Marcel Babin

Hak-Ryul Kim

Simon T Belt

Use of palmitoleic acid and its oxidation products for monitoring the degradation of ice algae in Arctic waters and bottom sediments

Keywords:

Degradation of palmitoleic acid (C 16:1x7 ), the main fatty acid component of sea ice-associated (sympagic) diatoms, was monitored in Arctic sea ice at the beginning of ice melting and in the underlying sinking particles and superficial bottom sediments. In sea ice, degradation of sympagic algae involved biotic oxidation induced by 10S-DOX-like lipoxygenase of unknown salinity-stressed attached bacteria, while photo-and autoxidation were limited. In the water column, strong hydratase and Z/E isomerase activity were observed. Hydration of unsaturated fatty acids seems to be a detoxification strategy, which is essential for bacterial survival when associated with free fatty acid-rich environments such as ice algae. In contrast, Z/E isomerisation of palmitoleic acid was attributed to the release of Fe 2+ ions during radicalinduced damage of the active site of the bacterial 10S-DOX-like lipoxygenase and Z/E isomerases. Due to the poor physiological state of their attached bacteria resulting from salinity stress in brine channels or toxicity of free ice algae fatty acids, sympagic algae appeared to be only very weakly biotically degraded within the water column. In bottom sediments, free radicals resulting from 10S-DOX-like lipoxygenase activity induced a strong autoxidation of the ice algal material. The presence in bottom sediments of a significant proportion of oxidation products resulting from 10S-DOX-like lipoxygenase activity attested to the strong contribution of sea ice-derived OM released during the early stages of ice melt prior to deposition in the sediments. However, on the basis of the highest fatty acid photooxidation state observed in these sediments, an additional contribution of highly photooxidized material (ice algal material released at the end of ice melting or open water phytoplankton) seems likely. The degradation of hydroperoxides, resulting from biotic and abiotic degradation of palmitoleic acid, appeared to involve: (i) homolytic cleavage of the peroxyl group affording the corresponding hydroxy-and oxoacids, (ii) reduction to the corresponding hydroxyacids by peroxygenases, (iii) heterolytic proton-catalysed cleavage and (iv) conversion to allylic 1,4-diols by diol synthases and hydroperoxide isomerases.

Introduction

The thinning and retreat of Arctic sea ice, which is one of the most striking consequences of recent climate change, will likely have a significant impact on Arctic ecosystem functioning in the future [START_REF] Wassmann | Footprints of climate change in the Arctic marine ecosystem[END_REF]. Continued changes in the polar seas and sea ice retreat will also affect marine biogeochemical cycles, with possibly important feedbacks to the climate and marine ecosystems [START_REF] Arzel | Sea ice evolution over the 20th and 21st centuries as simulated by current AOGCMs[END_REF].

Reduction of sea ice in the Arctic under the effect of global warming should significantly alter the relative fluxes of phytoplankton and ice algae to the seafloor [START_REF] Carroll | The Arctic seas[END_REF]. A climate-change-mediated shift in primary producers would thus impact the structure and function of the sea floor community, which is strongly dependent upon the deposition of organic material from the overlying water column for its energy requirements [START_REF] Mcmahon | Benthic community response to ice algae and phytoplankton in Ny Ålesund, Svalbard[END_REF]. Moreover, under the effect of global warming the carbon sink potential of ice algae (resulting from their strong aggregation and the stress state of their associated bacteria, should be gradually replaced by the carbon source potential of open water phytoplankton (weakly aggregated and mineralized before the bottom) [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF].

Determining the fate of sympagic (ice-associated) derived particulate organic matter (POM) following its release from sea ice during spring melting is thus an important research objective [START_REF] Tedesco | 21st century projections of surface mass balance changes for major drainage systems of the Greenland ice sheet[END_REF], but partitioning processes such as growth of the water column phytoplankton community, grazing, remineralization and export is complicated and currently not well constrained. The strong pulse of particulate OM released into the water column during ice melt provides essential energy at the base of the Arctic food web [START_REF] Michel | Carbon budget of sea-ice algae in spring: evidence of a significant transfer to zooplankton grazers[END_REF][START_REF] Lizotte | The contributions of sea ice algae to Antarctic marine primary production[END_REF], although some sinks to the seafloor [START_REF] Riebesell | Aggregation of algae released from melting sea ice-implications for seeding and sedimentation[END_REF][START_REF] Fortier | Climatic and biological forcing of the vertical flux of biogenic particles under seasonal Arctic sea ice[END_REF][START_REF] Renaud | Carbon cycling by seafloor communities on the eastern Beaufort Sea shelf[END_REF]. Ultimately, the material that is not grazed or re-mineralized during its descent through the water column, can feed the benthos [START_REF] Boetius | ARK27-3-Shipboard Science PartyExport of algal biomass from the melting Arctic sea ice[END_REF] or be stored in sediments.

In practice, many factors influence the quantity and the quality of the sympagic OM reaching the seafloor. These include the physical characteristics of the water column, including water depth and whether it is well mixed or stratified, the latter being a particular feature of Arctic waters due to the lower salinity caused by ice melt in spring and freshwater runoff. In addition, the fate of sympagic OM is influenced by physiological, morphological and ecological factors such as cell agglomeration and grazing by under-ice and water column heterotrophs [START_REF] Vancoppenolle | Role of sea ice in global biogeochemical cycles: emerging views and challenges[END_REF]. A particular feature of sea ice biota is their ability to produce large quantities of so-called extracellular polymeric substances (EPS), which serve as a cryoprotectant within the ice matrix. Indeed, EPS concentrations in Arctic sea ice are typically at least an order of magnitude higher than for under-ice and open water environments [START_REF] Krembs | Abundance and variability of microorganisms and transparent exopolymer particles across the ice-water interface of melting first-year sea ice in the Laptev Sea (Arctic)[END_REF][START_REF] Meiners | Vertical distribution of exopolymer particles in sea ice of the Fram Strait (Arctic) during autumn[END_REF], which promotes aggregation of sympagic algae and thus enhances export efficiency to the seafloor. The contribution of ice algae to the underlying sediments also depends strongly on the remineralization potential of their associated bacteria. Recently, it was observed that during the early stage of ice melting, hypersaline conditions in brine channels strongly impact the bacterial community associated with ice algae [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF]. The resulting poor physiological state of these bacteria should likely contribute to the preservation of the algal material prior to deposition on the ocean bottom.

