Species-Specific Associations Between Bacterioplankton and Photosynthetic Picoeukaryotes

Photosynthetic picoeukaryotes are significant contributors to marine primary productivity. Interactions between marine bacterioplankton and picoeukaryotes frequently occur and can have large biogeochemical impacts. Currently, partly due to methodological difficulties for studying microbial associations in situ, these ecological interactions are poorly characterized. Here we use flow cytometry sorting to identify novel bacterial phylotypes found in physical association with photosynthetic picoeukaryotes. Samples were collected on eight occasions at the Santa Cruz wharf on Monterey Bay during summer and fall, 2014. The phylogeny of associated microbes was assessed through clone libraries and Illumina MiSeq sequencing of amplicons of the 16S rRNA gene. In addition, 16 bacterial isolates comprised of 14 taxa were obtained from sorted photosynthetic picoeukaryote cells. The most frequently detected bacterioplankton phyla were Alphaproteobacteria, Bacteriodetes, and Gammaproteobacteria. The sequences from the sorted populations were a community distinct from the unsorted seawater samples suggesting species-specific functional associations. These species-specific patterns were further supported by re-occurring patterns between replicates and sampling dates. The finding of sequences from the free-living genera Synechococcus and Pelagibacter also suggest that photosynthetic picoeukaryotes can be bacterivores, possibly feeding on some of the most numerically abundant bacteria. The results show that specific bacterial phylotypes are found in association with photosynthetic picoeukaryotes. Taxonomic identification of these associations is a prerequisite for further characterizing the interactions, their metabolic pathways and ecological functions.