The shared and distinct roles of Prochlorococcus and co-occurring heterotrophic bacteria in regulating community dynamics

Prochlorococcus is the world's most abundant photosynthetic organism with over 1e27 cells distributed across much of Earth's oceans, and is collectively responsible for almost 10 percent of marine carbon fixation. Naturally co-occurring heterotrophic bacteria at roughly 1e5 to 1e6 cells per mL in the oceans have been shown to increase Prochlorococcus fitness and productivity. Despite this massive scale, our understanding of these globally important interactions remains limited, with past research largely focused on single Prochlorococcus-heterotroph pairings involving only a few species. In this study, we extend this perspective by using synthetic communities containing multiple diverse heterotrophic strains isolated from Prochlorococcus enrichment cultures. Specifically, we isolated the four most abundant co-occurring heterotroph species and used absolute quantification of RNA, DNA, and cell counts over the growth curve to examine both individual Prochlorococcus-heterotroph interactions and interactions within a synthetic community comprising Prochlorococcus and all four heterotrophs.