ALBERT

Description: Cell-specific leucine incorporation rates were determined in early summer across the open stratified Mediterranean Sea along vertical profiles from 0 to 200 m. During the period of our study, the bulk leucine incorporation rate was on average 5.0 ± 4.0 (n=31) pmol leu l−1 h−1. After 3H-radiolabeled leucine incorporation and SyBR Green I staining, populations were sorted using flow cytometry. Heterotrophic prokaryotes (Hprok) were divided in several clusters according to the cytometric properties of side scatter and green fluorescence of the cells: the low nucleic acid content cells (LNA) and the high nucleic acid content cells (HNA), with high size and low size (HNA-hs and HNA-ls, respectively). LNA cells represented 45 to 63% of the Hprok abundance between surface and 200 m, and significantly contributed to the bulk activity, from 17 to 55% all along the transect. The HNA/LNA ratio of cell-specific activities was on average 2.1 ± 0.7 (n=31). Among Hprok populations from surface samples (0 down to the deep chlorophyll depth, DCM), HNA-hs was mostly responsible for the leucine incorporation activity. Its cell-specific activity was up to 13.3 and 6.9-fold higher than that of HNA-ls and LNA, respectively, and it varied within a wide range of values (0.9–54.3×10−21 mol leu cell−1 h−1). At the opposite, ratios between the specific activities of the 3 populations tended to get closer to each other, below the DCM, implying a potentially higher homogeneity in activity of Hprok in the vicinity of nutriclines. Prochlorococcus cells were easily sorted near the DCM and displayed cell-specific activities equally high, sometimes higher than the HNA-hs group (2.5–55×10−21 mol leu cell−1 h−1). We then showed that all the sorted populations were key-players in leucine incorporation into proteins. The mixotrophic feature of certain photosynthetic prokaryotes and the non-negligible activity of LNA cells all over Mediterranean were reinforced.

Description: Radiolabeled orthophosphate (Pi) incubations coupled with cell sorting were conducted in the Mediterranean Sea to assess the contribution of picoplanktonic groups to total Pi uptake and to potentially explain their spatial distribution. Under natural Pi concentrations (P-deficient and stratified conditions during the survey), total Pi uptake was dominated in the surface by heterotrophic prokaryotes (Hprok; up to 82%) and shifted to a cyanobacterial dominance around the deep chlorophyll maximum depth. These experiments were completed with concentration bioassays at 4 stations to determine Pi uptake kinetic constants (maximum cell-normalized uptake rates Vmax and the half-saturation constant plus natural concentration K +Sn) in picophytoeukaryotes (Pic), Synechococcus (Syn), Prochlorococcus (Proc) and Hprok. Syn cells were the most efficient in the uptake of Pi at the cellular level at saturating concentration (100 nmol L-1). Indeed, Syn displayed the highest Vmax (up to 132 amol P cell−1 h-1) compared to other groups like Hprok (Vmax up to 2 amol P cell−1 h-1), and the lowest K + Sn. This suggests that they could be quickly reactive to a pulsed supply of Pi. Hprok and Proc cells seemed more adapted to take up Pi at low concentrations, with low K + Sn values (2.9 to 22.7 nmol P L-1), conferring them a nutritional advantage under P-deficient conditions. Such characteristics may explain the successful year-long coexistence of osmotrophic autotrophs and heterotrophs in the Mediterranean Sea and their vertical distribution in the photic zone.

Description: Heterotrophic bacterioplankton abundance and production were determined along vertical (down to bathypelagic layers) and latitudinal (from 4.9° E to 32.7° E) gradients across the Mediterranean Sea in early summer 2008. Abundance and flow cytometric characteristics (green fluorescence and side scatter signals) of high nucleic acid (HNA) and low nucleic acid (LNA) bacterial cells were investigated using flow cytometry. Contrarily to what is generally observed, the percentage of total bacteria represented by HNA cells (%HNA, range 30–69%) decreased with increased bacterial production (range 0.15–44 ng C l−1 h−1) although this negative relation was poorly explained (log-log regression r2=0.19). The %HNA as well as the mean side scatter of this group increased significantly with depth in the meso and bathypelagic layers. Our results demonstrated that vertical stratification with regard to chlorophyll distribution above, within or below the deep chlorophyll maximum plays an important role in influencing the distribution of cells, and in the relationships between the flow cytometric parameters and environmental variables such as chlorophyll a or bacterial production. Relationships between green fluorescence and side scatter of both HNA and LNA cells depended largely on chlorophyll distribution over the water column, suggesting that the dynamic link between HNA and LNA cells differs vertically.

