The ocean's skin is a massive, living chemical reactor covering most of our planet.

Nature · Published 3 October 2026 Swipe more like this →

In short: Spanning 70% of Earth's surface, the sea surface microlayer is far more than just water. This thin, biofilm-rich interface acts as a global gatekeeper, regulating how the ocean and atmosphere exchange gases, heat, and carbon. It is a bustling, microscopic nursery that remains one of science's most overlooked frontiers.

The sea surface microlayer, or SML, is the boundary interface between the atmosphere and the ocean. While its thickness is operationally defined between 1 and 1000 micrometers, its physical, chemical, and biological properties are distinct from the deeper water below. For decades, researchers often viewed the SML as a niche phenomenon, doubting its presence under typical oceanic conditions. However, modern studies have confirmed its global ubiquity, revealing a complex, aggregate-enriched biofilm teeming with unique microbial communities.

In 1983, Sieburth proposed that this layer consists of a hydrated, gel-like matrix composed of lipids, proteins, and carbohydrates. This view has since been validated, supporting the idea that the SML functions as a biochemical microreactor. Because it is exposed to intense solar radiation and contains high concentrations of organic matter, the SML facilitates unique interfacial reactions that occur much more slowly, if at all, in the bulk water beneath. Ellison et al. estimated in 1999 that 200 teragrams of carbon accumulate here annually, a figure comparable to the amount of carbon settling on the deep ocean floor, though the SML's carbon likely cycles much faster.

The SML is not just a chemical site; it is a critical biological habitat. Zaitsev described the surface and the layer extending down to 5 centimeters as a nursery for the eggs and larvae of various aquatic organisms. As anthropogenic pressures like ocean acidification, warming, and chemical pollution intensify, the SML’s role as a gatekeeper for gas exchange becomes increasingly vital. Despite its importance to climate science and global biogeochemical processes, the SML remains poorly represented in current numerical models, leaving a significant gap in our understanding of how the ocean responds to a changing climate.

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