Shifts in the microbial community in the Baltic Sea with increasing CO2

Ocean acidification, due to dissolution of anthropogenically produced carbon dioxide is considered a major threat to marine ecosystems. The Baltic Sea, with extremely low salinity and thus low pH buffering capacity, is likely to experience stronger variation in pH than the open ocean with increasing atmospheric carbon dioxide. We examined the effects of ocean acidification on the microbial community during summer using large volume in situ mesocosms to simulate present to future and far future scenarios. We saw distinct trends with increasing CO2 in each of the 6 groups of phytoplankton with diameters below 20 μm that we enumerated by flow cytometry. Of these groups two picoeukaryotic groups increased in abundance whilst the other groups, including prokaryotic Synechococcus spp., decreased with increasing CO2. Gross growth rates increased with increasing CO2 in the dominant picoeukaryote group sufficient to double their abundances whilst reduced grazing allowed the other picoeukaryotes to flourish at higher CO2. Significant increases in lysis rates were seen at higher CO2 in these two picoeukaryote groups. Converting abundances to particulate organic carbon we saw a large shift in the partitioning of carbon between the size fractions which lasted throughout the experiment. The heterotrophic prokaryotes largely followed the algal biomass with responses to increasing CO2 reflecting the altered phytoplankton community dynamics. Similarly, higher viral abundances at higher CO2 seemed related to increased prokaryote biomass. Viral lysis and grazing were equally important controlling prokaryotic abundances. Overall our results point to a shift towards a more regenerative system with potentially increased productivity but reduced carbon export.

Crawfurd K. J., Brussaard C. P. D. & Riebesell U., 2016. Shifts in the microbial community in the Baltic Sea with increasing CO2. Biogeosciences Discussions. Article.

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