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.

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Recent experimental evidence indicates the importance in some pelagic systems of mixotrophic protists that combine photosynthetic ability with the ability to ingest bacteria. If both bacteria and phytoplankton are mineral nutrient limited, this should provide the mixotrophs with the double benefit of combining removal of their competitor with ingestion of the limiting nutrient in pelleted form. It is the objective of this study to expand the classical theories of competition and predation to explore the effect on the microbial food web of one trophic group possessing both strategies. In a chemostat scenario, we analyzed the two—species situation of a mixotroph preying on mineral—nutrient—limited bacteria, and also the situations when the mixotroph in addition has to compete with specialized photoautotrophic and phagotrophic protists, each superior to the mixotroph in their specialized nutritional modes. In the mixotroph—bacteria relationship, somewhat paradoxically, high predatory abilities will reduce the quantitative importance of predation in the mixotroph's nutrition. The reason is a strong reduction in prey abundance, allowing the mixotroph to survive as a photoautotroph despite its low competitive ability. In the three—species case with mixotrophs, bacteria, and specialized phagotrophs, it is shown that the mixotroph can compensate for a "price" paid in reduced affinity for bacterial prey by a sufficiently high affinity for mineral nutrients. In the other three—species case where the mixotroph has to compete with a specialized photoautotroph, the situation is more complex; there is an optimum value for the mixotroph's predatory ability at which mixotroph biomass is maximized. In the general situation with all four species (bacteria, mixotrophs, and specialized auto— and phagotrophs) potentially present, different mixotrophic strategies will alter the equilibrium composition of the consortium, with the mixotroph being most successful with a high affinity for nutrients and an intermediate affinity for bacteria. In the simple form used here, the model predicts no equilibrium with all four species simultaneously present. The theory is in principle directly applicable to laboratory experimentation.

Abundance and bacterivory of mixotrophic flagellates were examined in a vertical profile during 1 wk in June 1992 in the Bay of Aarhus, Denmark. A stable pycnocline separated a n upper water mass with low salinity, low inorganic nutrient concentration (< 0.1 pm01 1-l) and low bacterial abundance (<106 ml-') from a bottom water mass with higher salinity, inorganic nutrient concentration, and bacterial abundance (>106 1111-l). In the upper layer, bacterivorous, pign~ented flagellates (mixotrophs) accounted for 49% of the pigmented biomass. In addition to their function as primary producers, mixotrophic flagellates were responsible for 86% of the entire flagellate bacterivory The abundance of bacterial food particles (<106 m]-') was probably not sufficient to sustain growth of most bacterivorous, colourless flagellates, and the nutrient-depleted water body prevented the strict phototrophs from dominating the environment. Below the pycnocline, nutrients were present, bacterial abundance exceeded 106 ml-l, and mixotrophic flagellates made up only 9 % of the pign~ented biomass and accounted for 1 9 % of the flagellate bacterivory.

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