Structure of MFW
Hypothesis: We hypothesize that microbial community structure highly depends on trophic status and temperature and salinity regimes of the marine environment. We will test the hypothesis that in nutrient limited environments heterotrophic picoplankton overdominate autotrophic picoplankton; autotrophic prokaryotic picoplankton (cyanobacteria) overdominate eukaryotic autotrophs; contribution of Prochlorococcus in the total autotrophic picoplankton increases; and contribution of LNA bacteria in total heterotrophic bacteria increases. Analyses of specific ratios and relationships between different trophic or functional groups within microbial community have potential in developing of specific microbial indicators of the changes and disturbances in the marine ecosystems.
Bacterial carbon flux
Hypothesis: In this part of the Project, we will test the hypothesis that microbial food web structure and carbon flow are highly dependent on temperature and phosphorus. The aim of this investigation is to compare the relative contributions of viral lysis and nanoflagellate grazing on bacterial mortality throughout the year, which dominantly determined the fate of bacterial carbon. We hypothesize that the low-temperature suppresses bacterial growth and reduces ‘microbial loop’ activity and that viral lysis may play a large role in determining the flux of organic carbon in the ‘microbial loop’, outweighing the impact of protists on bacteria in the cold seasons. We also hypothesize that relative importance of “bottom-up” and “top down” control of bacteria is significantly changing on trophic and seasonal scale.
Global warming perspective
Hypothesis: We hypothesized that phosphorus and temperature are both limiting factors for BP and BR in this system. Accordingly, we further hypothesize that temperature increase will probably result in an increase in BR of organic carbon and of bacterial losses to grazers (GB), which would then increase the biomass flux between these two trophic levels within the microbial food web. If enough resources are available, BP would also increase, at rates probably higher than GB. In that case, bacterial abundance would also increase, and a higher proportion of inorganic nutrients would accumulate as bacterial biomass, thus strengthening the already dominant role of microbes in the carbon cycle in a warmer marine environment. We will also test the hypothesis that an increase in temperature reduces BGE in oligotrophic conditions, while the addition of phosphorus would lead to an increase in BGE.