Marine foams represent compressed sea-surface microlayer with distinctive bacterial communities. Foam

Abstract The occurrence of foams at the oceans’ surface is patchy and generally short-lived. Here we investigated if marine foams as important sea surface phenomena represent a compressed form of the sea-surface microlayer (SML), a <1 mm thick film at the air-sea interface. To date, the bacterial community composition of sea foams, the bacterial role in the foam formation process and community differences to SML and underlying water remains predominantly unknown. Our study deals with the microbial composition of foams, SML and subsurface samples (1-m depth) collected from the North Sea and Indian Ocean. Foams were characterized by increasing abundance of small phototrophic and prokaryotic cells as well as high concentrations of surface-active substances (SAS). Amplicon sequencing revealed distinctive bacterial communities in foams compared to the other two habitats including species of Persicirhabdus and Winogradskyella belonging to particle-attached communities. Comparison of rRNA and DNA based sequenced data suggested that Pseudoalteromonas sp. are highly active and thus might enhance foam formation and stability by producing SAS. Presence of motile Vibrio might argue for active migration towards nutrient-rich foams. Our study shows that foam bacteria might contribute to foam formation with major implications for air-sea exchange processes, biogeochemical cycling and food web functioning.