The Chaetocerosphere: Chaetoceros Species and Associated Prokaryotes in a Coastal Area

The phycosphere, the organic-rich microenvironment surrounding phytoplankton cells, mediates interactions between algae and other microorganisms, mainly prokaryotes. We investigated associations between Chaetoceros spp., one of the most abundant diatom genera in coastal waters, and particle-attached prokaryotic communities. Monthly samples were collected from October 2017 to September 2018 at a fixed station in the Gulf of Naples (Italy). Light microscopy counts of Chaetoceros were integrated with 16S rRNA gene metabarcoding data, and bipartite network analyses were applied to infer putative species-specific associations. Most inferred associations involved taxa previously reported in diatom-associated assemblages, such as Alphaproteobacteria and Bacteroidetes and Archaeal taxa including Thermoplasmata and Marine Group II. The networks included both single and (2-3) grouped Chaetoceros species. Interestingly, most of the species involved were chain-forming, although the functional role of this trait in shaping prokaryotic associations remains unclear. Overall, our results broaden the range of prokaryotic taxa potentially associated with Chaetoceros and highlight new possible interkingdom interactions in coastal marine ecosystems.