Lipid biomarkers, although only representing a relatively minor fraction of the total OM, can convey important information on the source (terrigenous, marine or bacterial) and degradation state of OM, and their distributions are often more informative than bulk parameters such as total organic carbon [START_REF] Saliot | Transport and fate of Danube Delta terrestrial organic matter in the Northwest Black Sea mixing zone[END_REF]. Furthermore, the oxidation products of monounsaturated fatty acids appear to be especially useful for differentiating the involvement of photo-and autoxidative degradation processes in suspended and sinking particulate matter and bottom sediment samples (for a review see [START_REF] Rontani | Photo-and free radical-mediated oxidation of lipid components during the senescence of phototrophic organisms[END_REF]. More recently, analysis of these compounds provided evidence of lipoxygenase-induced degradation in suspended [START_REF] Galeron | Lipoxygenase-induced autoxidative degradation of terrestrial particulate organic matter in estuaries: a widespread process enhanced at high and low latitudes[END_REF] and sinking [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF] particles.

Since the integrity of the strong pulse of particulate organic matter (POM) released during Arctic ice melt remains largely unexamined, we monitored biotic and abiotic degradation processes in ice algae released from melting Arctic sea ice and their transfer to the ocean bottom using pathway-specific degradation tracer products of palmitoleic acid (C 16:1x7 ). Indeed, although present in several bacteria (e.g., [START_REF] De Carvalho | Fatty acids as a tool to understand microbial diversity and their role in food webs of Mediterranean temporary ponds[END_REF], palmitoleic acid is also the major fatty acid of diatoms [START_REF] Fahl | Lipid content and fatty acid composition of algal communities in sea-ice and water from the Weddell Sea (Antarctica)[END_REF][START_REF] Leu | Increased irradiance reduces food quality of sea ice algae[END_REF], which strongly dominate the biomass in sea ice.

Experimental

Sampling

Ice cores, sinking particles and superficial bottom sediments were collected during the spring of 2015 at a landfast ice station located near Broughton Island (67°28.766 0 N; 63°47.579 0 W; water column depth: 379 m) north of Davis Strait, Canada (GreenEdge ice camp 2015). Sea ice and sinking particles were sampled during the early stage of ice melt [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF]). An ice core was collected on 4 June 2015 using a 9 cm internal diameter Mark II coring system (Kovacs Enterprises) and sliced in the field. The bottom 1 cm slice, which corresponds to the ice-ocean interface and contains the largest amount of ice algae, was allowed to melt in filtered seawater (FSW; 0.2 mm) using at least 2 parts FSW for 1 part of ice in order to limit osmotic stress and filtered (Whatman GF/F 47 mm). Short-term sediments traps were deployed to collect sinking particles over 48 h at 2 m and 25 m with two mooring lines. Sediment traps were made of polyvinyl chloride (PVC) and had an aperture diameter of 15 cm. The recovery of the samples analyzed in the present study took place on 8-10 June 2015 (at the time of highest flux of sinking material measured during this time series, Lalande, unpublished data). A sub-sample for lipid analysis (corresponding to 4.5% of the total sample) was filtered onto Whatman GF/F 47 mm filter. Superficial bottom sediment samples (ca. 0-1 cm) were also collected using box corers at the same sampling location on board the CCGS Amundsen in October 2015 as part of the GreenEdge project. All the samples were kept frozen (-20 °C) until analysis.

Treatment

Samples (filters or sediments) were reduced with excess NaBH 4 or NaBD 4 after addition of MeOH (25 ml; 30 min) to reduce labile hydroperoxides to alcohols which are more amenable to analysis using gas chromatography-mass spectrometry. Water (25 ml) and KOH (2.8 g) were then added and the resulting mixture saponified by refluxing (2h). After cooling, the mixture was acidified (HCl, 2 N) to pH 1 and extracted with dichloromethane (DCM; 3 Â 20 ml). The combined DCM extracts were dried over anhydrous Na 2 -SO 4 , filtered and concentrated via rotary evaporation at 40 °C to give total lipid extracts (TLEs). TLEs were derivatized by dissolving them in 300 ml of pyridine/bis(trimethylsilyl)trifluoroacetamide (BSTFA; Supelco; 2:1, v:v) and silylated (50 °C, 1 h). After evaporation to dryness under a stream of N 2 , the derivatized residue was dissolved in ethyl acetate/BSTFA (to avoid desilylation) and analyzed using gas chromatography-electron ionization quadrupole time of flight mass spectrometry (GC-QTOFMS).

A different treatment was employed to estimate the relative proportions of hydroperoxides and their ketonic and alcoholic degradation products. Subsamples were extracted three times with chloroform-MeOH-H 2 O (1:2:0.8, v:v:v) using ultrasonication. The supernatant was separated by centrifugation at 3500g for 9 min. To initiate phase separation, purified H 2 O was added to the combined extracts. The upper aqueous phase was extracted three times with DCM and the combined organic phase and DCM extracts were filtered and the solvent removed via rotary evaporation. The residue obtained after extraction was dissolved in 4 ml of DCM and separated in two equal subsamples. After evaporation of the solvent, degradation products were obtained for the first subsample after acetylation, which induced conversion of hydroperoxides to the corresponding ketones [START_REF] Mihara | Photosensitized oxygenation reactions of phytol and its derivatives[END_REF], and saponification. The second subsample was reduced with NaBD 4 and saponified. Comparison of the amounts of alcohols present after acetylation and NaBD 4 reduction made it possible to estimate the amount of hydroperoxides and alcohols present in the samples, while deuterium labeling after NaBD 4 reduction allowed us to estimate the proportion of ketones present in the samples [START_REF] Marchand | Visible light-induced oxidation of lipid components of purple sulfur bacteria: a significant process in microbial mats[END_REF].

Acetylation was carried out in 300 ll of a mixture of pyridine and acetic anhydride (2:1, v:v), which was allowed to react at 50 °C overnight and then evaporated to dryness under a stream of N 2 .

