Virginie Riou

Julien Para

Marc Garel

Badr Catherine Guigue

Al Ali

Chiara Santinelli

Dominique Lefèvre

Jean-Pierre Gattuso

Madeleine Goutx

Stephanie Jacquet

Biodegradation of Emiliania huxleyi aggregates by a natural Mediterranean prokaryotic community under increasing hydrostatic pressure

Keywords: biological carbon pump, mesopelagic, mineral ballast, coccolithophorid, prokaryotes, biodegradation, hydrostatic pressure 2 3

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

Biodegradation of Emiliania huxleyi Aggregates by a Natural Mediterranean Prokaryotic Community under Increasing

Hydrostatic Pressure

Virginie Riou 1,2 , Julien Para 1 , Marc Garel 1 , Catherine Guigue 1 , Badr Al Ali 1,3 , Chiara Santinelli 4 , Dominique Lefèvre 1 , Jean-Pierre Gattuso 5,6 , Madeleine Goutx 1 , Stephanie Jacquet

Introduction

The ocean's biological carbon pump transports carbon into the deep ocean in the form of organic matter [START_REF] Eppley | Particulate organic matter flux and planktonic new production in the deep ocean[END_REF]. Between 1-40% of the organic carbon newly formed from CO 2 fixation at the sea surface is transported to the deep sea [START_REF] Ducklow | Upper ocean carbon export and the biological pump[END_REF], away from direct contact with the atmosphere. This process absorbs a large portion of the anthropogenic CO 2 emissions in the atmosphere, limiting greenhouse CO 2 gas build-up (Le [START_REF] Quéré | Impact of climate change and variability on the global oceanic sink of CO 2, Global Biogeochem[END_REF]. However, many climate-sensitive factors can influence organic carbon export and remineralization by mesopelagic heterotrophs, such as the mineral content of sinking particles [START_REF] Armstrong | A new, mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of POC with ballast minerals[END_REF][START_REF] Kwon | The impact of remineralization depth on the air-sea carbon balance[END_REF].

Globally, approximately 5-20 Pg C yr -1 leaves the euphotic zone (20-120m depth) in the form of sinking particulate organic carbon (POC) [START_REF] Henson | A reduced estimate of the strength of the ocean's biological carbon pump[END_REF][START_REF] Siegel | Global assessment of ocean carbon export by combining satellite observations and food-web models[END_REF]. However, most of this POC is potentially remineralized to CO 2 in the upper mesopelagic zone [START_REF] Buesseler | Shedding light on processes that control particle export and flux attenuation in the twilight zone of the open ocean[END_REF][START_REF] Giering | Reconciliation of the carbon budget in 25 49 the ocean's twilight zone[END_REF], where the activity of particle-attached prokaryotes and zooplankton play an important role in POC attenuation [START_REF] Cho | Major role of bacteria in biogeochemical fluxes in the ocean's interior[END_REF][START_REF] Giering | Reconciliation of the carbon budget in 25 49 the ocean's twilight zone[END_REF]Turley andMackie, 1994, 1995). On one hand, sinking POC collected in situ can be rapidly remineralized by attached prokaryotes, as demonstrated by incubations done at atmospheric pressure [START_REF] Goutx | Composition and degradation of marine particles with different settling velocities in the northwestern Mediterranean Sea[END_REF][START_REF] Panagiotopoulos | Bacterial degradation of large particles in the southern Indian Ocean using in vitro incubation experiments[END_REF][START_REF] Sempéré | Microbial decomposition of large organic particles in the northwestern Mediterranean Sea: an experimental approach[END_REF]. On the other hand, the remineralisation of sinking particles (diatom aggregates or faecal pellets) and dissolution of biogenic minerals (opal) by fresh natural surface prokaryotic assemblages were observed to slow down under increasing pressure [START_REF] Tamburini | Pressure effects on surface Mediterranean prokaryotes and biogenic silica dissolution during a diatom sinking experiment[END_REF][START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF]. Correlation between POC and mineral fluxes (calcium carbonate, opal, clays) in the deep ocean has lead to the interpretation that "ballast" minerals exert an important control over POC export [START_REF] Armstrong | A new, mechanistic model for organic carbon fluxes in the ocean based on the quantitative association of POC with ballast minerals[END_REF][START_REF] Francois | Factors controlling the flux of organic carbon to the bathypelagic zone of the ocean[END_REF][START_REF] Klaas | Association of sinking organic matter with various types of mineral ballast in the deep sea: Implications for the rain ratio[END_REF].

Oceanic provinces in which calcifying plankton dominates display lower mesopelagic remineralisation fluxes than systems dominated by opal-producing diatoms [START_REF] Cardinal | Geochemistry of suspended particles in the 23 45 Subantarctic and Polar Frontal zones south of Australia: Constraints on export and advection processes[END_REF][START_REF] Cardinal | Variations of carbon remineralisation in the Southern Ocean illustrated by the Baxs proxy[END_REF][START_REF] Jacquet | Barium distribution across the Southern Ocean frontal system in the Crozet-Kerguelen Basin[END_REF]. In sinking particles collected in situ, the presence of minerals has been suggested to preserve 3 5 organic matter from degradation [START_REF] Hedges | Evidence for nonselective preservation of organic matter in sinking marine particles[END_REF][START_REF] Ingalls | The role of biominerals in the sinking flux and preservation of amino acids in the Southern Ocean along 170°W[END_REF]. These observations suggest that mineral and POC dissolutions in the water column may be inter-related. To understand these interactions, a few biodegradation studies have been conducted using aggregates obtained from cultures of opal-protected diatoms, which form important seasonal blooms [START_REF] Treguer | Global change. Silica control of carbon dioxide[END_REF]. Biologically-mediated biogenic opal dissolution has been observed to increase with organic matter degradation in surface waters (Bidle andAzam, 1999, 2001;[START_REF] Goutx | Composition and degradation of marine particles with different settling velocities in the northwestern Mediterranean Sea[END_REF] and to diminish with temperature decrease [START_REF] Bidle | Regulation of oceanic silicon and carbon preservation by temperature control on bacteria[END_REF] or hydrostatic pressure increase [START_REF] Tamburini | Pressure effects on surface Mediterranean prokaryotes and biogenic silica dissolution during a diatom sinking experiment[END_REF]. Adding another ballast mineral (CaCO 3 ) to diatom aggregates diminished diatom aggregate remineralisation by zooplankton [START_REF] Le Moigne | Enhanced rates of particulate organic matter remineralization by microzooplankton are diminished by added ballast minerals[END_REF]. Regarding CaCO 3 , biologically-mediated dissolution has also been suggested [START_REF] Antia | Dissolution of coccolithophorid calcite by microzooplankton and copepod grazing[END_REF][START_REF] Harris | Zooplankton grazing on the coccolithophore Emiliania huxleyi and its role in inorganic carbon flux[END_REF][START_REF] Jansen | Carbonate dissolution in copepod guts: A numerical model[END_REF][START_REF] Milliman | Biologically mediated dissolution of calcium carbonate above the chemical lysocline?[END_REF] to explain the fact that significant CaCO 3 dissolution is occurring in the upper 1000 m of the water column above the calcite or aragonite saturation horizon (Wollast andChou, 1998, 2001). However, there is still no or contradictory evidence that this dissolution could be microbiologically-mediated [START_REF] Bissett | Dissolution of calcite in the twilight zone: bacterial control of dissolution of sinking planktonic carbonates is unlikely[END_REF], and the link between CaCO 3 dissolution and POC degradation remains unclear.

