Maria Montserrat Sala

Changes in Bacterioplankton Composition under Different Phytoplankton Regimens

ABSTRACT The results of empirical studies have revealed links between phytoplankton and bacterioplankton, such as the frequent correlation between chlorophyll a and bulk bacterial abundance and production. Nevertheless, little is known about possible links at the level of specific taxonomic groups. To investigate this issue, seawater microcosm experiments were performed in the northwestern Mediterranean Sea. Turbulence was used as a noninvasive means to induce phytoplankton blooms dominated by different algae. Microcosms exposed to turbulence became dominated by diatoms, while small phytoflagellates gained importance under still conditions. Denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments showed that changes in phytoplankton community composition were followed by shifts in bacterioplankton community composition, both as changes in the presence or absence of distinct bacterial phylotypes and as differences in the relative abundance of ubiquitous phylotypes. Sequencing of DGGE bands showed that four Roseobacter phylotypes were present in all microcosms. The microcosms with a higher proportion of phytoflagellates were characterized by four phylotypes of the Bacteroidetes phylum: two affiliated with the family Cryomorphaceae and two with the family Flavobacteriaceae. Two other Flavobacteriaceae phylotypes were characteristic of the diatom-dominated microcosms, together with one Alphaproteobacteria phylotype (Roseobacter) and one Gammaproteobacteria phylotype (Methylophaga). Phylogenetic analyses of published Bacteroidetes 16S rRNA gene sequences confirmed that members of the Flavobacteriaceae are remarkably responsive to phytoplankton blooms, indicating these bacteria could be particularly important in the processing of organic matter during such events. Our data suggest that quantitative and qualitative differences in phytoplankton species composition may lead to pronounced differences in bacterioplankton species composition.

Metabolic diversity of heterotrophic bacterioplankton over winter and spring in the coastal Arctic Ocean

Metabolic diversity of heterotrophic bacterioplankton was tracked from early winter through spring with Biolog Ecoplates under the seasonally ice covered arctic shelf in the Canadian Arctic (Franklin Bay, Beaufort Sea). Samples were taken every 6 days from December 2003 to May 2004 at the surface, the halocline where a temperature inversion occurs, and at 200 m, close to the bottom. Despite the low nutrient levels and low chlorophyll a, suggesting oligotrophy in the winter surface waters, the number of substrates used (NSU) was greater than in spring, when chlorophyll a concentrations increased. Denaturing gradient gel electrophorisis analysis also indicated that the winter and spring bacterial communities were phylogenetically distinct, with several new bands appearing in spring. In spring, the bacterial community would have access to the freshly produced organic carbon from the early phytoplankton bloom and the growth of rapidly growing specialist phenotypes would be favoured. In contrast, in winter bacterioplankton consumed more complex organic matter originated during the previous years phytoplankton production. At the other depths we tested the NSU was similar to that for the winter surface, with no seasonal pattern. Instead, bacterioplankton metabolism seemed to be influenced by resuspension, advection, and sedimentation events that contributed organic matter that enhanced bacterial metabolism.

Ectoenzymatic activities and heterotrophic bacteria decomposing detritus

One of the largest fluxes of carbon in most of the ecosystems is that from the detritus to microorganisms. Microbial ectoenzymes play a basic role in the degradation of detritus. However, the role of ectoenzymes in dependence of the detritus composition has not been studied. In a microcosm experiment we have followed the development of nine ectoenzymatic activities and hydrolytic bacteria during the degradation of four sources of detritus (macrophytes, algae, leaves and chitin). Throughout the degradation of algae and macrophytes, a succession of ectoenzymatic activities could be observed. This succession started with the hydrolysis of oligosaccharides and starch (high α-glucosidase, β-glucosidase, exocellulase and amylase activities), and was followed by the hydrolysis of plant structural polysaccharides (endocellulase and endoxylanase activities). Such a succession was neither found in the enrichments with leaves, with lower peaks of activity, nor of chitin. This latter was characterized by a high chitinolytic activity and the highest alkaline phosphatase/peptidase ratio. Along the experiment, the number of hydrolytic colonies (amylolytic, cellulolytic, xylanolytic, chitinolytic) varied between 2-52 % of the total CFUs, amylolytic colonies generally being the most abundant (up to 35 % of total CFUs). For 20 isolates, their ability to hydrolyze starch, cellulose, xylane and chitin when offered as single carbon source was checked. Of the isolates, 55 % could use more than one polymer. Very likely, the ability to hydrolyze several carbon sources offers these bacteria the possibility to shift or even express simultaneously various enzymes. During the process of microbial decomposition of detritus shown here, characterized by changes in the available molecules, bacteria with the ability to hydrolyze several carbohydrates would have an advantage to persist in the system in contrast to bacteria that could only hydrolyze one of the tested polymers. In aquatic environments exposed to changing inputs of organic matter such as the littoral zones, bacteria with multiple hydrolytic potential would very likely show a better adaptation.

