The sea surface microlayer (SML) is the boundary layer between the atmosphere and ocean, spanning the uppermost ~1 mm of the ocean. The SML is typically enriched with organic matter and microbial cells creating a distinct organic film between the ocean and atmosphere. The existence of the SML is a global phenomenon, and due to its unique position, all material and energy exchanged between the ocean and atmosphere has to pass through this interfacial boundary layer. A new emerging consensus in the literature describes the SML as biofilm-like and microbial-rich habitat. The global prevalence of the SML, its unique position between the ocean and atmosphere and re-current biofilm-like features has recently pushed the SML into a central role in ocean and climate science.

Our knowledge about the SML processes remains rudimentary, because the quantification of processes is difficult to achieve at the required scales and due to logistical constraints in sampling and in situ measurements with an interdisciplinary approach. The dynamic interaction of the SML with both the atmosphere and the ocean is largely unknown and currently does not allow any assessment of the extent to which the SML influences biogeochemical cycling in the upper ocean and chemo-physical processes in the lower atmosphere. Many of these questions are related to the dynamics of the formation of biofilm-like matrixes, their reactivity, and air–sea exchange processes (gas, heat, momentum, particles).

Our overall objective is to explore the significance of the SML as a biogeo- and photochemical reactor and how its reactivity influences air–sea interactions. The proposed research unit will provide new insights into the enrichment of organic matter in the SML, and how well-adapted microbes and (photo)- chemistry transform organic matter. Moreover, the photochemical production of trace gases and the presence of surfactants will connect these processes to air-sea exchange processes. BASS will also investigate the coupling between the SML and bulk water as such coupling determines the distribution of unique products of bio- and photochemical processes in the SML into the upper ocean.

Eight subprojects will cooperate in joint field campaigns in the open North Sea, a joint mesocosm study at the Sea Surface Facility (University Oldenburg), and a joint experiment at the wind-wave tunnel Hamburg (University Hamburg). BASS will bundle interdisciplinary expertise, advanced technologies for observations at millimeter scales or below, core infrastructures for controlled experiments, and state-of-the-art analytical facilities on molecular and cellular levels. With this framework, BASS will push observations of the SML towards unprecedented spatial and temporal scales and establish a mechanistic description of the biogeochemistry of the SML and its effect on exchange processes, including physical models.