Ocean acidification experiments using diatom aggregates supplemented with CaCO 3 ballast indicate that CaCO 3 dissolution and POC degradation will potentially increase with increasing pCO 2 , a phenomenon that is exacerbated under increasing pressure (de Jesus Mendes and Thomsen, 2012). However, little is known about the biodegradation of calcifiers aggregates. Among the calcifiers, coccolithophorids contribute to air-sea CO 2 exchange, through their combined photosynthesis (sink of CO 2 ) and calcification (source of CO 2 ) activities [START_REF] Poulton | Relating coccolithophore calcification rates to phytoplankton community dynamics: Regional differences and implications for carbon export, 29 57 Deep[END_REF]. Coccolithophorid aggregates formation has been reported from many recent lab studies [START_REF] Biermann | Effect of CO 2 on the properties and sinking velocity of aggregates of the coccolithophore <I>Emiliania huxleyi<[END_REF][START_REF] Chow | The influence of extracellular polysaccharides, growth rate, and free coccoliths on the coagulation efficiency of Emiliania huxleyi[END_REF]Engel et al., 2009aEngel et al., , 2009b;;[START_REF] Iversen | Ballast minerals and the sinking carbon flux in the ocean: carbon-specific respiration rates and sinking velocity of marine snow aggregates[END_REF], but they are considered as being more sensitive to fragmentation than diatom aggregates [START_REF] De La Rocha | Factors influencing the sinking of POC and the efficiency of the biological carbon pump[END_REF].

Laboratory biodegradation experiments using aggregates from the coccolithophore Emiliania huxleyi showed that heavily calcified strains have higher settling velocities and are better preserved over time 4 7 than poorly calcified strains (Engel et al., 2009a(Engel et al., , 2009b)). Oxygen consumption within E. huxleyi aggregates can be 4 to 7 times larger than that of marine snow and diatom aggregates, indicating that they may be more prone to degradation [START_REF] Ploug | Ballast, sinking velocity, and apparent diffusivity within marine snow and zooplankton fecal pellets: Implications for substrate turnover by attached bacteria[END_REF]. Nevertheless, the carbon-specific respiration rate per meter settled was found to be lower in E. huxleyi aggregates than in diatom aggregates of similar size [START_REF] Iversen | Ballast minerals and the sinking carbon flux in the ocean: carbon-specific respiration rates and sinking velocity of marine snow aggregates[END_REF]. This potentially confers coccolithophorid aggregates a larger potential for transporting organic carbon deeper in the ocean relative to diatom aggregates. It is important to note that the biodegradation experiments cited above were conducted in the presence of culture-associated prokaryotes or natural prokaryotic assemblages stored for a long time before use and the effect of fresh natural prokaryotic assemblages remains to be assessed.

In addition, the increase in pressure experienced by aggregates sinking through the water column has yet to be included in such experiments.

The objective of the present study was therefore to investigate the effect of hydrostatic pressure on the lipid composition (an indicator of cell degradation) and CaCO 3 dissolution within E. huxleyi aggregates in the presence of fresh free-living natural prokaryotic assemblages collected at 200 m depth in the NW Mediterranean Sea. Incubations were performed under controlled pressure conditions simulating a sinking rate of 150 m d -1 , in which we monitored the structure of the prokaryotic communities as well as the degradation of E. huxleyi lipids over 10 days following the experimental design presented in [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF]. Total alkalinity, particulate inorganic and organic carbon (PIC and POC) and total organic carbon (TOC) concentrations were also monitored as indicators of CaCO 3 dissolution and organic carbon remineralization.

Results

Two sets of incubations (Exp A and Exp B) were performed at 13 °C, with two pressure conditions applied in parallel: constant atmospheric pressure (ATM) and increasing hydrostatic pressure corresponding to a sinking simulation from 200 to 1700 m depth (HP). The E. huxleyi aggregate dilutions in either sterile-filtered (Exp A) or fresh GF/F-filtered (Exp B) seawater were sampled at t0 and after 10 days in the incubation vessels (t10). Exp A was performed with sterile-filtered seawater to monitor the development of prokaryotes associated with the E. huxleyi culture, while Exp B was designed to observe the development on E. huxleyi aggregates of a natural freshly collected seawater free-living prokaryotic community, and its effect on the organic and inorganic carbon contents of the aggregates.

Prokaryotic development

The prokaryotic abundance associated with E. huxleyi culture aggregates (Exp A) did not change significantly between the start (t0: 7.7 ± 1.8 10 3 cells mL -1 , average ± SD) and end of the incubation (ATMt10: 6.5 ± 0.2 10 3 cells mL -1 ; HPt10: 9.4 ± 3.92 10 3 cells mL -1 ). Normalizing cell concentrations by the total organic carbon (TOC) concentration measured in each replicate to account for potential variations in the initial amount of aggregates added to each bottle, no significant difference was observed between both pressure conditions (Fig. 1A).

In Exp B, the total prokaryotic abundance increased by two orders of magnitude after 10 days (ATMt10: 2.2 ± 0.4 10 6 cells mL -1 , HPt10: 2.7 ± 0.0 10 6 cells mL -1 ), with no significant difference between pressure treatments, when normalised by TOC (Fig. 1A). The growth of prokaryotes attached to E. huxleyi aggregates was stimulated under increasing pressure incubation (HP), where large aggregates were observed at the end (Suppl. Fig. 1). In this condition, attached prokaryotes (> 2 µm) were significantly more abundant (35.9 ± 14.7 % of total DAPI positive cells) than under constant atmospheric pressure (20.9 ± 4.7 %, Mann-Whitney-MW p < 0.05).

