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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.

Ecophysiology of Nitrogen‐Metabolising Microbes.-- 15 pages, 7 figures, 1 table, supporting information https://doi.org/10.1111/1462-2920.14581, The impact of grazing, resource competition and light on prokaryotic growth and taxonomic composition in subtropical and tropical surface waters were studied through 10 microcosm experiments conducted between 30°N and 30°S in the Atlantic, Pacific and Indian oceans. Under natural sunlight conditions, significant changes in taxonomic composition were only observed after the reduction of grazing by sample filtration in combination with a decrease in resource competition by sample dilution. Sunlight exposure significantly reduced prokaryote growth (11 ± 6%) and community richness (14 ± 4%) compared to continuous darkness but did not significantly change community composition. The largest growth inhibition after sunlight exposure occurred at locations showing deep mixed layers. The reduction of grazing had an expected and significant positive effect on growth, but caused a significant decrease in community richness (16 ± 6%), suggesting that the coexistence of many different OTUs is partly promoted by the presence of predators. Dilution of the grazer‐free prokaryotic community significantly enhanced growth at the level of community, but consistently and sharply reduced the abundance of Prochlorococcus and SAR11 populations. The decline of these oligotrophic bacterial taxa following an increase in resource availability is consistent with their high specialization for exploiting the limited resources available in the oligotrophic warm ocean, This study was supported by the by the Spanish Ministerio de Ciencia e Innovación through project Consolider‐Ingenio Malaspina 2010 (CSD2008‐00077). Sample sequencing was funded by grant M'OMICS‐MALAN (CTM2011‐15461‐E PI) of the Spanish Ministerio de Economía y Competitividad

19 pages, 7 figures, 1 table, Small eukaryotes (0.2–20 μm cell‐size) represent a significant fraction of the microbial plankton community in shelf waters of NW‐Spain. The community composition of small eukaryotes living at the surface and at the base of the photic zone was analysed by means of 18S rDNA high‐throughput sequencing on a circa‐monthly basis over a 23 months period. Ostreococcus was the most abundant taxon in surface waters, showing marked peaks in read abundance in spring and late summer, while Syndiniales dominated at the base of the photic zone. A well‐defined seasonal pattern of community composition, linked to the succession of the dominant taxa, was found in surface waters. Seasonality was less apparent at the base of the euphotic zone. Temporal changes in abiotic factors significantly correlated with changes in community composition in surface (r = 0.71) and at the base of the photic zone (r = 0.38). Changes in community composition significantly correlated with changes in community function‐related variables (including biomass, primary production and respiration) only in surface water (r = 0.36). Co‐occurrence network analyses revealed 45 significant interspecies associations among the 50 most abundant taxa with highly connected OTUs belonging to cryptophyceans. The network topology, with small‐world characteristics, suggests a stabilizing role of biotic interactions to environmental disturbance, This research was supported by the Xunta the Galicia through DIMENSION project (grant EM2013/023) and by Spanish Ministry of Economy, Industry and Competitivity (MINECO) trough ENVISION project (grant CTM2014‐59031‐P) in collaboration with the ‘Instituto Español de Oceanografía’ (IEO) RADIALES project. Esther Barber‐Lluch was funded by an F.P.I. MINECO fellowship

Even though compelling evidences indicate that marine microbes show biogeographic patterns, very little is known on the mechanisms driving those patterns in aquatic ecosystems. In the present study, bacterial community structure was examined in epipelagic waters of a highly hydrodynamic area of the Southern Ocean to gain insight into the role that biogeochemical factors and water mass mixing (a proxy of dispersal) have on microbial biogeography. Four water masses that converge and mix around the South Shetland Islands (northern tip of the Antarctic Peninsula) were investigated. Bacterioplankton communities were water-mass specific, and were best explained by dispersal rather than by biogeochemical factors, which is attributed to the relatively reduced environmental gradients found in these cold and nutrient rich waters. These 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 the ocean, analysis of water mass mixing is a more suitable approach to investigate the role of dispersal on the biogeography of planktonic microorganisms rather than geographical distance.