GC-QTOFMS

Oxidation products of palmitoleic acid were identified and quantified using an Agilent 7890B/7200A GC-QTOF System (Agilent Technologies, Parc Technopolis -ZA Courtaboeuf, Les Ulis, France). A cross-linked 5% phenyl-methylpolysiloxane (Macherey Nagel; Optima 5-MS Accent) column (30 m  0.25 mm, 0.25 lm film thickness) was employed. Analysis was performed with an injector operating in pulsed splitless mode at 280 °C and the oven temperature programmed from 70 °C to 130 °C at 20 °C/min, then to 250 °C at 5 °C/min and then to 300 °C at 3 °C/min. The carrier gas (He) was maintained at 0.69  10 5 Pa until the end of the temperature program. Instrument temperatures were 300 °C for the transfer line and 230 °C for the ion source. Accurate mass spectra were obtained across the range m/z 50-700 at 4 GHz. The QTOFMS instrument provided a typical resolution ranging from 8009 to 12,252 from m/z 68.9955 to 501.9706. Perfluorotributylamine (PFTBA) was utilized for daily MS calibration. Compounds were identified by comparison of their TOF mass spectra, accurate masses and retention times with those of standards, either purchased or synthesized in the laboratory (see following Section). Quantification of each compound involved extraction of specific accurate fragment ions, peak integration and determination of individual response factors using external standards.

Standard compounds

(8-11)-Hydroperoxyhexadec-(8-10)-enoic acids (Z and E) acids (1-6) (see Appendix A) were produced by Fe 2+ /ascorbate induced autoxidation [START_REF] Loidl-Stahlhofen | a-Hydroxyaldehydes, products of lipid peroxidation[END_REF] of palmitoleic acid. Subsequent reduction of these different hydroperoxides in methanol with excess NaBH 4 afforded the corresponding hydroxyacids. Hydrogenation of these hydroxyacids was carried out in methanol with Pd/CaCO 3 as catalyst. Treatment of palmitoleic acid with meta-chloroperoxybenzoic acid in dry methylene chloride yielded 9,10-epoxyhexadecanoic acid (20). Hydrolysis of this epoxide in HCl (2N) afforded the corresponding chlorohydrins, while methanolysis yielded the corresponding methoxyhydrins. Threo and erythro 9,10-dihydroxyhexadecanoic acids (17) were respectively obtained after stereospecific oxidation of the double bond of palmitelaidic (the trans isomer of palmitoleic acid: hexadec-9 (E)-enoic acid) and palmitoleic acids with OsO 4 in pyridinedioxane [START_REF] Mccloskey | Mass spectra of O-isopropylidene derivatives of unsaturated fatty esters[END_REF]. Threo and erythro 8,11-dihydroxyoctadec-9(E and Z)-enoic acids (22 and 23) were produced in low yield (5-10%) by oxidation of palmitoleic acid with SeO 2 -t-butylhydroperoxide [START_REF] Knothe | Allylic mono-and dihydroxylation of isolated double bonds with selenium dioxide-tert-butyl hydroperoxide. NMR characterization of long-chain enols, allylic and saturated 1,4-diols, and enones[END_REF]. A standard of threo 7,10-dihydroxyoctadec-8(E)-enoic acid (42) containing 10% of threo 7,10-dihydroxyhexadec-8(E)-enoic acid (21) previously 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 Dr. H.R. Kim (School of Food Science and Biotechnology, Kyungpook National University, Daegu, Korea).

Estimation of autoxidative, photooxidative and 10S-DOX degradation

The role played by autoxidation in the degradation of palmitoleic acid was estimated by considering the proportion of its specific Z-oxidation products [START_REF] Frankel | Lipid Oxidation[END_REF] and the water temperature according to the approach previously described by [START_REF] Marchand | Characterisation of photooxidation 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 (8) and 10-E (9) oxidation products during the photooxidation of the D 9 monounsaturated fatty acids [START_REF] Frankel | Lipid Oxidation[END_REF] and their specific allylic rearrangement to 11-E (5) and 8-E (6) isomers, respectively [START_REF] Porter | Mechanisms of free radical oxidation of unsaturated lipids[END_REF], the contribution of photooxidative degradation was estimated to be 2 Â (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 7,10dihydroxyhexadec-8(E)-enoic acid ( 21) formed [START_REF] Galeron | Lipoxygenase-induced autoxidative degradation of terrestrial particulate organic matter in estuaries: a widespread process enhanced at high and low latitudes[END_REF].

Results

Significant proportions (increasing from 3.5% relative to the residual parent palmitoleic acid in sea ice to 25% in sediments) of C 16 isomeric allylic hydroperoxyacids (11-hydroperoxyhexadec-9(E)-enoic (1), 9-hydroperoxyhexadec-10(E)-enoic (2), 10-hydroperoxyhexadec-8(E)-enoic (3), 8-hydroperoxyhexadec-9(E)-enoic (4), 11-hydroperoxyhexadec-9(Z)-enoic ( 5) and 8-hydroperoxyhexadec-9(Z)-enoic (6) acids) (see Appendix A) and of the corresponding hydroxy-and oxoacids could be detected in the different samples investigated (Table 1). After NaBH 4 reduction, the mixtures of the corresponding isomeric hydroxyacids appeared to be strongly dominated by 10-hydroxyhexadec-8(E)-enoic acid (9) in sea ice (Fig. 1a) and by 10-hydroxyhexadec-8(E)-enoic ( 9) and 8-hydroxyhexadec-9(E)-enoic (10) acids in sinking particles (Fig. 1b). This dominance relative to the other isomers was less apparent in superficial bottom sediments (Fig. 1c).

Saturated hydroxyacids (9-hydroxyhexadecanoic (13) and 10hydroxyhexadecanoic (14) acids), methoxyhydrins (9-hydroxy-10-methoxyhexadecanoic (15) and 9-methoxy-10-hydroxyhexadecanoic (16) acids), diols (diastereoisomeric 9,10-dihydroxyhexadecanoic acids ( 17)) and chlorohydrins (9-hydroxy-10chlorohexadecanoic (18) and 9-chloro-10-hydroxyhexadecanoic (19) acids), all derived from the degradation of (Z and E) 9,10epoxyhexadecanoic acids (20) during the sample treatment (Fig. 2; [START_REF] Marchand | Characterisation of photooxidation and autoxidation products of phytoplanktonic monounsaturated fatty acids in marine particulate matter and recent sediments[END_REF], could be also detected (Fig. 1). NaBH 4 reduction of mid-chain epoxides is generally slow and not regioselective [START_REF] Zabeti | Potential alteration of U 37 K' paleothermometer due to selective degradation of alkenones by marine bacteria isolated from the haptophyte Emiliania huxleyi[END_REF]; the higher proportions of 10-hydroxyhexadecanoic acid ( 14) relative to 9-hydroxyhexadecanoic acid (13) observed (Fig. 1) thus suggest the involvement of an additional source of the 10-isomer. The concentration of epoxides was thus estimated on the basis of their degradation product concentrations according to the following equation:

(2 Â [9-hydroxyhexadecanoic acid] + [methoxyhydrins] + [diols] + [chlorohydrins]
). Interestingly, their percentage relative to the sum of the parent palmitoleic acid and its degradation products increased from 1% in sea ice to 17% in superficial bottom sediments.

