SP 1.5 Molecular characterization of dissolved organic matter in the sea surface microlayer (SML) and its influence on the inorganic carbon cycle

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Our motivation is to understand the role of dissolved organic matter (DOM) in the SML as a key component influencing air-sea gas exchange, carbonate chemistry, and ecosystem functioning of associated organisms (Engel et al., 2017). During our preliminary work, we have found indications for a yet unknown link between DOM and carbonate chemistry in the SML, as well as for high spatio-temporal dynamics in DOM composition. However, despite its apparent ephemeral character, the SML-DOM geometabolome (i.e., the entirety of the DOM pool that is produced and modified by biotic and abiotic processes) is still largely uncharted territory. Further, a mechanistic understanding of the driving forces behind DOM compartmentalization into distinct chemical fractions in the SML is hindered by the limited availability of data from a larger range of study sites under differing environmental and experimental conditions, as well as a lack of interdisciplinary studies combining physics, geochemistry, and biology. In other words, we are missing basic (organo-)geochemical information from the largest air-water interface on earth, with unknown consequences for the associated exchange of climate-relevant gases. In this project, we seek to fill this gap by complementary measurements of DOM composition and inorganic carbon system parameters. Relevance to the research unit BASS stems from the aim of our subproject to provide the missing basic biogeochemical information of the SML-DOM inventory, and set it into context with SML ecosystem processes including DOM production (SP1.1) as well as microbial (SP1.2) and photochemical (SP1.4) turnover. Furthermore, we will examine the contribution of the DOM geometabolome to the acid-base balance of the SML, which we expect to affect gas equilibria in the interface—within the carbonic acid system in particular—and therefore, impact greenhouse gas fluxes (SP2.1).