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The capability of different bacterial populations to degrade abundant polymers, such as algal-derived polysaccharides, or to utilize preferentially polymers over monomers, remains largely unknown. In this study, microautoradiography was combined with fluorescence in situ hybridization (MAR-FISH) to evaluate the ability of Bacteroidetes, SAR11, Roseobacter spp., Gammaproteobacteria and SAR86 cells to use bicarbonate, leucine and starch under natural light conditions at two locations in shelf surface waters off NW Spain. The percentage of cells incorporating bicarbonate was relatively high (mean 32% ± 4%) and was positively correlated with the intensity of solar radiation. The proportion of cells using starch (mean 56% ± 4%) or leucine (mean 47% ± 4%) was significantly higher than that using bicarbonate. On average, SAR11, Roseobacter spp. and Gammaproteobacteria showed a similarly high percentage of cells using leucine (47%-65% of hybridized cells) than using starch (51%-64% of hybridized cells), while Bacteroidetes and SAR86 cells preferentially used starch (53% of hybridized cells) over leucine (34%-40% of hybridized cells). We suggest that the great percentage of bacteria using starch is related to a high ambient availability of polymers associated to algal cell lysis, which, in turn, weakens the short-term coupling between phytoplankton release and bacterial production.

Even though compelling evidences indicate that marine microbes show biogeographic patterns, very little is known on the mechanisms driving those patterns in aquatic ecosystems. Here we examined bacterial community structure in epipelagic waters of a highly hydrodynamic area of the Southern Ocean to gain insight into the role of biogeochemical factors and water mass mixing, as a proxy for dispersal, on microbial biogeography. We targeted four water masses that converge and mix around the South Shetland Islands (northern tip of the Antarctic Peninsula). Bacterioplankton communities were water-mass specific and best explained by dispersal rather than by biogeochemical factors, which we attribute to the relatively reduced environmental gradients found in these cold and nutrient rich waters. Our results support the notion that currents and water mixing may have a considerable effect in connecting and transforming different water bodies, and consequently, in shaping communities of microorganisms. Considering the multidimensional and dynamic nature of our oceans, we recommend the analysis of water mass mixing as a more suitable approach to investigate the role of dispersal on microbial plankton biogeography rather than geographical distance. This article is protected by copyright. All rights reserved.

Oxygen consumption during in vitro dark bottle incubations is the most common method to estimate planktonic respiration. This method is time consuming and labor instensive and, consequently, the database of planktonic respiration rates is scarce. Electron Transport System (ETS) activity measurement has gained acceptance as a routine technique to estimate respiration due to its high sensitivity. However, the in vitro ETS assay commonly used yields potential rates. Hence, the empirically determined ratio between in situ respiration and potential ETS activity is not constant, varying over two orders of magnitude, highly limiting the routine application of this technique to estimate microbial respiration. We hypothesized that in vivo ETS activity should be a reliable estimator of in situ respiration in marine plankton communities. The aim of this study was to develop a methodology to estimate in vivo ETS activity rates by using tetrazolium salt 2-para (iodo-phenyl)-3(nitrophenyl)-5(phenyl)tetrazolium chloride (INT) as electron acceptor. We established a procedure to apply the in vivo INT reduction method to natural marine microplankton samples. Optimum incubation times were found to be lower than 2-6 h, even for oligotrophic waters. The method was tested in natural waters, covering a very wide range of respiration rates and trophic conditions. A significant linear relationship was found between in situ respiration and in vivo INT reduction. Moreover, the ratio between oxygen consumption and INT reduction was constant across contrasting environments, which reveals the INT reduction method as a simple, quick, sensitive, and robust technique suitable to substantially improve the microbial plankton respiration database.

