Acidification and warming affect prominent bacteria during two seasonal phytoplankton bloom mesocosms

In contrast to clear stimulatory effects of rising temperature, recent studies reported conflicting results of CO2 effects on planktonic bacteria. To obtain a better understanding of the impact of future climate scenarios on the development of bacterial communities, we performed bifactorial mesocosm experiments (pCO2 and temperature) with Baltic Sea water, during a diatom bloom in autumn and during a summer bloom. The development of bacterial community composition BCC followed well-known bloom dynamics with Alphaproteobacteria dominating during the bloom peak and Bacteroidetes dominating during bloom decay. A principle coordinate analysis (PCoA) of bacterial OTUs (operational taxonomic units) revealed that phytoplankton succession and temperature were the major bacterial community structuring variables whereas only a weak impact of pCO2 was found. This was corroborated by the trends in bacterial bulk parameters. However, significant and potentially direct effects of pCO2 on the relative abundance of several dominant OTUs occurred and were in some cases accompanied by an antagonistic temperature effect. Our results suggest the necessity of high-resolution BCC analyses and statistical analyses on OTU level to observe strong effects of CO2 on specific bacterial groups, which in turn might influence also particular organic matter degradation processes.