Isomeric allylic dihydroxyhexadecenoic acids could be also detected (Table 1, Fig. 3). Threo 7,10-dihydroxyhexadec-8(E)enoic (21), threo and erythro 8,11-dihydroxyhexadec-9(E)-enoic ( 22) and erythro 8,11-dihydroxyhexadec-9(Z)-enoic acids ( 23) could be formally identified by comparison of their accurate mass spectra (Fig. 4a andc) and retention times with those of reference compounds. The compound eluting just after threo 7,10dihydroxyhexadec-8(E)-enoic acids (21) (Fig. 3), and exhibiting exactly the same mass spectrum, was attributed to the erythro diastereoisomer. Indeed, it was observed previously that threo and erythro diastereoisomers of TMS derivatives of dihydroxyhexadecanoic acids eluted on an apolar column in the same order [START_REF] Hansel | Formation of dihydroxy acids from Zmonounsaturated alkenoic acids and their use as biomarkers for the processing of marine commodities in archaeological pottery vessels[END_REF]. Erythro 9,12-dihydroxyhexadec-10 (E)-enoic acids (24) were tentatively identified on the basis of the accurate mass fragmentations of their trimethylsilyl derivatives (Fig. 4b) and the abundance of the m/z 147 peak in their mass spectra, which is similar to those of the other E isomers (Fig. 4a andc). Indeed, this fragment ion, which arises from interaction between two TMS groups, is always more intense in the EI mass spectrum of the Z isomer compared to its E counterpart [START_REF] Zaikin | A Handbook of Derivatives for Mass Spectrometry[END_REF]. The percentage of these isomeric dihydroxyhexadecenoic acids increased from 0.1% in sea ice to 6% in superficial bottom sediments.

A significant increase of the proportion of hexadec-9(E)-enoic (palmitelaidic) acid relative to palmitoleic acid was apparent in the deeper sediment trap and in superficial bottom sediments (Table 1).

Discussion

Degradation of palmitoleic acid in sea ice (0-1 cm)

Type II photosensitized oxidation (i.e. involving singlet oxygen) of palmitoleic acid produces equal proportions of isomeric 9-and 10-hydroperoxides (2 and 3) 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 hydroperoxides (1 and 4), respectively (Fig. 5; [START_REF] Porter | Mechanisms of free radical oxidation of unsaturated lipids[END_REF]. In contrast, autoxidation (i.e. free radical induced oxidation) affords a mixture of 9-E (2), 10-E (3), 11-E (1), 11-Z (5), 8-E (4), and 8-Z (6) hydroperoxides, also exhibiting equal proportions of the major 9-E (2) and 10-E (3) isomers (Fig. 5; [START_REF] Frankel | Lipid Oxidation[END_REF]. The strong predominance of the 10-E hydroxyacid (9) observed in sea ice after NaBH 4 reduction (Fig. 1) is thus very surprising. Similar profiles of palmitoleic acid oxidation products were observed previously in other sea ice samples (Amiraux et al., submitted for publication) and in sinking particles underlying melting sea ice [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF], also in the Canadian Arctic, and attributed to the involvement of a specific bacterial enzymatic process under the hypersaline conditions found in brine channels. Indeed, a 10S-DOX-like lipoxygenase able to convert palmitoleic acid to 10(S)hydroperoxyhexadec-8(E)-enoic acid (3) was previously 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 42A2 responsible for the biotransformation of oleic acid into (S)-(E)-10-hydroxy-8octadecenoic acid[END_REF]. Palmitoleic acid is a major constituent of numerous bacteria [START_REF] Oliver | Extractable lipids of gram-negative marine bacteria: Fatty acid composition[END_REF][START_REF] Viso | Fatty acids from 28 marine microalgae[END_REF], although it is also the predominant fatty acid in sea ice diatoms [START_REF] Fahl | Lipid content and fatty acid composition of algal communities in sea-ice and water from the Weddell Sea (Antarctica)[END_REF], which make up the major part of the biomass in sea ice. The presence of high proportions of 10hydroperoxyhexadec-8(E)-enoic acid (3) in sea ice was thus attributed to the degradation of ice algae by some of their attached bacteria [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF], consistent with the autonomous action of prokaryotic lipoxygenases on the membrane phospholipids of their eukaryotic cell hosts [START_REF] Vance | The opportunistic pathogen Pseudomonas aeruginosa carries a secretable arachidonate-15lipoxygenase[END_REF][START_REF] Garreta | Structure and interaction with phospholipids of a prokaryotic lipoxygenase from Pseudomonas aeruginosa[END_REF].

The involvement of 10S-DOX-like lipoxygenase enzymatic activity in the sea ice sample investigated herein is further supported by the detection of threo 7,10-dihydroxyhexadec-8(E)enoic acid (21) (Table 1, Fig. 3) formed from the specific action of 7S,10S-hydroperoxide diol synthase (linked to the 10S-DOX-like lipoxygenase enzymatic activity) (Fig. 5; [START_REF] Estupiñán | Unveiling the genes responsible for the unique Pseudomonas aeruginosa oleate-diol synthase activity[END_REF][START_REF] Estupiñán | In silico/ in vivo insights into the functional and evolutionary pathway of Pseudomonas aeruginosa oleate-diol synthase. Discovery of a new bacterial di-heme cytochrome C peroxidase subfamily[END_REF] on 10(S)-hydroperoxyhexadec-8(E)-enoic acid (3). This enzyme appeared to be inactive towards reaction with 9hydroperoxyhexadec-10(E)-enoic acid (2), since there was an absence of threo 9,12-dihydroxyhexadec-10(E)-enoic acid (24) (Fig. 5), although it converted 12-hydroperoxyoctadec-10(E)enoic acid (25) (arising from vaccenic acid oxidation) to threo 9,12-dihydroxyoctadec-10(E)-enoic acid (31) (Table 2). These contrasting results suggest that the diol synthase present in the Arctic bacteria is only active on allylic hydroperoxides allowing the formation of a diol between the carboxyl and the peroxyl groups and not on those where the diol is formed between the methyl and the peroxyl groups (Table 3). Consequently, the formation of threo 8,11-dihydroxyoctadec-9(E)-enoic acid (22) (Fig. 5) likely results from the reaction of this enzyme with 11hydroperoxyhexadec-9(E)-enoic acid (1) rather than with 8hydroperoxyhexadec-9(E)-enoic acid (4) (Table 2).