Under both atmospheric and increasing pressure conditions, abundances of Archaea and Bacteria were two and three orders of magnitude higher than at t0, respectively. Although the proportion of Euryarchaea was 4.1 times lower under pressure (Fig. 1B, significant, MW p < 0.05), increasing pressure had no effect on the proportion and abundance of Crenarchaea (ATMt10: 2.3 ± 0.2 10 4 cell mL -1 , HPt10: 2.6 ± 0.4 10 4 cell mL -1 , MW p = 0.51) and Bacteria (ATMt10: 1.3 ± 0.2 10 6 cell mL -1 , HPt10:

1.4 ± 0.2 10 6 cell mL -1 , MW p = 0.83). Among the Bacteria, Alpha-and Gamma-proteobacteria cell 6 11 concentrations increased by three orders of magnitude during the incubations, and the fraction of Alphaproteobacteria was twice as low in conditions of increasing HP than at ATM (not significant, MW p = 0.13). The proportion of Bacteroidetes also tended to be on average 1.9 times lower in HP than in ATM conditions (not significant, MW p = 0.83), but their concentrations increased only by one to two orders of magnitude.

E. huxleyi detritus organic carbon content and lipid degradation

During the Exp B, TOC and particulate organic carbon (POC) concentrations tended to decrease over the 10 days incubation under atmospheric pressure only (Fig. 2A,ATMt10, not significant, p = 0.13).

The POC pigment content decreased significantly after 10 days of exposure to natural prokaryotic 7 13 assemblages, with no effect of pressure (Fig. 2B). In contrast, the lipid content (excluding pigments) decreased significantly in HPt10 but not in ATMt10 (Fig. 2C). At the start of Exp B, the lipid composition in E. huxleyi aggregates was similar to the one described in an earlier study with the same strain of E. huxleyi in exponential growth [START_REF] Bell | Lipid composition during growth of motile and coccolith forms of Emiliania huxleyi[END_REF]; see suppl. Table 1). Overall, chloroplast lipids (including pigments and membrane monogalactosyldiacylglycerol-MGDG) and cell membrane lipids (phospholipids) dominated the lipid composition (61.9-63.1 %). Sterols, hydrocarbons, neutral lipids (wax esters, triglycerides, methyl esters, ketones, free fatty acids and alcohols) and metabolites (degradation lipids: mono-and di-acylglycerols) accounted for 35.1-37.2 % of the particle lipids.

Within the non-pigment lipids, a significant decrease in the proportion of chloroplast membrane and whole cell membrane lipids (MGDG and PL, t0 9.8-18.2 % and 25.4-33.5 %, respectively) was observed in both ATMt10 (2.5-6.8 % and 4.8-6.8 %) and HPt10 samples (0.0 % and 4.7-8.5 %).

Meanwhile, the proportion of hydrocarbons, methyl esters and ketones increased significantly in both 8 15 conditions (Fig. 3). HPt10 samples experienced significant losses of membrane sterols and MGDG relative to ATMt10 samples. At the same time, the relative proportion of reserve wax esters and triglycerides were significantly higher in HP (HP: 5.9-12.5 %; ATM: 0.0-5.5 %), mainly due to the appearance of triacylglycerides.

Inorganic carbon content variations

A significant drop of particulate inorganic carbon (PIC, Fig. 4A) was observed in both ATM and HP conditions after 10 d (p < 0.05). It was larger under HP than at ATM (not significant, p = 0.13). Total alkalinity (A T ) increased significantly between t0 and ATMt10 (Fig. 4B, p < 0.05) but the increase was significantly lower under HP. The increase in A T observed between t0 and HPt10 was indeed not statistically significant (p = 0.27). 9 17

Discussion

Effect of hydrostatic pressure increase on the prokaryotic community

During the present E. huxleyi aggregate biodegradation experiment, initial natural mesopelagic Mediterranean Sea prokaryotic concentrations increased by up to two orders after 10 days in controlled temperature and hydrostatic pressure (HP) conditions simulating particle sinking from 200m to 1700m depth at a rate of 150 m d -1 . Such a high prokaryotic development has recently been observed with deep Mediterranean seawater natural prokaryotic communities amended with labile marine broth, both under atmospheric (ATM) and HP conditions [START_REF] Wannicke | Measuring bacterial activity and community composition at high hydrostatic pressure using a novel experimental approach: A pilot study[END_REF]. This might therefore indicate that E. huxleyi aggregates constitute a relatively labile substrate for mesopelagic Mediterranean prokaryotic communities to grow on.

