The Role of Phylogeny in Associations Between Marine Phytoplankton and Heterotrophic Bacteria

Empirical studies show coupling between phytoplankton and heterotrophic bacterioplankton across marine ecosystems through correlations between bacterial cell abundances and bulk phytoplankton measurements of chlorophyll-a or primary production. However, it is still not known whether associations between heterotrophic and autotrophic microbes are mediated solely through bulk carbon pools or if specific cellular interactions are involved. We examined the coupling of heterotrophic bacteria and specific phytoplankton populations in a dynamic, coastal ecosystem and explored the phylogenetic diversity of heterotrophic bacteria associated with a model phytoplankton taxon. Using flow cytometry to count distinct microbial groups, we found heterotrophic bacteria and phytoplankton abundances exhibit similar seasonal dynamics at the coastal sampling site, indicative of close coupling between these organisms. Heterotrophic bacterial abundances were more tightly correlated with small eukaryotic phytoplankton than either cyanobacteria group, Prochlorococcus or Synechococcus. Given our finding that associations between bacteria and eukaryotic phytoplankton drive ecologically important heterotroph-phytoplankton coupling, we examined the community composition of heterotrophic bacteria in culture with a model diatom, Thalassiosira rotula using 16S rRNA gene sequencing. Results indicate that strains of T. rotula support unique heterotrophic bacterial taxa but that all T. rotula also associate with a common set of bacterial phylotypes, comprised of members of the Alphaproteobacteria. Characterizing the phylogenetic associations between heterotrophic bacteria and phytoplankton is essential for identifying factors shaping phytoplankton-bacteria interactions and their role in organic matter processing, trophic dynamics and biogeochemical cycles in marine ecosystems.