Sea foams are ephemeral hotspots for distinctive bacterial communities contrasting sea-surface microlayer and underlying surface water.

The occurrence of foams at oceans' Surfaces is patchy and generally short-lived but a detailed understanding of bacterial communities inhabiting sea foams is lacking. Here we investigated how marine foams differ from the sea-surface microlayer (SML), a <1 mm thick layer at the air-sea interface and underlying water from 1 m depth. Samples of sea foams, SML and underlying water collected from the North Sea and Timor Sea indicated that foams were often characterized by a high abundance of small eukaryotic phototrophic and prokaryotic cells as well as a high concentration of surface-active substances (SAS). Amplicon sequencing of 16S rRNA (gene) revealed distinctive foam bacterial communities compared to SML and underlying water, with high abundance of Gammaproteobacteria. Typical SML dwellers such as Pseudoalteromonas and Vibrio were highly abundant, active foam inhabitants and thus might enhance foam formation and stability by producing SAS. Despite a clear difference in the overall bacterial community composition between foam and SML, the presence of SML bacteria in foams supports previous assumptions that foam is strongly influenced by the SML. We conclude that active and abundant bacteria from interfacial habitats potentially contribute to foam formation and stability, carbon cycling and air-sea exchange processes in the ocean.