The natural prokaryotic diversity was dominated by Bacteria, with a lower proportion of Archaea at the end of the incubation, as previously observed in similar experiments (Tamburini et al., 2006, 10 19 2009), despite a significant increase in absolute abundance. The initial fraction of Bacteria, Crenarchaea and Euryarchaea were lower in the present experiment (30%, 7% and 4%, respectively) than in previous experiments with 200m depth seawater sampled in different areas of the Mediterranean Sea [START_REF] Tamburini | Pressure effects on surface Mediterranean prokaryotes and biogenic silica dissolution during a diatom sinking experiment[END_REF][START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF]. Different sea surface mixing conditions at the time of the natural prokaryotic community sampling for the different experiments might have resulted in different initial community compositions (Pinazo, pers. Com). When focusing on the classes of Bacteria generally found associated to marine aggregates [START_REF] Acinas | Diversity of free-living and attached bacteria in offshore Western Mediterranean waters as depicted by analysis of genes encoding 16S rRNA[END_REF][START_REF] Delong | Phylogenetic diversity of aggregate-attached vs. freeliving marine bacterial assemblages[END_REF][START_REF] Moeseneder | Horizontal and vertical complexity of attached and free-living bacteria of the eastern Mediterranean Sea, determined by 16S rDNA and 16S rRNA 28 55 fingerprints[END_REF][START_REF] Ploug | Bacterial production and respiration in suspended aggregates -a matter of the incubation method[END_REF][START_REF] Simon | Microbial ecology of organic aggregates in aquatic ecosystems[END_REF][START_REF] Thiele | Colonization in the Photic Zone and Subsequent Changes during Sinking Determine Bacterial Community Composition in Marine Snow[END_REF] , only Bacteroidetes andAlphaproteobacteria tended to be sensitive to increasing hydrostatic pressure conditions (HP). Similar trends were observed during previous particle sinking simulation experiments with natural faecal pellets [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF]. The latter authors proposed that the sensitivity of Bacteroidetes to pressure may have reduced the amount of particulate carbohydrate hydrolysed. This could result in a lower production of dissolved carbohydrates as substrate for Alphaproteobacteria [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF]. A similar mechanism might be at play in the present experiment with E. huxleyi detritus, which is supported by the observation of large sticky particles and aggregates under HP condition only, as in [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF](Suppl. Fig. 1). These large sticky aggregates might indeed be the result of less degradation of transparent exopolymeric particles (TEP) under HP relative to ATM [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF], associated with the fact that more dissolved carbohydrates may have been released under HP by E. huxleyi, possibly resisting prokaryotic degradation [START_REF] Nanninga | Importance of light for the formation of algal blooms by Emiliania huxleyi[END_REF] and scavenged by TEP [START_REF] Engel | Polysaccharide aggregation as a potential sink of marine dissolved organic carbon[END_REF]. Interestingly, we observed that prokaryotes grew significantly more on > 2 µm aggregates under increasing pressure (HP) than at atmospheric pressure (ATM). This pattern was not observed in a previous similar controlled experiment studying the biodegradation of natural aggregates collected in the Mediterranean Sea (mainly faecal pellets) during 6.5 days, although the formation of visible sticky aggregates had also been observed under HP [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF]. The fact that the present simulation lasted longer and reached higher pressure levels could explain that the pattern observed 11 21 here may not have been detected in the previous study. Finally, it also is also possible that coccolithophorid cells are more sensitive to pressure than faecal pellets or diatom aggregates [START_REF] Tamburini | Pressure effects on surface Mediterranean prokaryotes and biogenic silica dissolution during a diatom sinking experiment[END_REF][START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF]. Hence, more labile intracellular organic matter would be released in the aggregates surroundings, favouring higher prokaryotic growth. To verify the latter hypothesis, we analysed the aggregate lipid contents and calculated a lipolysis index.

Degradation of E. huxleyi aggregates organic matter

Assuming a prokaryotic cellular carbon content of 12.4-53.0 fg C cell -1 [START_REF] Fukuda | Direct determination of carbon and nitrogen contents of natural bacterial assemblages in marine environments[END_REF][START_REF] Kogure | Particle counter determination of bacterial biomass in seawater[END_REF], prokaryotes made up for 0.05-0.21% of the TOC at t0, which increased to 1.84-11.44 % at ATMt10 and 2.32-11.94 % at HPt10. Heterotrophic growth could have been supported by the dissolved organic matter present at the start of the incubation, and/or by the degradation of E. huxleyi aggregates. A lipolysis index defined as the ratio of metabolites resulting from acyl-lipid degradation to the sum of acyl lipids can be used to characterize the degradation stage of labile organic matter [START_REF] Goutx | High-frequency fluxes of labile compounds in the central Ligurian Sea, northwestern Mediterranean, Deep Sea Res. Part I[END_REF][START_REF] Goutx | Triacylglycerol biodegradation experiment in marine environmental conditions: definition of a new lipolysis index[END_REF]. Although the lipid index in E. huxleyi aggregates increased significantly between the beginning (t0: 0.72±0.06) and the end of the experiment (ATMt10: 2.07±1.22, HPt10: 1.93±0.32), there was no significant pressure effect (MW p = 0.51). However, the lipid fraction of E. huxleyi detritus organic carbon decreased significantly after ten days under HP, contrasting with previous observations using natural aggregates, mainly consisting in faecal pellets [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF]. In addition, compared to the lipid composition at ATM, membrane lipids (phospholipids, sterols, MGDG) disappeared from the particulate fraction significantly more under HP than at ATM, which could indicate higher E. huxleyi cell lysis under pressure [START_REF] Baldi | Cell lysis and release of particulate polysaccharides in extensive marine mucilage assessed by lipid biomarkers and molecular probes[END_REF]. E. huxleyi aggregates might therefore be more vulnerable to degradation under pressure than faecal pellets. 12 23

Changes in mineral content

Particulate inorganic carbon (PIC) decreased more under HP than at ATM, suggesting that more calcite dissolution may have occurred under increased hydrostatic pressure (Fig. 4). Enhanced PIC dissolution under HP has been observed in a previous study with diatom-mineral (carbonatekaolinite-smectite) aggregates, together with a decreased POC decay rate (de Jesus Mendes and Thomsen, 2012). We here show that increased CaCO 3 dissolution can occur in calcite-sheathed cell aggregates sinking through mesopelagic waters in the presence of natural prokaryotic communities.

According to the stoichiometry of calcification, two moles of total alkalinity are produced per mole of CaCO 3 dissolved (approximated as moles of PIC in Fig. 4C). However, although there was more calcite dissolution under pressure, the increase in total alkalinity was larger at atmospheric pressure.

Nonetheless, values were within the range expected for calcite dissolution, considering the starting total alkalinity and PIC conditions (Fig. 4C), except for the two outliers circled in Fig. 4 (namely ATM2 and HP3 incubations, also circled in Fig. 2). Processes other than CaCO 3 dissolution may explain the outlying PIC-AT pairs measured in replicates ATM2 and HP3, which respectively also displayed the lowest prokaryotic abundance (1.77 10 6 cell mL -1 ) and POC concentration (30.7 µmol L -1 ).

Total alkalinity may indeed be affected by microbial aerobic and anaerobic organic matter remineralization/transformation through the consumption or release of ammonium, nitrate, phosphate and sulphate [START_REF] Gattuso | Measurement of community metabolism and significance in the coral reef CO2 source-sink debate[END_REF]. The processes of respiration, nitrification and sulphate reduction have adverse effects on protons release/uptake. However, the difference of +31 µmol L -1 AT between the value measured in ATM2 and the value expected from the triplicate t0 PIC concentrations cannot be solely explained by dissimilation of ammonium by organic matter respiration. This process would result in a AT increase of 0.17 moles per mole of TOC respired [START_REF] Gattuso | Measurement of community metabolism and significance in the coral reef CO2 source-sink debate[END_REF], when the difference in TOC between t0 replicates and ATM2 was only 5.4 to 21.6 µmol L - 1 (which was comparable to the other 2 ATM replicates). Since the t0 replicates consisted in independent splits of the initial aggregate-seawater mixture that was further split into ATM and HP replicate incubations, another explanation for this outlying pair could be that the ATM2 incubation 13 25 may have started with a higher PIC concentration around 65 µM (higher concentration of detached coccoliths).

