Prochlorococcus predation by a globally abundant filter feeder

Prochlorococcus is the most abundant photosynthetic cell on Earth and is critical to primary productivity and biogeochemical cycles of the open ocean. Appendicularians are ubiquitous gelatinous filter-feeding zooplankton that feed on marine microorganisms including Prochlorococcus. However, the details of this feeding interaction are extremely understudied relative to its potential importance in top-down controls on Prochlorococcus. This is the first study to experimentally examine several dimensions of the feeding interaction between cultivated appendicularians and Prochlorococcus. We found that Prochlorococcus retention rates by the appendicularian Oikopleura dioica increased with prey concentration and predator age. We found that appendicularians grazed equally on the two most globally abundant Prochlorococcus ecotypes HLI and HLII and that the presence of larger diatom prey did not change Prochlorococcus retention rates. The quantitative insight and retention rates provided by this study will help fill gaps in models of the marine carbon cycle and marine microbial community dynamics and biogeography, and expand the knowledge of Prochlorococcus ecology. ImportanceAppendicularians are a known predator of picocyanobacteria, but the details of their feeding on the globally abundant picocyanobacterium Prochlorococcus have not been investigated. We quantified Prochlorococcus retention rates over a range of ecologically relevant conditions, which will inform microbial community predictions and carbon flux models and lead to improved understanding of carbon transfer in the ocean, microbial ecology, and microbial communities.