We determined the distribution and activity of the major prokaryotic groups (Bacteria, Cren-, and Euryarchaeota) inhabiting the deep water masses of the North Atlantic by following the path of the North Atlantic Deep Water (NADW) from its formation in the Greenland-Iceland-Norwegian (GIN) Sea along its two major branches covering approximately the first 50 yr of the NADW in the oceanic conveyor belt system. The relative abundance of Eury- and Crenarchaeota, assessed by catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH), was significantly higher in the western branch (17% and 24% of 49,69-diamidino-2- phenylindole (DAPI)-stained cells, respectively) than in the eastern (9% and 17%, respectively) branch of the NADW. In contrast, the relative abundance of Bacteria (30% of DAPI-stained cells) did not differ between the western and the eastern basin. Prokaryotic production and turnover rates, however, were higher in the western than the eastern basin. Generally, the contribution of Euryarchaeota to total picoplankton was correlated positively with oxygen concentrations ( p < 0.001) and negatively with salinity ( p < 0.001) and temperature ( p < 0.001). The contribution of Crenarchaeota to total picoplankton correlated positively with oxygen ( p < 0.05) and negatively with salinity ( p < 0.001). There was a positive correlation between the crenarchaeotal contribution to picoplankton and nitrite concentration ( p < 0.001), especially in the oxygen minimum layer, suggesting their potential involvement in the marine nitrogen cycle as nitrifiers. The observed variability in archaeal abundance in relation to bulk prokaryotic activity supports the emerging notion that Archaea are a highly dynamic and metabolically active component of the deep ocean prokaryotic community.

We determined the contribution of the three major prokaryotic groups (Bacteria, Crenarchaeota, and Euryarchaeota) on the uptake of D- and L-aspartic acid (Asp) in the major water masses of the North Atlantic (from 100to 4,000-m depth) with the use of microautoradiography combined with catalyzed reporter deposition fluorescence in situ hybridization (MICRO-CARD-FISH). The percentage of prokaryotic cells that assimilated D- and L-Asp ranged from ,5% to 25%. In the meso- and bathypelagic waters of the North Atlantic, Archaea are more abundant (42% 6 2% of 49,69-diamino-2-phenylindole [DAPI]‐stained cells) than Bacteria (30% 6 1% of DAPI-stained cells), and more archaeal than bacterial cells are actively incorporating D-Asp (62% 6 2% vs. 38% 6 2% of total D-Asp active cells). In contrast, Bacteria and Archaea almost equally contribute to L-Asp use in the deep waters of the North Atlantic (47% 6 2% vs. 53% 6 2% of total L-Asp active cells). The increase in the D-Asp : L-Asp uptake ratio in the prokaryotic community with depth appears to be driven by the efficient uptake of D-Asp by, especially, the Crenarchaeota in the deep waters. Because Archaea, and particularly Crenarchaeota, commonly dominate the prokaryotic communities in the ocean’s interior, we suggest that they represent a previously unrecognized sink of D‐amino acids in the deep ocean.

Dissolved organic carbon (DOC) production by microbial populations was measured at 19 stations in the Atlantic Ocean to quantify the fraction of photoassimilated carbon that flows through the dissolved organic pool at basin scale and to assess the relationship between the percentage of DOC production, phytoplankton size structure, and rates of net community production. Experiments were conducted during four cruises carried out between May 1998 and October 1999, covering three upwelling regions: Benguela (SW Africa), Mauritania (NW Africa) and NW Spain, and the oligotrophic North Atlantic subtropical gyre between 308N and 368N. Photic zone integrated particulate organic carbon (POC) production rates ranged from 10 to 1,178 mg C m 22 h 21 , thus covering a wide productivity spectrum. The percentage of DOC production with respect to total integrated primary production ranged from 4 to 42%, being larger in oligotrophic, picoplankton-dominated waters, where a balanced metabolism of the microbial community was observed, than in productive, net autotrophic waters, where large-sized cells formed the bulk of the phytoplankton biomass. A highly significant relationship was calculated between DOC and POC production rates in upwelling conditions. By contrast, the relationship between these variables in oligotrophic environments was weak, which suggests that different processes could be controlling the release of dissolved organic matter in productive and unproductive waters. Dissolved organic matter (DOM) is one of the least understood pools of marine matter and represents a major reservoir of organic carbon in the ocean. A large fraction of the dissolved organic carbon (DOC) present in the ocean ultimately derives from primary producers. However, a great deal of controversy still exists on the ecological significance and the ultimate control of DOC production in the ocean. The magnitude of DOC-related fluxes remains still largely uncertain, especially in oligotrophic regions. The high rates of DOC uptake by heterotrophic bacteria in relation to primary production recently measured in unproductive waters (e.g., Hansell et al. 1995; del Giorgio et al. 1997), largely justifies the growing biogeochemical interest of measuring and modeling DOC production in planktonic ecosystems. Initially, the radioactive carbon method for primary production estimation was modified for the measurement of direct excretion of dissolved organic compounds from algal cells. More recently, it has been recognized that several processes, besides direct excretion from intact algal cells, are 1