| Summary: | Ocean acidification resulting from the uptake ofanthropogenic carbon dioxide (CO2) by the ocean is considereda major threat to marine ecosystems. Here we examinedthe effects of ocean acidification on microbial communitydynamics in the eastern Baltic Sea during the summerof 2012 when inorganic nitrogen and phosphorus werestrongly depleted. Large-volume in situ mesocosms wereemployed to mimic present, future and far future CO2 scenarios.All six groups of phytoplankton enumerated by flowcytometry (<20 μm cell diameter) showed distinct trends innet growth and abundance with CO2 enrichment. The picoeukaryoticphytoplankton groups Pico-I and Pico-II displayedenhanced abundances, whilst Pico-III, Synechococcusand the nanoeukaryotic phytoplankton groups were negativelyaffected by elevated fugacity of CO2 (fCO2). Specifically,the numerically dominant eukaryote, Pico-I, demonstratedincreases in gross growth rate with increasing fCO2sufficient to double its abundance. The dynamics of theprokaryote community closely followed trends in total algalbiomass despite differential effects of fCO2 on algal groups.Similarly, viral abundances corresponded to prokaryotic hostpopulation dynamics. Viral lysis and grazing were both importantin controlling microbial abundances. Overall our resultspoint to a shift, with increasing fCO2, towards a moreregenerative system with production dominated by small picoeukaryoticphytoplankton.
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