Faithfulness and promiscuity in phytoplankton-bacteria relationships

Ecological interactions between phytoplankton and bacteria govern the productivity and biogeochemistry of aquatic ecosystems. Yet much of our understanding of these partnerships comes from a narrow set of model systems, meaning that little is known about the diversity of lifestyles among phytoplankton-associated bacteria. Here we single-cell isolated 15 different phytoplankton from the ocean and characterised the evolution of their microbiome for 400 days. We identified: (i) bacterial specialists, associated with only 1-2 phytoplankton, which increased ten-fold in relative abundance through time; (ii) generalists, associated with 3 or more phytoplankton, and (iii) transients, which steadily declined in relative abundance over time. Important functional traits such as chemotaxis, vitamins/secondary metabolite production, and transport of organic compounds were enriched in generalists and specialists compared to transients. Moreover, generalists had greater potential to move between hosts (motility) and release antimicrobial compounds, while specialists displayed a heightened capacity to attach, and catabolise phytoplankton metabolites (DHPS, DMSP, putrescine). Finally, the abundance of generalists and specialists mirror their phytoplankton hosts in the environment. The divergent genomic characteristics of these discrete categories of phytoplankton associates highlight different life history strategies that likely shape phytoplankton microbiomes and govern the ecological and biogeochemical implications of phytoplankton-bacteria partnerships.