Organic Alkalinity in the Sea-Surface Microlayer: Implications for Ocean Acid-Base Chemistry

The air-sea CO₂ exchange is a critical process in regulating Earth's carbon cycle. At the ocean's surface, the sea-surface microlayer (SML) - a thin, organic-rich layer - serves as the critical interface between the air and sea and acts as a microreactor where unique chemical transformations occur, driven by sunlight, biological activity and surface-active materials. However, its role in air-sea CO₂ exchange is not well explored. In this study, we present the first direct measurements of organic alkalinity (OA) in the SML during a mesocosm experiment simulating a coccolithophore bloom of Emiliania huxleyi, aiming to better understand the contribution of organic matter to the air-sea CO₂ exchange.

Our every third day-resolution data on dissolved inorganic carbon (DIC), total alkalinity (TA), pH, and OA, quantified using back-titration, reveal significant differences between the SML and the underlying water (ULW). OA concentrations in the SML were consistently higher, contributing 8.07% ± 2.60 of TA, 2.58 times higher than the 3.12% ± 1.24 contribution observed in the ULW. This enrichment suggests that the SML serve as a significant reservoir for OA, influencing the overall acid-base balance.

During the coccolithophore bloom phase, we observed that photosynthesis and calcification—the dominant biogeochemical processes—resulted in decreases in both TA and DIC in the SML. Normally, changes in DIC would lead to a decrease in pH (increased acidity), while changes in TA might buffer this effect. However, the observed pH variability could not be explained by DIC and TA alone. Only by considering OA concentrations we can explain the observed pH variability. A strong negative correlation between the OA contribution to TA and pH (r = -0.82, p < 0.05) highlighted OA's role in modulating pH only in the SML. While calcification produces CO₂ and lowers pH through the dissociation of carbonic acid, coccolithophores also release organic acids, including humic-like fluorescent dissolved organic matter (fDOM). These acids may contribute to TA, but their primary effect is to release H⁺ ions, further acidifying the surface layer.

The increased OA in the SML contributes to its buffering capacity, but it does not fully counteract the acidification induced by calcification. These findings underscore the importance of incorporating OA dynamics in studies of the SML, particularly in the context of intense biological activity, such as coccolithophore blooms. Our results suggest that pH changes in the SML cannot be fully explained by TA alone, highlighting the need to consider OA in the analysis of the marine carbon system and the air-sea CO₂ exchange. While the specific organic acids contributing to OA remain unidentified for this work, future research into these compounds will be essential for improving our understanding of OA’s role in modulating the Earth's carbon cycle.