Similarly, the -31 µmol L -1 difference in AT between the value measured in HP3 and the expected value cannot be explained solely by the dissimilation of nitrate by organic matter respiration. Such a process would lead to a decrease in AT of 0.13 moles per mol of CO 2 released [START_REF] Gattuso | Measurement of community metabolism and significance in the coral reef CO2 source-sink debate[END_REF], whereas the difference in TOC concentration between t0 replicates and HP3 comparable to that with ATM2 was observed (Fig. 2). Given the large AT variability observed between the independent splits sampled at t0, it is more likely that the HP3 incubation started with a much lower AT than the other HP replicates (around 2635 µmol L -1 , or 2561 µmol kg -1 at 50 µM PIC).

Implications for the carbon pump and the ballast hypothesis

In the Mediterranean Sea, CaCO 3 -sheathed coccolithophores can dominate the phytoplankton community [START_REF] Ignatiades | Diversification of phytoplankton community structure and related parameters along a large-scale longitudinal east-west transect of the Mediterranean Sea[END_REF]. They are highly diverse, including large numbers of the ubiquitous species E. huxleyi [START_REF] Cros | Atlas of Northwestern Mediterranean Coccolithophores[END_REF][START_REF] Knappertsbusch | Geographic-Distribution of Living and Holocene Coccolithophores in the Mediterranean-Sea[END_REF]. Several studies have stressed the need for controlled experiments to validate the hypothesis that CaCO 3 may be a more efficient POC "carrier" than biogenic silica [START_REF] De La Rocha | Factors influencing the sinking of POC and the efficiency of the biological carbon pump[END_REF][START_REF] Passow | Accumulation of mineral ballast on organic aggregates[END_REF][START_REF] Passow | Switching perspectives: Do mineral fluxes determine particulate organic carbon fluxes or vice versa?[END_REF]. Our experiment contributes to a better understanding of this issue. In contrast with previous experiments using natural marine snow/faecal pellets [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF][START_REF] Turley | The effect of pressure on leucine and thymidine incorporation by free-living bacteria and by bacteria attached to sinking oceanic particles[END_REF], organo-mineral aggregates (de Jesus [START_REF] De | Pressure effects on the biological degradation of organo-mineral aggregates in submarine canyons[END_REF] or diatom culture aggregates (de Jesus [START_REF] De | Effects of Ocean Acidification on the Ballast of Surface Aggregates Sinking through the Twilight Zone[END_REF][START_REF] Tamburini | Pressure effects on surface Mediterranean prokaryotes and biogenic silica dissolution during a diatom sinking experiment[END_REF], CaCO 3 -containing E. huxleyi culture aggregates appear more sensitive to the increase in hydrostatic pressure. Our results suggest that increased pressure results in higher cell lysis and CaCO 3 dissolution relative to ATM. The protecting effect of coccoliths may therefore not remain effective throughout the water column, since coccoliths might detach from cells and dissolve more readily under pressure. Calcium carbonate may therefore not be an efficient POC carrier to the deep ocean in the form of intact coccolithophorid cells.

27

Coccolithophores are the main producers of the pelagic CaCO 3 biogenically produced in the global ocean (0.8-1.4 Gt year -1 ) [START_REF] Feely | Impact of anthropogenic CO2 on the CaCO3 system in the oceans[END_REF] and may contribute to about 50% of pelagic CaCO 3 deposition in sediments [START_REF] Broecker | Ratio of coccolith CaCO3 to foraminifera CaCO3 in late Holocene deep sea sediments[END_REF][START_REF] Poulton | Relating coccolithophore calcification rates to phytoplankton community dynamics: Regional differences and implications for carbon export, 29 57 Deep[END_REF]. Yet, the processes and rates of coccolith CaCO 3 dissolution in the upper layers of the ocean are poorly identified and quantified.

Since calcification decreases the ocean CO 2 uptake capacity by reducing total alkalinity, the depth of calcite dissolution likely has important implications on the net gradient in pCO 2 between the ocean and atmosphere [START_REF] Antia | Basin-wide particulate carbon flux in the Atlantic Ocean: Regional export patterns and potential for atmospheric CO2 sequestration[END_REF][START_REF] Barrett | Calcium carbonate dissolution in the upper 1000 m of the eastern North Atlantic[END_REF].

The dissolution of pelagic CaCO 3 particles has long been assumed to occur primarily at great depths below the calcite and aragonite saturation depths [START_REF] Broecker | Recommendations of the working group on carbonate dissolution, in The fate of fossil fuel CO2 in the oceans[END_REF]. In fact, up to 80% of the CaCO 3 that is exported out of the surface ocean might actually dissolve in the upper 1000 m, well above the lysocline [START_REF] Chung | Calcium carbonate budget in the Atlantic Ocean based on water column inorganic carbon chemistry[END_REF][START_REF] Feely | Impact of anthropogenic CO2 on the CaCO3 system in the oceans[END_REF][START_REF] Iglesias-Rodriguez | Progress made in study of ocean's calcium carbonate budget[END_REF][START_REF] Jansen | Carbonate dissolution in copepod guts: A numerical model[END_REF][START_REF] Milliman | Biologically mediated dissolution of calcium carbonate above the chemical lysocline?[END_REF]Wollast andChou, 1998, 2001). Grazing by microzooplankton has been shown to play an important role in coccolithophorid calcite dissolution [START_REF] Antia | Dissolution of coccolithophorid calcite by microzooplankton and copepod grazing[END_REF][START_REF] Harris | Zooplankton grazing on the coccolithophore Emiliania huxleyi and its role in inorganic carbon flux[END_REF][START_REF] Jansen | Carbonate dissolution in copepod guts: A numerical model[END_REF][START_REF] Milliman | Biologically mediated dissolution of calcium carbonate above the chemical lysocline?[END_REF]. In contrast, carbonate dissolution on individual calcifying cells by microbes was deemed negligible [START_REF] Bissett | Dissolution of calcite in the twilight zone: bacterial control of dissolution of sinking planktonic carbonates is unlikely[END_REF], but the presence of chemical gradients within sinking aggregates [START_REF] Ploug | Oxygen diffusion limitation and ammonium production within sinking diatom aggregates under hypoxic and anoxic conditions[END_REF] may create micro-environments with lower pH and calcite saturation state that could facilitate (a)biotic dissolution. We show here that calcium carbonate dissolution in the presence of natural prokaryotic communities might be higher under increasing hydrostatic pressure. Pressure was recently suggested to affect dissolution kinetics over and above its influence on the saturation constants [START_REF] Dong | Kinetics of Inorganic Calcite Dissolution in Seawater under Pressure[END_REF]. The combined effect of pressure and microbial activities therefore remains to be assessed in a more mechanistic manner.

