Over the last 2 decades, studies have shown that the sea-surface microlayer covers large areas of the global ocean and, under certain conditions, can even form biofilm-like (and thus unique) habitats between the ocean and the atmosphere. Due to its recurrent biofilm-like features and unique position at the atmosphere–ocean interface, the microlayer is central to a range of global biogeochemical and climate-related processes, including marine carbon cycling, air–sea gas exchange, physical surface processes, and aerosol production. Despite the fact that processes occurring in and across the microlayer have received increasing attention during recent years, our knowledge about the microlayer remains rudimentary. New approaches and technologies have been developed to understand the formation and function of the microlayer on a mechanistic level. For example, the BASS (Biogeochemical processes and Air–sea exchange in the Sea-Surface microlayer; https://uol.de/en/bass) group has been funded to explore the significance of the microlayer as a bio- and photochemical reactor in an unprecedented comprehensive and interdisciplinary approach, and BASS findings will contribute to this special issue (SI). The purpose of the SI is twofold: (1) to present outcomes of a BASS mesocosm study and (2) to present further contributions on the microlayer from the wider research community based on field, mesocosm, modelling, and laboratory studies. This SI will bring together the latest scientific studies to describe the microlayer as an interface between the ocean and atmosphere across different disciplines.

Review process: all papers of this special issue underwent the regular interactive peer-review process of Biogeosciences handled by members of the editorial board as well as guest editors designated by the BG co-editors-in-chief.