The relatively low proportions of 9E-and 11E-hydroxyacids (Fig. 1a) is consistent with the low efficiency of Type II photooxidation processes on ice algae, attributed previously to the good physiological state of algae at the time of sampling [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF]. Autoxidation also appeared to be limited in the sea ice sample as shown by the low amounts of specific 8Z-and 11Zhydroxyacids (Fig. 1a). The degradation of all these labile isomeric hydroperoxyacids arising from photooxidative, autoxidative and enzymatic processes in sea ice involves: (i) homolytic cleavage of the peroxyl group affording the corresponding hydroxy-and oxoacids [START_REF] Frimer | The reaction of singlet oxygen with olefins: the question of mechanism[END_REF], (ii) reduction to the corresponding hydroxyacids by peroxygenases, which catalyze an intermolecular transfer of oxygen from hydroperoxides to the double bonds of fatty acids yielding alcohols and epoxides (Blée and Schubert, 1995;[START_REF] Blée | Phytooxylipins and plant defense reactions[END_REF], (iii) heterolytic proton-catalysed cleavage leading to the formation of two carbonyl fragments (Hock cleavage) initiated by migration of groups to positive oxygen [START_REF] Frimer | The reaction of singlet oxygen with olefins: the question of mechanism[END_REF], and (iv) isomerization to allylic 1,4-diols by hydroperoxide isomerases (Fig. 6). It is important to note that this last process affords erythro isomers (Jernerén et al., 2010), which are different to the threo isomers resulting from diol synthase activity.

Intermolecular transfer of oxygen from hydroperoxides to the double bond of palmitoleic acid by peroxygenases affords Z-9,10epoxyhexadecanoic acid (20) (Fig. 7; Blée and Schubert, 1995;[START_REF] Blée | Phytooxylipins and plant defense reactions[END_REF]. This Z-configuration may also be produced by stereospecific Z-epoxidation of palmitoleic acid by a hydroperoxide (m/z 199.1518, 213.1675, 225.1670, 317.1968, 329.1968, 331.2127 and 343.2125) showing the presence of palmitoleic acid oxidation products in sea ice (0-1 cm) (a), sediment trap (25 m) (b) and superficial bottom sediments (0-1 cm) (c). species in the presence of transition metal ions (Fig. 7; [START_REF] Hansel | Formation of dihydroxy acids from Zmonounsaturated alkenoic acids and their use as biomarkers for the processing of marine commodities in archaeological pottery vessels[END_REF]. In contrast, hydroperoxyl radical addition to one end of the double bond of palmitoleic acid is not stereoselective. Indeed, in this case intramolecular homolytic substitution [START_REF] Fossey | Free Radicals in Organic Chemistry[END_REF] can occur on the resulting free radical after rotation about the position of the original double bond affording Z-and Eepoxides (Fig. 7). Isomerization of palmitoleic acid to hexadec-9 (Z)-enoic acid (palmitelaidic acid) catalyzed by thiyl radicals [START_REF] Ferreri | Thiyl radicals in biosystems: effects on lipid structures and metabolisms[END_REF], metal ions [START_REF] Holtwick | Cis-trans isomerization of unsaturated fatty acids: Cloning and sequencing of the cti gene from Pseudomonas putida P8[END_REF] or Z/E bacterial isomerases [START_REF] Heipieper | The cis-trans isomerase of unsaturated fatty acids in Pseudomonas and Vibrio: biochemistry, molecular biology and physiological function of a unique stress adaptive mechanism[END_REF] and subsequent epoxidation by the three processes described above, constitute other potential sources of (Z and E) 9,10-epoxyhexadecanoic acids (20) (Fig. 7).

The higher abundance of 10-hydroxyhexadecanoic acid ( 14) relative to its 9-isomer (13) (Fig. 1a) was attributed to the involvement of hydratase. Indeed, several bacteria are able to hydrate, stereospecifically, D 9 double bonds of fatty acids to 10hydroxyacids [START_REF] El-Sharkawy | Microbial oxidation of oleic acid[END_REF][START_REF] Yang | Stereospecificity of microbial hydrations of oleic acid to 10-hydroxystearic acid[END_REF][START_REF] Hou | Production of 10-ketastearic acid from oleic acid by Flavobacterium sp. strain DS5 (NRRL B-14859)[END_REF], which are often subsequently dehydrogenated by alcohol dehydrogenases to the corresponding 10-ketoacids [START_REF] El-Sharkawy | Microbial oxidation of oleic acid[END_REF]. Reduction of the samples with NaBD 4 instead of NaBH 4 allowed us to estimate the relative proportions of 10hydroxyhexadecanoic acid (14) and 10-oxohexadecanoic acid (35) in sea ice (9:1).

Degradation of palmitoleic acid in sinking particles

The dominance of Nitzschia frigida (1-20%) and Fragilariopsis oceanica (2-24%) (well-known diatom species in sea ice; [START_REF] Poulin | The pan-Arctic biodiversity of marine pelagic and sea-ice unicellular eukaryotes: a first-attempt assessment[END_REF][START_REF] Ren | Diatom distributions in northern North Pacific surface sediments and their relationship to modern environmental variables[END_REF] in the sediment trap material (Lalande, unpublished data) confirms a sea ice origin for the majority of sinking particles. The high contribution of ice diatoms to this material is also supported by the strong dominance of palmitoleic acid in TLEs [START_REF] Fahl | Lipid content and fatty acid composition of algal communities in sea-ice and water from the Weddell Sea (Antarctica)[END_REF]. Ice algae appeared to be very weakly degraded within the water column, with degradation percentages of palmitoleic acid of only 4.8%, 5.4%, and 4.3% in ice, upper and deeper sediment traps, respectively. This relative preservation is likely attributed to the poor physiological state of bacterial communities associated with ice algae resulting from salinity stress in brine channels [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF].

