Species-specific exudates from marine phytoplankton drive microbial community composition

Phytoplankton-bacteria interactions are governed by the transformation of algal-derived organic matter, exchange of metabolites and infochemicals. However, the assembly of the bacterial community within the phycosphere is not well understood. In this study, we performed experiments to tease apart the role of species-specific exometabolites, bacterial/bacterial interactions and algal/bacterial interactions in shaping bacterial community within the phycophere. Twelve bacterial isolates, representative of the main members of the phycosphere microbiome were selected and exposed to phytoplankton extracellular fractions (PEF) from Alexandrium minutum, Prymnesium parvum and Tisochrysis lutea. While all bacterial isolates were able to grow individually on the different PEF, the bacterial community analysis indicated that some species were selected and others were not when grown in a synthetic community (SynCom). In addition, we compared the SynCom assembly in the absence or presence of the harmful algal cells. If PEF, irrespective of the producing alga, allowed the growth of copiotrophic strains, we found that exometabolites of the two different species led to distinct SynCom composition and that the presence of algal cells also influenced the bacterial assemblage. Overall, our results support the assertion that algal exudates and the presence of algae are the main factors driving the composition of the bacterial community.