Marine microbial food web processes in global warming perspective (MICROGLOB)

Microbial community includes single-celled organisms that cover a wide size range (from 0.02 to 200 µm) and according to the trophic status includes autotrophic, heterotrophic and mixotrophic organisms. This project will study the structure of microbial community and microbial food web, as well as its quantitative, qualitative (structural) and functional responses to natural and man-induced changes in marine environment and will try to detect potential indicators of these changes. The main focus of the project will be to study what changes in the structure and functioning of marine microbial food web in terms of global warming we can expect, and how these influences are manifested with regard to the trophic status of the area. Project activities will be carried out through three main topics: (1) Structure of microbial food web on spatial (different marine environments), trophic (trophic gradient) and temporal (intra- and inter-annual scale, and possibly temperature and salinity scales (cross comparisons between natural systems under different temperature and salinity regimes), (2) Bacterial carbon flux which include grazing experiments and study of bacterial carbon flux through the microbial food web) and (3) Experimental impact of temperature increase and phosphate additions on microbial community structure and carbon flux toward the higher trophic levels.

  • Research project of the Croatian Science Foundation.

  • Microbial food web.

  • Viral loop.

  • Pelagic food web.

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.