Stereospecific allylic rearrangement of 10-hydroperoxyhexadec-8(E)-enoic acid (3) to 8-hydroperoxyhexadec-9(E)-enoic acid (4), which appeared to be very limited in sea ice (Fig. 1a), acted very intensively in sinking particles (Fig. 1b). The extent of this rearrangement increased with time [START_REF] Porter | Mechanisms of free radical oxidation of unsaturated lipids[END_REF], with higher proportions of the 8-E isomer observed in sinking particles confirming the aging of the material collected in the sediment traps.

A significant increase of the proportion of 10hydroxyhexadecanoic acid ( 14) was observed in sinking particles (Fig. 1, Tables 1 and3) suggesting an enhancement in the activity of hydratases within the water column. Interestingly, it was previously demonstrated that myosin cross-reactive antigen (MCRA) proteins acting as a hydratase on 9(Z) double bonds of fatty acids, play a role in adherence of bacteria [START_REF] Volkov | Myosin cross-reactive antigen of Streptococcus pyogenes M49 encodes a fatty acid double bond hydratase that plays a role in oleic acid detoxification and bacterial virulence[END_REF]. Indeed, free unsaturated fatty acids (and notably palmitoleic acid, [START_REF] Wille | Palmitoleic acid isomer (C 16:1D6 ) in human skin sebum is effective against gram-positive bacteria[END_REF] are toxic for many bacteria due to their deleterious effect on bacterial cellular membranes [START_REF] Greenway | Mechanism of the inhibitory action of linoleic acid on the growth of Staphylococcus aureus[END_REF]. They also inhibit enoyl-ACP reductase and thus disrupt bacterial fatty acid synthesis [START_REF] Zheng | Disruptions of the Arabidopsis enoyl-CoA reductase gene reveal an essential role for very-long-chain fatty acid synthesis in cell expansion during plant morphogenesis[END_REF]. Hydration of unsaturated fatty acids may thus represent a detoxification mechanism in bacteria harboring MCRA enzymes. Such detoxification could be essential for bacterial colonization and survival on free fatty acid-rich environments such as ice algae [START_REF] Falk-Petersen | Lipids and fatty acids in ice algae and phytoplankton from the Marginal Ice Zone in the Barents Sea[END_REF]. However, it may be noted that such enhancement of hydratase activity was not previously observed in sinking particles collected in spring 2012 at a landfast ice station in Resolute Passage (Nunavut, Canada) [START_REF] Rontani | Monitoring abiotic degradation in sinking versus suspended Arctic sea ice algae during a spring ice melt using specific lipid oxidation tracers[END_REF], probably due to the higher aggregation state of the ice algae [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF], resulting from an enhanced EPS content. In this case, the presence of high amounts of EPS around ice algae cells could have limited the toxicity of unsaturated fatty acids toward bacteria. The intense Z/E isomerisation of palmitoleic acid observed in the deeper sediment trap samples (Table 1) may result from the involvement of thiyl radicals. Indeed, functionalised aliphatic thiols (glutathione, methionine-containing proteins), which are present in living organisms in considerable amounts [START_REF] Ferreri | Thiyl radicals in biosystems: effects on lipid structures and metabolisms[END_REF], are extraordinarily efficient antioxidants that protect the cells against consequences of damage induced by free radicals and hydroperoxides [START_REF] Wlodek | Beneficial and harmful effects of thiols[END_REF]. However, this role as repairing agents is counterbalanced by the formation of thiyl radicals, which are efficient catalysts for Z/E isomerisation of lipids in biological membranes [START_REF] Ferreri | Thiyl radicals in biosystems: effects on lipid structures and metabolisms[END_REF]. Due to the presence of significant amounts of hydroperoxyacids in sinking particles (Table 1), an induction of Z/E isomerisation of palmitoleic acid by thiyl radicals resulting from the reaction of thiols with these compounds is therefore possible. However, the involvement of such an isomerisation process is not supported by the very low E/Z ratios of unsaturated fatty acids measured previously in highly photooxidized sinking particles (with very high hydroperoxide contents) collected in spring 2012 at a landfast ice station in Resolute Passage (Nunavut, Canada) [START_REF] Rontani | Monitoring abiotic degradation in sinking versus suspended Arctic sea ice algae during a spring ice melt using specific lipid oxidation tracers[END_REF].

Bacterial enzymatic reactions with unsaturated fatty acids constitutes another possibility to explain Z/E isomerisation ( HeipieperRetention time (min) /z 197.1357, 211.1513, 225.1670, 327.1807, 341.1963 and 355.2120) showing the presence of isomeric allylic dihydroxyhexadecenoic acids in superficial bottom sediments (0-1 cm). et al., 2003, 2007) (Fig. 5). Such an adaptive mechanism appears to be an alternative way to regulate membrane fluidity when the growth of bacteria is inhibited by toxic compounds or environmental stress (such as high salinity; [START_REF] Heipieper | The cis-trans isomerase of unsaturated fatty acids in Pseudomonas and Vibrio: biochemistry, molecular biology and physiological function of a unique stress adaptive mechanism[END_REF]. The strong isomerization of vaccenic acid observed previously in the sinking particles investigated herein was thus attributed to salinityinduced bacterial stress in brine channels during the early stages of ice melting [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF]. However, it is generally con-sidered that bacterial Z/E isomerase is the only enzymatic system known to alter the double bond stereochemistry of phospholipids, whereas such enzymatic isomerisation is unknown for eukaryotes [START_REF] Ferreri | Thiyl radicals in biosystems: effects on lipid structures and metabolisms[END_REF]. Consequently, enzymatic isomerisation of palmitoleic acid (arising mainly from algal material in sinking particles) cannot be occurring in ice algae. Moreover, due to the location of the Z/E bacterial isomerase in the periplasm and its hydrophilic properties [START_REF] Heipieper | The cis-trans isomerase of unsaturated fatty acids in Pseudomonas and Vibrio: biochemistry, molecular biology and physiological function of a unique stress adaptive mechanism[END_REF][START_REF] Heipieper | Solvent-tolerant bacteria for biotransformations in two-phase fermentation systems[END_REF], an activity of this enzyme on palmitoleic acid in diatoms to which the bacteria are attached, is very unlikely, even in the case of lysis of both organisms.