Description: Aerobic anoxygenic phototrophic (AAP) bacteria are photoheterotrophic prokaryotes able to use both light and organic substrates for energy production. They are widely distributed in coastal and oceanic environments and may contribute significantly to the carbon cycle in the upper ocean. To better understand questions regarding links between the ecology of these photoheterotrophic bacteria and the trophic status of water masses, we examined their horizontal and vertical distribution and the effects of nutrient additions on their growth along an oligotrophic gradient in the Mediterranean Sea. Concentrations of bacteriochlorophyll-a (BChl-a) and AAP bacterial abundance decreased from the western to the eastern basins of the Mediterranean Sea and were linked with concentrations of chlorophyll-a, nutrient and dissolved organic carbon. Inorganic nutrient and glucose additions to surface seawater samples along the oligotrophic gradient revealed that AAP bacteria were nitrogen- and carbon-limited in the ultra-oligotrophic eastern basin. The intensity of the AAP bacterial growth response generally differed from that of the total bacterial growth response. BChl-a quota of AAP bacterial communities was significantly higher in the eastern basin than in the western basin, suggesting that reliance on phototrophy varied along the oligotrophic gradient and that nutrient and/or carbon limitation favors BChl-a synthesis.

Description: The chemical and biological characteristics of the surface microlayer were determined during a transect across the South Pacific Ocean in October-December 2004. Concentrations of particulate organic carbon (1.3 to 7.6-fold) and nitrogen (1.4 to 7), and POC:PON ratios were consistently higher in the surface microlayer as compared to subsurface waters (5 m). The large variability in particulate organic matter enrichment was negatively correlated to wind speed. No enhanced concentrations of dissolved organic carbon were detectable in the surface microlayer as compared to 5 m, but chromophoric dissolved organic matter was markedly enriched (by 2 to 4-fold) at all sites. Based on pigment analysis and cell counts, no consistent enrichment of any of the major components of the autotrophic and heterotrophic microbial community was detectable. CE-SSCP fingerprints and CARD FISH revealed that the bacterial communities present in the surface microlayer had close similarity (〉76%) to those in subsurface waters. By contrast, bacterial heterotrophic production (3H-leucine incorporation) was consistently lower in the surface microlayer than in subsurface waters. By applying CARD-FISH and microautoradiography, we observed that Bacteroidetes and Gammaproteobacteria dominated leucine uptake in the surface microlayer, while in subsurface waters Bacteroidetes and Alphaproteobacteria were the major groups accounting for leucine incorporation. Our results demonstrate that the microbial community in the surface microlayer closely resembles that of the surface waters of the open ocean. However, even short time periods in the surface microlayer result in differences in bacterial groups accounting for leucine incorporation, probably as a response to the differences in the physical and chemical nature of the two layers.

Description: Aerobic anoxygenic phototrophic (AAP) bacteria are photoheterotrophic prokaryotes able to use both light and organic substrates for energy production. They are widely distributed in coastal and oceanic environments and may contribute significantly to the carbon cycle in the upper ocean. To better understand questions regarding links between the ecology of these photoheterotrophic bacteria and the trophic status of water masses, we examined their horizontal and vertical distribution and the effects of nutrient additions on their growth along an oligotrophic gradient in the Mediterranean Sea. Concentrations of bacteriochlorophyll-a (BChl-a) and AAP bacterial abundance decreased from the western to the eastern basin of the Mediterranean Sea and were linked with concentrations of chlorophyll-a, nutrient and dissolved organic carbon. Inorganic nutrient and glucose additions to surface seawater samples along the oligotrophic gradient revealed that AAP bacteria were nitrogen- and carbon-limited in the ultraoligotrophic eastern basin. The intensity of the AAP bacterial growth response generally differed from that of the total bacterial growth response. BChl-a quota of AAP bacterial communities was significantly higher in the eastern basin than in the western basin, suggesting that reliance on phototrophy varied along the oligotrophic gradient and that nutrient and/or carbon limitation favors BChl-a synthesis.

Description: Heterotrophic bacterioplankton abundance and production were investigated with depth (down to bathypelagic layers) and with longitude (from 4.9° E to 32.7° E) along a cruise track across the Mediterranean Sea in early summer 2008. Abundances and flow cytometric characteristics (green fluorescence and side scatter signals) of high nucleic acid (HNA) and low nucleic acid (LNA) bacterial cells were determined using flow cytometry. Contrary to what is generally observed, the relative importance of HNA cells, as a percent of total cells, (%HNA, range 30–69 %) was inversely related to bacterial production (range 0.15–44 ng C l−1 h−1) although the negative relation was weak (log–log regression r2=0.19). The %HNA as well as the mean side scatter of HNA group increased significantly with depth in the meso and bathypelagic layers. Vertical stratification played an important role in influencing the distribution and characteristics of bacterial cells especially with regard to layers located above, within or below the deep chlorophyll maximum. Within a given layer, the relationships between the flow cytometric characteristics and environmental variables such as chlorophyll-a, nutrients or bacterial production changed. Overall, the relationships between HNA and LNA cells and environmental parameters differed vertically more than longitudinally.

