Identifying biological sources and indicators of surfactants in the sea surface microlayer and subsurface waters using fluorescence and microbial community profiling

Abstract. Marine surfactants accumulate in the sea surface microlayer (SML) where they influence gas transfer velocities and air-sea exchange of climate relevant gases. Despite their importance, the biological sources and reliable proxies of marine surfactants remain poorly constrained. This study paired measurements of surfactant concentrations and fluorescent dissolved organic matter (FDOM) composition with bacterial and protist community profiles in the SML and subsurface waters (SSW) across three hydrographic regions and two seasons in the northwestern Atlantic Ocean. Surfactant concentrations were significantly enriched in the SML relative to the SSW, and temporal differences are a major driver of variability in surfactant concentrations, as demonstrated by the highest values recorded during a fall picoeukaryotic bloom. Surfactant concentrations showed a positive correlation with tryptophan-like FDOM, supporting a biogenic source of marine surfactants. Microbial community composition differed between the SML and SSW, with the SML enriched in small, resource-efficient taxa adapted to high-light and low-nutrient conditions. Significant correlations were observed between surfactant concentrations and diverse microbial taxa, including picoeukaryotic phytoplankton such as Ostreococcus, Bathycoccus, and Micromonas; heterotrophic bacteria such as the NS5 marine group, Pseudoalteromonas, SAR86, and Candidatus Actinomarina; as well as phagotrophic protists such as MAST-7B and Cercozoa, and parasitic dinoflagellates from the Dino-Group-I and Dino-Group-II clades. The diversity of surfactant associated microbial candidates suggests that surfactant production reflects a broader range of microbial processes than phytoplankton activity alone, with tryptophan-like FDOM capturing this integrated biological process more effectively than chlorophyll-a concentrations. These findings establish tryptophan-like FDOM as a promising proxy for predicting marine surfactant concentrations and highlight the need for an in-depth understanding of the diverse microbial sources of surfactants.