It may be noted that Z/E isomerisation of unsaturated fatty acids can also be catalyzed non-enzymatically by Fe 2+ ions [START_REF] Holtwick | Cis-trans isomerization of unsaturated fatty acids: Cloning and sequencing of the cti gene from Pseudomonas putida P8[END_REF][START_REF] Li | Iron(II)-induced isomerization of (all-E)-xanthophyll pigments lutein, zeaxanthin and b-cryptoxanthin in acetone[END_REF]. The potential release of Fe 2+ ions during radical-induced damage of the active site of the bacterial 10S-DOX-like lipoxygenase (Fuch and Spiteller, 2014) and Z/E isomerase (a cytochrome C-type protein containing an ion Fe 2+ , [START_REF] Holtwick | Cis-trans isomerization of unsaturated fatty acids: Cloning and sequencing of the cti gene from Pseudomonas putida P8[END_REF] could thus be at the origin of the Z/E isomerisation of palmitoleic acid in diatoms (Fig. 5).

Degradation of palmitoleic acid in superficial bottom sediments from north of Davis Strait

The dominance of 10-hydroxyhexadec-8(E)-enoic acid (9) (arising from 10S-DOX-like lipoxygenase activity) among palmitoleic acid oxidation products observed in the sediments investigated (Fig. 1c) was also previously detected in several surficial sediments collected in different zones of Arctic [START_REF] Amiraux | Ice biota degradation in the Arctic environment: impact of bacterial stress state on this material's preservation and burial[END_REF]. It may be attributed to a strong contribution of ice-derived OM released to Arctic sediments during the first stage of ice melting and containing bacteria stressed by salinity. It is well known that the generation of radicals in the course of the lipoxygenase catalytic cycle can act as a catalyst in autoxidation processes (Fuch and Spiteller, 2014). Moreover, when lipoxygenase activity becomes very high, increasing amounts of free radicals may damage the active site of these enzymes 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]Fuch and Spiteller, 2014), which may very efficiently catalyze the reduction of hydroperoxides to alkoxyl radicals [START_REF] Schaich | Lipid oxidation: Theoretical aspects[END_REF] and thereby induce free radical oxidation chains. The increase of autoxidation observed in surface sediments (Table 3) is thus likely initiated by the 10S-DOX-like lipoxygenase activity present in sinking ice algae [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF].

In contrast, these sediments exhibit a higher proportion of photooxidation products of palmitoleic acid than the particles settling during the early stages of ice melting (Table 3), suggesting the contribution of an additional source of highly photooxidized sinking material. Photooxidative damage is strongly dependent on the physiological state of phototrophic cells [START_REF] Nelson | Rates and possible mechanism of light-dependent degradation of pigments in detritus derived from phytoplankton[END_REF][START_REF] Merzlyak | Free radical metabolism, pigment degradation and lipid peroxidation in leaves during senescence[END_REF]. At the end of the ice melt process, downward percolation of melt water strongly reduces the overall salinity in brine channels, affecting the physiological state of ice algae and increasing their susceptibility towards photooxidation [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF]. A weak contribution of highly aggregated ice algae settling during the end of ice melting, where palmitoleic photooxidation percentage was very high (mean value at 30 m: 60%; [START_REF] Rontani | Monitoring abiotic degradation in sinking versus suspended Arctic sea ice algae during a spring ice melt using specific lipid oxidation tracers[END_REF] could thus explain the relatively high photooxidation state of palmitoleic acid observed in sediments. It was previously observed that strands of the under-ice diatom Melosira arctica, which can cover up to 40-80% of the underside of undisturbed ice floes [START_REF] Syvertsen | Ice algae in the Barents Sea: types of assemblages, origin, fate and role in the ice edge phytoplankton bloom[END_REF][START_REF] Gutt | The occurrence of sub-ice algae aggregations off northeast Greenland[END_REF] and contain a high proportion of palmitoleic acid [START_REF] Falk-Petersen | Lipids and fatty acids in ice algae and phytoplankton from the Marginal Ice Zone in the Barents Sea[END_REF], are not used as food in the pelagic zone and sink rapidly to the seafloor [START_REF] Boetius | ARK27-3-Shipboard Science PartyExport of algal biomass from the melting Arctic sea ice[END_REF]. A contribution of strongly photooxidized M. arctica could thus also explain the high photooxidation state of sedimentary palmitoleic acid. However, the high chlorophyll content measured in these algal deposits [START_REF] Boetius | ARK27-3-Shipboard Science PartyExport of algal biomass from the melting Arctic sea ice[END_REF] (attesting to their very weak photooxidation state) allows us to discard a significant contribution of this algal material to the superficial bottom sediments analyzed. Despite its lower aggregation and thus sedimentation rate, a weak contribution of open water phytoplankton, which is strongly affected by photooxidation processes in summer in the Arctic [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], cannot be totally excluded.

The relatively similar palmitelaidic acid/palmitoleic acid ratios observed in superficial bottom sediments and in the deeper sediment trap dominated by sympagic algae (Table 1) confirms the strong contribution of sea ice-derived OM to bottom sediments, a conclusion consistent with that derived following a previous analysis of vaccenic acid isomerisation in these samples [START_REF] Amiraux | Monitoring of photooxidative and osmotic bacterial stresses in Arctic using specific lipid tracers[END_REF].

Conclusions

In the present work, we detected an intense 10S-DOX-like lipoxygenase activity in sea ice, attributed to the involvement of a specific bacterial enzymatic process under the hypersaline conditions found in brine channels during the first stages of ice melting. This enzymatic activity, well-known to produce radicals, is probably at the origin of the strong autoxidation of ice algal material observed in the underlying sediments.

