An algicidal bacterium shapes the microbiome during outdoor diatom cultivation collapse

Biogeochemistry and productivity of algae-dominated environments is fundamentally influenced by the diversity and activity of bacteria. Namely, algicidal bacteria that prey on algal hosts can control elemental cycling and host populations within outdoor algal ponds used for biofuel production. In this study, we describe the genomic and proteomic signatures of a putative algicidal bacterium,Kordiasp. (familyFlavobacteriaceae), that bloomed during a population-wide crash of the biofuel diatom,Phaeodactylum tricornutum. ThisKordiasp. bloom occurred after 29 days of cultivation in outdoor algal raceway ponds inoculated withP. tricornutum, but not in parallel ponds inoculated withMicrochloropsis salina. Several signatures of antagonism expressed byKordiaduring diatom demise highlights previously unexplored mechanisms that may aid in algicidal activity or bacterial competition, including the type VI secretion system and hydrogen peroxide production. Analysis of accompanying downstream microbiota (primarily of theRhodobacteraceaefamily) provides evidence that cross-feeding is important in supporting microbial diversity during algal demise. Specifically,in situand laboratory data suggest thatKordiaacts as a primary degrader of biopolymers during algal demise, and co-occurringRhodobacteraceaeexploit degradation molecules or scavenge metabolic byproducts for carbon. Further, targeted analysis of 30Rhodobacteraceaemetagenome assembled genomes suggest algal pondRhodobacteraceaecommonly harbor pathways for carbon monoxide oxidation, a potential strategy to persist under competition. Collectively, these observations further constrain the role of algicidal bacteria in the aquatic ecosystem.