Bacterioplankton groups involved in the uptake of phosphate and dissolved organic phosphorus in a mesocosm experiment with P-starved Mediterranean waters.
ABSTRACT The use of inorganic phosphate (Pi) and dissolved organic phosphorus (DOP) by different bacterial groups was studied in experimental mesocosms of P-starved eastern Mediterranean waters in the absence (control mesocosms) and presence of additional Pi (P-amended mesocosms). The low Pi turnover times in the control mesocosms and the increase in heterotrophic prokaryotic abundance and production upon Pi addition confirmed that the bacterial community was originally P-limited. The bacterioplankton groups taking up Pi and DOP were identified by means of microautoradiography combined with catalysed reporter deposition fluorescence in situ hybridization. Incubations with leucine were also performed for comparative purposes. All the probe-identified groups showed a high percentage of cells taking up Pi and DOP in the control, P-limited, mesocosms throughout the experiment. However, in response to Pi addition two contrasting scenarios in Pi use were observed: (i) on day 1 of the experiment Pi addition caused a clear reduction in the percentage of SAR11 cells taking up Pi, whereas Gammaproteobacteria, Roseobacter and Bacteroidetes showed similar percentages to the ones in the control mesocosms and (ii) on day 4 of the experiment, probably when the bacterial community had fully responded to the P input, all the probe-identified groups showed low percentages of cells taking up the substrate as compared with the control mesocosms. These differences are likely related to different P requirements among the bacterial groups and point out to the existence of two contrasting strategies in P use.
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ABSTRACT: Subtropical oceanic gyres are the most extensive biomes on Earth where SAR11 and Prochlorococcus bacterioplankton numerically dominate the surface waters depleted in inorganic macronutrients as well as in dissolved organic matter. In such nutrient poor conditions bacterioplankton could become photoheterotrophic, that is, potentially enhance uptake of scarce organic molecules using the available solar radiation to energise appropriate transport systems. Here, we assessed the photoheterotrophy of the key microbial taxa in the North Atlantic oligotrophic gyre and adjacent regions using (33)P-ATP, (3)H-ATP and (35)S-methionine tracers. Light-stimulated uptake of these substrates was assessed in two dominant bacterioplankton groups discriminated by flow cytometric sorting of tracer-labelled cells and identified using catalysed reporter deposition fluorescence in situ hybridisation. One group of cells, encompassing 48% of all bacterioplankton, were identified as members of the SAR11 clade, whereas the other group (24% of all bacterioplankton) was Prochlorococcus. When exposed to light, SAR11 cells took 31% more ATP and 32% more methionine, whereas the Prochlorococcus cells took 33% more ATP and 34% more methionine. Other bacterioplankton did not demonstrate light stimulation. Thus, the SAR11 and Prochlorococcus groups, with distinctly different light-harvesting mechanisms, used light equally to enhance, by approximately one-third, the uptake of different types of organic molecules. Our findings indicate the significance of light-driven uptake of essential organic nutrients by the dominant bacterioplankton groups in the surface waters of one of the less productive, vast regions of the world's oceans-the oligotrophic North Atlantic subtropical gyre.The ISME Journal advance online publication, 25 October 2012; doi:10.1038/ismej.2012.126.The ISME Journal 10/2012; · 8.95 Impact Factor
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ABSTRACT: The heterotrophic bacterial community of the Eastern Mediterranean Sea is believed to be limited by phosphorus (P) availability. This observation assumes that all bacterial groups are equally limited, something that has not been hitherto examined. To test this hypothesis, we performed nutrient addition experiments and investigated the response of probe-identified groups using microautoradiography combined with catalyzed reporter deposition fluorescence in situ hybridization. Our results show contrasting responses between the bacterial groups, with Gammaproteobacteria being the group more affected by P availability. The Roseobacter clade was likely colimited by P and nitrogen (N), whereas Bacteroidetes by P, N and organic carbon (C). In contrast, SAR11 cells were active regardless of the nutrient concentration. These results indicate that there is high heterogeneity in the nutrient limitation of the different components of the bacterioplankton community.The ISME Journal advance online publication, 14 March 2013; doi:10.1038/ismej.2013.42.The ISME Journal 03/2013; · 8.95 Impact Factor
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ABSTRACT: The coastal northern Adriatic Sea receives pulsed inputs of riverine nutrients, causing phytoplankton blooms and seasonally sustained dissolved organic carbon (DOC) accumulation-hypothesized to cause episodes of massive mucilage. The underlying mechanisms regulating P and C cycles and their coupling are unclear. Extensive biogeochemical parameters, processes and community composition were measured in a 64-day mesocosms deployed off Piran, Slovenia. We followed the temporal trends of C and P fluxes in P-enriched (P+) and unenriched (P-) mesocosms. An intense diatom bloom developed then crashed; however, substantial primary production was maintained throughout, supported by tightly coupled P regeneration by bacteria and phytoplankton. Results provide novel insights on post-bloom C and P dynamics and mechanisms. 1) Post-bloom DOC accumulation to 186μM remained elevated despite high bacterial carbon demand. Presumably, a large part of DOC accumulated due to the bacterial ectohydrolytic processing of primary productivity that adventitiously generated slow-to-degrade DOC; 2) bacteria heavily colonized post-bloom diatom aggregates, rendering them microscale hotspots of P regeneration due to locally intense bacterial ectohydrolase activities; 3) Pi turnover was rapid thus suggesting high P flux through the DOP pool (dissolved organic phosphorus) turnover; 4) Alpha- and Gamma-proteobacteria dominated the bacterial communities despite great differences of C and P pools and fluxes in both mesocosms. However, minor taxa showed dramatic changes in community compositions. Major OTUs were presumably generalists adapted to diverse productivity regimes.We suggest that variation in bacterial ectohydrolase activities on aggregates, regulating the rates of POM→DOM transition as well as dissolved polymer hydrolysis, could become a bottleneck in P regeneration. This could be another regulatory step, in addition to APase, in the microbial regulation of P cycle and the coupling between C and P cycles.Science of The Total Environment 11/2013; 470-471C:1173-1183. · 3.16 Impact Factor