Seasonal dynamics of pelagic and benthic (littoral and profundal) bacterial abundances and activities in a deep prealpine lake (L. Constance)

In order to obtain a first insight into the characteristics of pelagic, littoral and profundal bacterial communities and their potential role for the degradation of organic matter in the deep prealpine Lake Constance, we compared the seasonal dynamics of bacterial abundances and activities during the annual cycle in the epilim- netic water of a central station as well as in surface layers of sediments at a central pro- fundal and a littoral site. For this purpose, bacterial abundances (DAPI counts), rates of leucine incorporation, respiration of 14 C-labeled substrates (glucose, phenol) and ectoenzymatic activities were measured biweekly-monthly at the different sampling sites. Bacterial densities in lake sediments generally exceeded those of epilimnetic waters by at least two orders of magnitude with higher values in profundal sediments. A seasonal pattern was well expressed for bacterioplankton, less pronounced for ben- thic littoral bacteria and hardly visible for profundal bacteria. Compared to pelagic bacteria, benthic bacteria exhibited lower specific rates of leucine incorporation, espe- cially during the warmer periods. For profundal benthic communities specific rates of glucose respiration and of enzyme activities were permanently low with little seasonal fluctuation. In contrast, littoral benthic communities showed the highest values for these specific metabolic activities among the three sites tested. Additionally, the capac- ity of metabolism of phenol (used here as a representative of more recalcitrant aro- matic substances) was by far highest respired for littoral communities, and almost lacking for pelagic communities. Evidence from the literature suggests that the qualita- tively and quantitatively enhanced level of degradation capacities observed for littoral bacteriobenthos may be predominantly a result of the combined effect of increased supply of additional organic matter (e. g. allochthonous matter and macrophytes), an enriched bacterial gene reservoir in sediments, and frequent resuspension. Littoral ben- thic communities showed the highest specific metabolic activities exceeding those of bacterioplankton and profundal bacteriobenthos by an order of magnitude. As a whole,

Coastal Bacterioplankton Metabolism Is Stimulated Stronger by Anthropogenic Aerosols than Saharan Dust

In oligotrophic regions, such as the Mediterranean Sea, atmospheric deposition has the potential to stimulate heterotrophic prokaryote growth and production in surface waters, especially during the summer stratification period. Previous studies focused on the role of leaching nutrients from mineral particles of Saharan (S) origin, and were restricted to single locations at given times of the year. In this study, we evaluate the effect of atmospheric particles from diverse sources and with a markedly different chemical composition [S dust and anthropogenic (A) aerosols] on marine planktonic communities from three locations of the northwestern Mediterranean with contrasted anthropogenic footprint. Experiments were also carried out at different times of the year, considering diverse initial conditions. We followed the dynamics of the heterotrophic community and a range of biogeochemical and physiological parameters in six experiments. While the effect of aerosols on bacterial abundance was overall low, bacterial heterotrophic production was up to 3.3 and 2.1 times higher in the samples amended with A and S aerosols, respectively, than in the controls. Extracellular enzymatic activities [leu-aminopeptidase (AMA) and β-glucosidase (β-Gl)] were also enhanced with aerosols, especially from A origin. AMA and β-Gl increased up to 7.1 in the samples amended with A aerosols, and up to 1.7 and 2.1 times, respectively, with S dust. The larger stimulation observed with A aerosols might be attributed to their higher content in nitrate. However, the response was variable depending the initial status of the seawater. In addition, we found that both A and S aerosols stimulated bacterial abundance and metabolism significantly more in the absence of competitors and predators.

Viruses and Protists Induced-mortality of Prokaryotes around the Antarctic Peninsula during the Austral Summer

During the Austral summer 2009 we studied three areas surrounding the Antarctic Peninsula: the Bellingshausen Sea, the Bransfield Strait and the Weddell Sea. We aimed to investigate, whether viruses or protists were the main agents inducing prokaryotic mortality rates, and the sensitivity to temperature of prokaryotic heterotrophic production and mortality based on the activation energy (Ea) for each process. Seawater samples were taken at seven depths (0.1–100 m) to quantify viruses, prokaryotes and protists abundances, and heterotrophic prokaryotic production (PHP). Viral lytic production, lysogeny, and mortality rates of prokaryotes due to viruses and protists were estimated at surface (0.1–1 m) and at the Deep Fluorescence Maximum (DFM, 12–55 m) at eight representative stations of the three areas. The average viral lytic production ranged from 1.0 ± 0.3 × 107 viruses ml−1 d−1 in the Bellingshausen Sea to1.3 ± 0.7 × 107 viruses ml−1 d−1 in the Bransfield Strait, while lysogeny, when detectable, recorded the lowest value in the Bellingshausen Sea (0.05 ± 0.05 × 107 viruses ml−1 d−1) and the highest in the Weddell Sea (4.3 ± 3.5 × 107 viruses ml−1 d−1). Average mortality rates due to viruses ranged from 9.7 ± 6.1 × 104 cells ml−1 d−1 in the Weddell Sea to 14.3 ± 4.0 × 104 cells ml−1 d−1 in the Bellingshausen Sea, and were higher than averaged grazing rates in the Weddell Sea (5.9 ± 1.1 × 104 cells ml−1 d−1) and in the Bellingshausen Sea (6.8 ± 0.9 × 104 cells ml−1 d−1). The highest impact on prokaryotes by viruses and main differences between viral and protists activities were observed in surface samples: 17.8 ± 6.8 × 104 cells ml−1 d−1 and 6.5 ± 3.9 × 104 cells ml−1 d−1 in the Weddell Sea; 22.1 ± 9.6 × 104 cells ml−1 d−1 and 11.6 ± 1.4 × 104 cells ml−1 d−1 in the Bransfield Strait; and 16.1 ± 5.7 × 104 cells ml−1 d−1 and 7.9 ± 2.6 × 104 cells ml−1 d−1 in the Bellingshausen Sea, respectively. Furthermore, the rate of lysed cells and PHP showed higher sensitivity to temperature than grazing rates by protists. We conclude that viruses were more important mortality agents than protists mainly in surface waters and that viral activity has a higher sensitivity to temperature than grazing rates. This suggests a reduction of the carbon transferred through the microbial food-web that could have implications in the biogeochemical cycles in a future warmer ocean scenario.