Biochemical characteristics of the sea surface microlayer in the central Baltic Sea and potential signatures of cyanobacterial blooms

Abstract

The sea surface microlayer (SML) forms the < 1 mm thin ocean’s boundary with the atmosphere and plays a critical role in mediating air–sea gas exchange and biogeochemical cycling. However, the biological processes shaping its molecular composition remain insufficiently understood. During a research cruise in the central Baltic Sea (Eastern Gotland Basin), we investigated how phytoplankton, including cyanobacteria, influence the biomolecular composition of the SML. Although no major bloom was detected, distinct shifts in phytoplankton composition were observed, leading to pronounced differences in biomolecular characteristics between the SML and underlying water (ULW), and between conditions characterized by high and low cyanobacteria abundance. While SML enrichment patterns and carbohydrate concentrations were comparable to those previously reported for the Western Baltic Sea, concentrations of total amino acids (TAA) and surfactants were substantially higher in this study and under cyanobacteria-dominated phytoplankton conditions. Distinct molecular signatures were associated with different phytoplankton size classes. During periods of high abundance of pico- and nanophytoplankton (P/NP; Synechococcus-dominated), the SML was characterized by elevated surfactant and total combined carbohydrate (TCCHO) concentrations. Furthermore, Synechococcus sp. co-varied with the non-protein amino acid γ -aminobutyric acid (GABA), particularly under high abundance of P/NP. This suggests that the production of surface-active organic matter may be linked to Synechococcus sp. In contrast, under high abundance of microphytoplankton (MP; filamentous and colonial cyanobacteria), particulate amino acids > 20 µm (PAA > 20 µm) and particulate combined carbohydrates > 20 µm (PCCHO > 20 µm) were elevated in the ULW, mirroring particulate organic carbon > 20 µm (POC > 20 µm) and filamentous/colonial cyanobacterial biomass patterns. The significant correlation between MP biomass and POC > 20 µm suggests that the particulate organic carbon pool was largely derived from filamentous/colonial cyanobacteria, even in the absence of a distinct bloom. Together, our results imply that phytoplankton size structure and taxonomy exert distinct biomolecular imprints on SML chemistry in the Central Baltic Sea. The contrasting roles of filamentous/colonial cyanobacteria (proteinaceous signatures) and Synechococcus sp. (carbohydrate/surface-activity imprint) imply community-dependent modulation of surface activity and indicate that future changes in biodiversity potentially impacts air–sea gas exchange in the ocean.