On the other hand, we show that increasing hydrostatic pressure potentially triggers the development of Mediterranean mesopelagic prokaryotic communities on E. huxleyi aggregates (as opposed to diatom detritus or natural aggregates, [START_REF] Tamburini | Pressure effects on surface Mediterranean prokaryotes and biogenic silica dissolution during a diatom sinking experiment[END_REF][START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF], as well as aggregation. The study by [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF] had already shown that particulate 15 29 carbohydrates:POC and Transparent Exopolymer Particles (TEP):POC ratios could be significantly higher in conditions of increasing hydrostatic pressure. The prokaryotes themselves may promote aggregation, through the production of sticky extracellular muco-polysaccharides [START_REF] Biddanda | Microbial aggregation and degradation of phytoplankton-derived detritus in seawater: 1. Microbial succession[END_REF][START_REF] Biddanda | Structure and function of marine microbial aggregates[END_REF][START_REF] Turley | Formation, vertical flux and remineralisation of aggregates in the ocean: a short review[END_REF]. Moreover, coccoliths have been proposed as aggregation nuclei for colloidal DOM, therefore offering organic matter protection from solubilization/remineralization (Engel et al., 2009a). The increased aggregation observed under pressure in our reaction vessels could therefore compensate the loss of mineral ballast at depth, by increasing aggregate sizes and/or mineral scavenging potential. To what extent pressure-induced aggregation balances calcite dissolution in situ remains to be observed and assessed, by including model calculations, for instance.

Methods

Production of Emiliania huxleyi aggregates and experimental design

The TW1 E. huxleyi strain (Algobank-Caen) was maintained in exponential growth phase in K/2 medium [START_REF] Keller | Media for the culture of oceanic ultraphytoplankton[END_REF] at 21°C, under a 12-12h light-dark cycle (intensity: 4300 Lux). The culture was supplemented with Provasoli's antibiotic concentrate (3% final concentration, Sigma) until the bacterial numbers reached a minimal proportion of one to two prokaryote per phytoplankton cell.

The antibiotic solution was then removed for two culture passagings before E. huxleyi cells in exponential phase were concentrated by centrifugation (3000 rpm, 15 min) [START_REF] Houdan | Cryopreservation of the coccolithophore, Emiliania huxleyi (Haptophyta, Prymnesiophyceae)[END_REF][START_REF] Sawada | Alkenone and alkenoic acid compositions of the membrane fractions of Emiliania huxleyi[END_REF]. Fresh detritus was obtained according to a procedure modified from [START_REF] Tamburini | Pressure effects on surface Mediterranean prokaryotes and biogenic silica dissolution during a diatom sinking experiment[END_REF] by exposing cell culture to 55 °C for 10 min only, which avoided cell disruption observed during freeze-thaw cycles. The detritus aliquots were immediately diluted in sterile-filtered or GF/F-filtered Mediterranean Seawater (for Exp A and Exp B, respectively, see details below) to reach a final concentration of about 600 µg POC L -1 (cell density of 5000 cell mL -1 ) close to natural coccolithophore bloom [START_REF] Holligan | A biogeochemical study of the coccolithophore, Emiliania huxleyi , in the North Atlantic[END_REF]. Aliquots were sampled at the starting point of the incubations (t0) for the various microbiological and chemical parameters described below. 16 31

Incubation in the PArticle Sinking Simulator (PASS)

The two experiments (Exp A and Exp B) were conducted in March 2007 using Mediterranean seawater (13°C) collected at 200 m depth, above the Marseilles Trench (43°01'48 N-05°11'158 E, 600m bottom depth). In Exp A, seawater was left in the dark at 4 °C for 15 d and gently filtered through 0.2 µm-porosity polycarbonate filters to subtract the prokaryotic community. In Exp B, fresh natural prokaryotic assemblages <0.7 µm were recovered in the filtrate of a gentle filtration onto precombusted GF/F filters of the freshly collected seawater sample.

Exp A and Exp B were incubated for 10 d at in situ temperature (13°C). The PASS system was used following a procedure described in [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF], except a peristaltic liquid dispenser (Jencons Scientific Ltd.) was used to obtain homogeneous replicate aliquots. Aliquots were incubated in 500 mL high-pressure bottles (HPBs) where pressure increased linearly using a piloted pressure generator in order to reproduce the increase in pressure that occurs when particles sink.

Coccolithophores detritus was kept in suspension by regular half-revolutions of the HPBs in water baths kept at in situ temperature [START_REF] Tamburini | Pressure effects on surface Mediterranean prokaryotes and biogenic silica dissolution during a diatom sinking experiment[END_REF]. For Exp A, two duplicate HPBs were kept at atmospheric pressure and two duplicate HPBs were exposed to increasing pressure (1.5 MPa d -1 corresponding to a sinking rate of 150 m d -1 ). This is applicable to mid-density particles, as discussed in [START_REF] Tamburini | Pressure effects on surface Mediterranean prokaryotes and biogenic silica dissolution during a diatom sinking experiment[END_REF] and corresponds to sinking rates observed in previous E. huxleyi aggregates studies (Engel et al., 2009b;[START_REF] Iversen | Ballast minerals and the sinking carbon flux in the ocean: carbon-specific respiration rates and sinking velocity of marine snow aggregates[END_REF][START_REF] Ploug | Ballast, sinking velocity, and apparent diffusivity within marine snow and zooplankton fecal pellets: Implications for substrate turnover by attached bacteria[END_REF]. Exp B was performed with triplicate HPBs. After 10 days of incubation (HPt10, corresponding to a depth of 1700 m), the pressurised coccolithophores detritus dilutions were decompressed and sampled as described in [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF] for the parameters described below.