Description: Microbial transformations are key processes in marine phosphorus cycling. In this study, we investigated the contribution of phototrophic and heterotrophic groups to phosphate (Pi) uptake fluxes in the euphotic zone of the low-Pi Mediterranean Sea and estimated Pi uptake kinetic characteristics. Surface soluble reactive phosphorus (SRP) concentrations were in the range of 6–80 nmol L−1 across the transect, and the community Pi turnover times, assessed using radiolabeled orthophosphate incubations, were longer in the western basin, where the highest bulk and cellular rates were measured. Using live cell sorting, four vertical profiles of Pi uptake rates were established for heterotrophic prokaryotes (Hprok), phototrophic picoeukaryotes (Pic) and Prochlorococcus (Proc) and Synechococcus (Syn) cyanobacteria. Hprok cells contributed up to 82% of total Pi uptake fluxes in the superficial euphotic zone, through constantly high abundances (2.7–10.2 × 105 cells mL−1) but variable cellular rates (6.6 ± 9.3 amol P cell−1 h−1). Cyanobacteria achieved most of the Pi uptake (up to 62%) around the deep chlorophyll maximum depth, through high abundances (up to 1.4 × 105 Proc cells mL−1) and high cellular uptake rates (up to 40 and 402 amol P cell−1 h−1, respectively for Proc and Syn cells). At saturating concentrations, maximum cellular rates up to 132 amol P cell−1 h−1 were measured for Syn at station (St.) C, which was 5 and 60 times higher than Proc and Hprok, respectively. Pi uptake capabilities of the different groups likely contribute to their vertical distribution in the low Pi Mediterranean Sea, possibly along with other energy limitations.

Description: The chemical and biological characteristics of the surface microlayer were determined during a transect across the South Pacific Ocean in October-December 2004. Concentrations of particulate organic carbon (1.3 to 7.6-fold) and nitrogen (1.4 to 7-fold), and POC:PON ratios were consistently higher in the surface microlayer as compared to surface waters (5 m). The large variability in particulate organic matter enrichment was negatively correlated to wind speed. No enhanced concentrations of dissolved organic carbon were detectable in the surface microlayer as compared to 5 m, but chromophoric dissolved organic matter was markedly enriched (by 2 to 4-fold) at all sites. Based on pigment analysis and cell counts, no consistent enrichment of any of the major components of the autotrophic and heterotrophic microbial community was detectable. CE-SSCP fingerprints and CARD FISH revealed that the bacterial communities present in the surface microlayer had close similarity (〉76%) to those in surface waters. By contrast, bacterial heterotrophic production (3H-leucine incorporation) was consistently lower in the surface microlayer than in surface waters. By applying CARD-FISH and microautoradiography, we observed that Bacteroidetes and Gammaproteobacteria dominated leucine uptake in the surface microlayer, while in surface waters Bacteroidetes and Alphaproteobacteria were the major groups accounting for leucine incorporation. Our results demonstrate that the microbial community in the surface microlayer closely resembles that of the surface waters of the open ocean. Even a short residence in the surface microlayer influences leucine incorporation by different bacterial groups, probably as a response to the differences in the physical and chemical nature of the two layers.

Description: The surface of the Mediterranean Sea is a low-phosphate-low-chlorophyll marine area where marine heterotrophic prokaryotes significantly contribute to the biogeochemical cycles of all biogenic elements such as carbon, notably through the mineralization of dissolved organic compounds. Cell-specific leucine incorporation rates were determined in early summer in the open stratified Mediterranean Sea. The bulk leucine incorporation rate was on average 5 ± 4 pmol leu l−1 h−1 (n=30). Cell-specific 3H-leucine incorporation rates were assayed using flow cytometry coupled to cell sorting. Heterotrophic prokaryotes (Hprok) were divided into cytometric groups according to their side scatter and green fluorescence properties: high nucleic acid containing cells (HNA) with high scatter (HNA-hs) and low scatter (HNA-ls) and low nucleic acid containing cells (LNA). Cell-specific leucine incorporation rates of these cytometric groups ranged from 2 to 54, 0.9 to 11, and 1 to 12 × 10-21 mol cell−1 h−1, respectively. LNA cells represented 45 to 63% of the Hprok abundance, and significantly contributed to the bulk leucine incorporation rates, from 12 to 43%. HNA/LNA ratios of cell-specific leucine incorporation were on average 2.0 ± 0.7 (n=30). In surface layers (from 0 m down to the deep chlorophyll depth, DCM), cell-specific rates of HNA-hs were elevated (7 and 13 times greater than LNA and HNA-ls, respectively). Nevertheless, on average HNA-hs (26%) and LNA (27%) equally contributed to the bulk leucine incorporation in these layers. Prochlorococcus cells were easily sorted near the DCM and displayed cell-specific leucine incorporation rates ranging from 3 to 55 × 10-21 mol leu cell−1 h−1, i.e. as high as HNA-hs'. These sorted groups could therefore be defined as key-players in the process of leucine incorporation into proteins. The mixotrophic features of certain photosynthetic prokaryotes and the high contribution of LNA cells to leucine incorporation within the microbial communities of the Mediterranean could be reinforced.