Distinct Lipid Compositions in Seawater and the Sea Surface Microlayer Measured with Liquid Chromatography and...

The sea surface microlayer (SML) plays an important role in the air-sea exchange of gases and particles. Surfactants accumulate at and modulate the properties of the SML, but specific chemical and physical properties driving variability in their distribution in the SML remain unclear. Lipids are a type of surfactant in the ocean and are biomarkers for dissolved organic matter (DOM) sources. We collected paired SML and associated subsurface seawater (SSW) samples over three time points in the North Atlantic Ocean in September 2022 and used liquid chromatography and high-resolution mass spectrometry (LC-MS) to identify 163 and 189 lipids in the SML and SSW, respectively. Of those, just 33 were identified in both layers. Lipids were classified as glycolipids, phospholipids, triglycerols, diacylglycerides, monoacylglycerides, and betaine lipids, as well as lipid byproducts, including free fatty acids and polyunsaturated aldehydes. Enrichment factors for each lipid class show overall enrichment in the SML (average of 1.68 ± 1.43 for all classes (n = 28) over all times (n = 3)), varying throughout the day (3.33 ± 4.13 for Night, 0.79 ± 1.76 for Afternoon, and 0.92 ± 1.61 for Evening). Based on these enrichments and ratios of specific lipids, we show that the SML and SSW are unique microenvironments, with bacterial lipids and degradation products enriched in the SML relative to the SSW. Bacteria and DOM degradation processes may be important for shaping the lipid and thus surfactant composition in the SML. Determining the types of lipids and their sources and degradation processes in the SML may improve our understanding of the sources and properties of surfactants and thus their influence on air-sea exchange processes.