Microbiological analyses

Samples for prokaryotic abundances (50-135 mL) were fixed using 0.2-µm filtered formaldehyde (2% final concentration), kept at room temperature for 15 min, and subsequently stored at 4°C in the dark for around 12h. For total (>0.2 µm) and free-living (0.2-2.0 µm) prokaryotic abundances, samples 17 33 Cren537 TGACCACTTGAGGTGCTG Crenarchaea 20 [START_REF] Teira | Combining Catalyzed Reporter Deposition-Fluorescence In Situ Hybridization and Microautoradiography To Detect Substrate 30 59 Utilization by Bacteria and Archaea in the Deep Ocean[END_REF]) Eury806 CACAGCGTTTACACCTAG Euryarchaea 20 [START_REF] Teira | Combining Catalyzed Reporter Deposition-Fluorescence In Situ Hybridization and Microautoradiography To Detect Substrate 30 59 Utilization by Bacteria and Archaea in the Deep Ocean[END_REF]) NegControl TAGTGACGCGCTCGA For non-specific probe binding 55 [START_REF] Karner | Determination of Active Marine Bacterioplankton: a Comparison of Universal 16S rRNA Probes, Autoradiography, and Nucleoid Staining[END_REF] a Including an unlabeled competitor probe BET42a (5'-GCCTTCCCACTTCGTTT-3'), see [START_REF] Manz | Phylogenetic Oligodeoxynucleotide Probes for the Major Subclasses of Proteobacteria: Problems and Solutions[END_REF] for details.

Chemical analyses

All glassware was pre-combusted before use. Additionally, glassware was rinsed with 1 N HCl and Milli-Q water after each sample. All plastic ware was rinsed with 1 N HCl and Milli-Q water and changed for each sample. Sample aliquots for particulate organic carbon (POC, 50-104 mL) and lipids (P-lips, 140-500mL) were filtered onto pre-combusted 0.7 µm GF/F filters (25 mm filter diameter) under a low vacuum (<50mm Hg) and preserved at -20°C or in liquid nitrogen, respectively.

Filters for particulate total carbon (PtotC) and POC were divided in two and analyzed as in Raimbault et al. (2008). The filter portions used for PtotC were analyzed directly, while filter portions for POC measurement were acidified with 100 µL H 2 SO 4 (0.5N) and subsequently analyzed using high combustion (1000°C) on a mass spectrometer (CN-Integra tracer-mass). The concentration of particulate inorganic carbon (PIC) was calculated from the difference between PtotC and POC.

Lipids were extracted from filters using a Bligh and Dyer (1959) protocol, and lipid classes were separated on chromarods and quantified on a thin layer chromatography/flame ionization detection (TLC/FID) Iatroscan apparatus model MK-6s (Iatron,Tokyo) coupled to a PC equipped with a Chromstar 6.1 integration system (Bionis, Paris), as in [START_REF] Goutx | Composition and degradation of marine particles with different settling velocities in the northwestern Mediterranean Sea[END_REF]. The separation of the 16 following lipid TOC concentrations were obtained by adding POC to DOC values. Samples for DOC were obtained and preserved as in [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF]. The DOC measurements were carried out in the laboratory at the Pisa section of the Institute of Biophysics (IBF), CNR, with a Shimadzu TOC-VCSN. Samples were acidified with 2N HCl and sparged for 3 minutes with CO 2 -free ultra-high purity air in order to remove inorganic carbon. From 3 to 5 replicate injections were performed until the analytical precision was lower than 1%. The system blank was measured every day at the beginning and the end of analyses

  1 , Frédéric A.C. Le Moigne 7 , Kazuyo Tachikawa 2 and Christian Tamburini 1,* (1) Aix Marseille Université, Université Toulon, CNRS, IRD, MIO UM 110, Mediterranean Institute of Oceanography, Marseille, France (2) CEREGE, UM34, Aix Marseille Université, CNRS, IRD, Aix-en-Provence Cedex, France (3) Ministry of Higher Education, Tishreen University, High Institute of Marine Research, LATTAKIA, Syria (4) C.N.R., Istituto di Biofisica, Pisa, Italy (5) Sorbonne Universités, UPMC Univ Paris 06, CNRS-INSU, Laboratoire d'Océanographie de Villefranche, 181 chemin du Lazaret, F-06230 Villefranche-sur-mer, France (6) Institute for Sustainable Development and International Relations, Sciences Po, 27 rue Saint Guillaume, F-75007 Paris, France (7) GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany

Figure 1 :

 1 Figure 1: (A) Total prokaryotic abundance (DAPI-stained cells) normalized by sample TOC concentration in Emiliania huxleyi aggregates biodegradation Exp A (no natural prokaryotes added) and Exp B (GF/F-filtered natural prokaryotic community added) initially (t0) and after 10 days under constant atmospheric pressure (ATMt10) or increasing hydrostatic pressure (HPt10). White bars: free-living prokaryotes; Black bars: >2µm particle-attached prokaryotes; (B) Percent of total prokaryotes (DAPI-stained cells) at the end of Exp B, detected by catalyzed reporter deposition coupled with fluorescence in situ hybridization (CARD-FISH). Eub mix (Eub338,Eub-II and Eub-III): Bacteria; CF319a: Bacteriodetes; Alf968, Gam42a: alpha-and gammasubclasses of Proteobacteria; Cren537: Crenarchaea; Eury806: Euryarchaea. Average±SD. Significant differences between ATM and HP are indicated as a (Mann-Whitney p < 0.05) for particle-attached (A) or total prokaryotes (B).

Figure 2 :

 2 Figure 2: Variation in organic carbon composition in Exp B: (A) total or particulate organic carbon (TOC-empty circles, or POC-filled circles, µmol L -1 ), (B) POC pigment fraction (%), (C) POC lipid fraction (pigments excluded, %). Significant differences (Mann-Whitney p < 0.05) are indicated as: a between t0 and t10. Circled data points belong to the same ATM and HP replicates circled in Fig. 4.

Figure 3 :

 3 Figure 3: Variation in the lipid composition of E. huxleyi detritus in Exp B: Proportions of the different lipids (pigments are excluded, % Ltot). MGDG: monogalactosyldiacylglycerol, PL: phospholipids, ST: sterols, HC: Hydrocarbons, WE+TG: wax esters and triglycerides, ME+KET: methyl esters and ketones, ; Average±SD. Significant differences (Mann-Whitney p < 0.05) are indicated as: a between t0 and t10, b between ATM and HP.