In the water column, the degradation of ice algal material appeared to be very limited, probably due to the presence of bacteria stressed by salinity in the overlying sea ice or by the high free fatty acid content of ice algae. The degradation of hydroperoxides (arising from 10S-DOX-like lipoxygenase oxidation, photo-and autoxidation in ice) involves peroxygenases, diol synthases, hydroperoxide isomerases and abiotic processes (homolytic and heterolytic cleavages, allylic rearrangement). The increasing hydratase activity (a well-known detoxification process in bacteria) observed in sinking particles, which attests to the toxicity of free unsaturated FA of ice algae toward their attached bacteria, constitutes another explanation of the relative preservation of ice algae within the water column. Z/E isomerization of palmitoleic acid was observed in the deeper trap and in surface sediments and was attributed to Fe 2+ ions released during the degradation of active sites of bacterial lipoxygenases and Z/E isomerases. These ions could then enhance autoxidation of ice algal material in the underlying sediments.

Comparison of the profiles of palmitoleic acid degradation products in superficial bottom sediments and in sinking particles allowed us to propose ice algae settling during the early stages of ice melting as important contributors to Arctic sediments. A weaker contribution of ice algal material, strongly photooxidized at the end of sea ice melting, also seems likely. Due to their strong aggregation and low remineralizing potential, ice algae seem to contribute more significantly than open water phytoplankton to the export of carbon to Arctic sediments.

Fig. 1 .

 1 Fig. 1. Partial ion chromatograms(m/z 199.1518, 213.1675, 225.1670, 317.1968, 329.1968, 331.2127 and 343.2125) showing the presence of palmitoleic acid oxidation products in sea ice (0-1 cm) (a), sediment trap (25 m) (b) and superficial bottom sediments (0-1 cm) (c).

Fig. 2 .

 2 Fig. 2. Degradation of 9,10-epoxyhexadecanoic acid during the treatment.

Fig. 3 .

 3 Fig. 3. Partial ion chromatograms (m/z 197.1357, 211.1513, 225.1670, 327.1807, 341.1963 and 355.2120) showing the presence of isomeric allylic dihydroxyhexadecenoic acids in superficial bottom sediments (0-1 cm).

Fig. 4 .Fig. 5 .

 45 Fig. 4. TOF MS mass spectra of: 7,10-dihydroxyhexadec-8(E)-enoic (a), 9,12-dihydroxyhexadec-10(E)-enoic (b) and 8,11-dihydroxyhexadec-9(E)-enoic (c) acid trimethylsilyl ether derivatives.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Proposed degradation pathways of isomeric hydroperoxyhexadecenoic acids.

Table 1

 1 Concentration of palmitoleic acid and its degradation products in sea ice, sinking particle and surficial bottom sediment samples collected in Davis Strait.

	Compounds	Sea ice	Sinking particles (trap 2 m)	Sinking particles (trap 25 m)	Surface sediment (0-1 cm)
		(0-1 cm)	(mg m À1 d À1 )	(mg m À1 d À1 )	(mg g À1 )
		(ng ml À1 )
	Hexadec-9(Z)-enoic acid (Palmitoleic acid)	10,105	345	437	7.5
	(8-11)-Hydroperoxyhexadec-(8-10)(Z and E)-enoic acids (1-6) 104.5	3.3	3.2	1.1
	(8-11)-Hydroxyhexadec-(8-10)(Z and E)-enoic acids (7-12)	137.8	4.4	4.3	1.0
	(8-11)-Oxohexadec-(8-10)(Z and E)-enoic acids (36-41)	130.3	4.1	4.0	1.8
	10-Hydroxyhexadecanoic acid (14)	19.4	2.2	2.8	0.6
	10-Oxohexadecanoic acid (35)	2.1	0.2	0.3	0.1
	9,10(Z and E)-Epoxyhexadecanoic acid 1 (20)	109.2	3.4	3.6	2.7
	7,10-Dihydroxyhexadec-8(E)-enoic acid (threo) (21)	1.8	0.5	0.3	0.2
	7,10-Dihydroxyhexadec-8(E)-enoic acid (erythro) (21)	1.5	0.3	0.3	0.1
	8,11-Dihydroxyhexadec-9(Z)-enoic acid (threo or erythro) (23) 2.5	0.8	0.3	0.2
	8,11-Dihydroxyhexadec-9(E)-enoic acid (threo) (22)	1.9	0.3	0.2	0.1
	8,11-Dihydroxyhexadec-9(E)-enoic acid (erythro) (22)	0.8	0.3	0.2	0.1
	9,12-Dihydroxyhexadec-10(E)-enoic acid (threo or erythro) (24)	1.9	0.1	0.1	0.3
	Ratio hexadec-9(E)-enoic acid/hexadec-9(Z)-enoic acid	0.03	0.04	0.22	0.27

Table 3

 3 Relative importance (%) of degradation processes of palmitoleic acid in sea ice, sinking particle and surficial bottom sediment samples collected in Davis Strait.

	Degradation processes	Sea ice	Sinking particles	Sinking particles	Surface sediment
		(0-1 cm)	(trap 2 m)	(trap 25 m)	(0-1 cm)
	Autoxidation	12.3	5.7	9.3	19.2
	Photooxidation	9.3	4.2	1.4	19.9
	Enzymatic 10S-DOX oxidation	47.6	52.1	48.0	13.2
	Enzymatic hydration	4.9	15.5	20.0	8.6
	Epoxidation (peroxygenase and autoxidation)	25.9	22.5	21.3	39.1

Sum of epoxide degradation products (alcohols + diols + chlorohydrins + methoxyhydrins) (Fig.

2).J

Acknowledgements

This work was supported by the Bacstress (funded by EC2COmicrobien) and GreenEdge (funded by ANR (Contract #111112), CNES (project #131425), IPEV (project #1164), CSA, Fondation Total, ArcticNet, LEFE and the French Arctic Initiative) projects. The GreenEdge project would not have been possible without the support of the Hamlet of Qikiqtarjuaq and the members of the community as well as the Inuksuit School and its principal Jacqueline Arsenault. The project was conducted under the scientific coordination of the Canada Excellence Research Chair on Remote sensing of Canada's new Arctic frontier and the CNRS & Université Laval Takuvik Joint International laboratory (UMI3376). We especially thank C. Nozais from Université du Québec à Rimouski for providing the sediment traps, C. Aubry and T. Dezutter from Université Laval Joint International laboratory Takuvik for allowing the 2015 short term sediment trap deployment and G. Massé for the seafloor surface sediment sampling. Thanks are due to Dr. H. J. Heipieper for helpful discussions. We also acknowledge the FEDER OCEANOMED (N°1166-39417) for the funding of the apparatus employed. Finally, we thank the two anonymous reviewers for their useful and constructive comments.

Appendix A