Figure 4 :

 4 Figure 4: Variation in (A) particulate inorganic carbon (PIC, µmol L -1 ), (B) total alkalinity (AT, meq kg -1 ), and (C) AT converted to µmol L -1 (using a seawater density of 1028.7 L kg -1 ) vs PIC in Exp B. Full and dashed lines on panel C model the expected co-evolution of AT and PIC during calcite dissolution. Significant differences (Mann-Whitney p < 0.05) are marked with an "a" between t0 and t10, "b" between ATM and HP. Circled data points point at the ATM2 and HP3 replicate incubations displaying values out of the expected range for calcite dissolution from the measured t0 values shown in panel C.

  classes was obtained: neutral lipids including hydrocarbons (HC), sterol esters co-eluting with wax esters (WE), ketones (KET), triacylglycerols (TG), free fatty acids (FFA), alcohols (ALC), sterols and diglycerides (DG); chloroplast lipids containing pigments (Pig), glycolipids and monogalactosyldiacylglycerol (MGDG); monoacylglycerols (MG); and phospholipids (PL) including diphosphatidylglycerides co-eluting with phosphatidylglycerides (DPG+PG), phosphatidylethanolamine (PE) and phosphatidylcholine (PC). A lipolysis index was calculated as in[START_REF] Goutx | High-frequency fluxes of labile compounds in the central Ligurian Sea, northwestern Mediterranean, Deep Sea Res. Part I[END_REF]Goutx et al. ( , 19 37 2003)), defined as the ratio of the sum of the metabolites (DG, MG, FFA, ALC) to the sum of the intact acyl-lipids (TG, PE, PI, DPG+PG, PC, WE, MGDG).

  using low-carbon water (2-3 µmol L -1 C) produced by a Milli-Q system. Measurement reliability was assessed twice a day by comparison of data with DOC Consensus Reference Waters (CRM; Hansell, 2005) (measured concentration = 42.7 µmol L -1 , nominal concentration: 41-44 µmol L -1 ). For further analytical details refer toSantinelli et al. (2013) Samples for total alkalinity (AT) were filtered on GF/F membranes (duplicate 50 mL filtrate aliquots from the PtotC and POC GF/F filtrations), immediately poisoned with 100 µl of a 50% saturated HgCl 2 solution (37 g HgCl 2 in 1 L distilled water) and stored at 4°C in the dark pending analysis. AT was determined on 20 mL subsamples using a home-made titration system composed of a 20 mL open temperature-controlled titration cell (kept at 25.0°C), an Orion 8103SC pH electrode calibrated on the National Bureau of Standards scale and a computer-driven Metrohm 665 Dosimat titrator, Switzerland. The seawater samples and the acid titrant (0.1 N HCl) were kept at a constant temperature of 25°C. Samples were weighted with a precision of 0.01 g before the titration to determine their exact volume from temperature and salinity. AT was calculated from a Gran function applied to pH variations from 4.2 to 3.0 as a function of added volume of HCl. AT measurements had abundances (N = 3 replicate incubation bottles per condition) were analyzed by the non-parametric pairwise Mann-Whitney test on raw data (i.e. individual replicate values), due to the low number of replicates, using the Statistica 6 software. Unless otherwise stated, data are reported as averages and standard deviation (SD) and statistical difference was accepted at p ≤ 0.05.

Acknowledgements

Thanks are due to M.-D. Pizay for performing the determination of total alkalinity, to C. Pinazo for her insights on the hydrological conditions at the sampling location, and the crew of the N/O Antedon for assitance in the field. I. This work is a contribution to the Mediterranean Institute of Oceanography "AT-EMBE", and to the Labex OT-Med (ANR-11-LABEX-0061, www.otmed.fr) funded by the Investissements d'Avenir, French Government project of the French National Research Agency (ANR, www.agence-nationale-recherche.fr) through the A*Midex project (ANR-11-IDEX-0001-02), funding Virginie Riou during preparation of the manuscript.

were filtered onto 0.2-µm-pore-size polycarbonate filters supported by 0.45-µm-pore-size cellulose nitrate filters directly, or after filtration by gravitation onto 2.0-µm-pore-size polycarbonate filters, as in [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF]. Cells were stained with diamidinophenylindole (DAPI) according to [START_REF] Porter | The use of DAPI for identifying and counting aquatic microflora[END_REF]. The fraction of "attached" or large prokaryotes >2 µm was calculated by subtracting values of the 0.2-2.0 µm fraction from the total fraction. Samples for catalyzed reporter deposition coupled with fluorescence in situ hybridization (CARD-FISH) were treated as in [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF]: briefly, samples were filtered onto 0.2-µm-pore-size polycarbonate filters as for the total prokaryotic abundances. Filters were washed twice with 0.2-µm filtered MilliQ water, air-dried, and stored in scintillation tubes at -20°C until analysis. Cells were embedded in low-gelling point 0.1% agarose (Sigma), dried at 37°C for 10min, and dehydrated with 95% ethanol. Bacterial cell walls were permeabilized by subsequent treatments with lysozyme (10 mg ml -1 , Sigma) [START_REF] Amann | Phylogenetic identification and in situ detection of individual microbial cells without cultivation[END_REF] and with achromopeptidase (60 U, Sigma) [START_REF] Sekar | An Improved Protocol for Quantification of Freshwater Actinobacteria by Fluorescence In Situ Hybridization[END_REF].

Archaeal cell walls were permeabilized with proteinase K (0.2 ml ml -1 , Fluka) [START_REF] Teira | Combining Catalyzed Reporter Deposition-Fluorescence In Situ Hybridization and Microautoradiography To Detect Substrate 30 59 Utilization by Bacteria and Archaea in the Deep Ocean[END_REF]. Acid treatment in 0.01M HCl was performed to denature endogenous peroxydase before hybridization with the horseradishperoxydase (HRP)-labeled oligonucleotide probes listed in Table1: for a detailed protocol see [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF]. The filter portions hydridized with each probe were mounted in a Citifluor:Vectashield:DAPI mix, and DAPI and CARD-FISH-stained cells were quantified under an Olympus BX61 microscope equipped with a 100-WHg-lamp and appropriate filter sets according to [START_REF] Tamburini | Effects of hydrostatic pressure on microbial alteration of sinking fecal pellets[END_REF].