Seasonal dynamics of DOM diversity and molecular composition in the NW Mediterranean Sea

In the Mediterranean Sea, an accumulation of dissolved organic matter (DOM) is observed during the stratified summer period, which is decoupled from the spring phytoplankton maxima. This has been classically explained by nutrient limitation, that hampers prokaryotic DOM uptake of the labile DOM released by phytoplankton. However, relatively high heterotrophic prokaryotic activity in summer led us to hypothesize that the accumulated DOM might be recalcitrant and derived from these microorganisms. To test this, we tracked the seasonality of DOM molecular composition and chemical diversity using FT-ICR MS from 2019 to 2021 in the NW Mediterranean Sea, and framed it using a broad suite of environmental and biological parameters. Our results reveal a clear seasonal variation in DOM molecular composition and diversity in the surface mixed layer. Changes in composition reflected a higher proportion of molecular formulae containing CHO in spring and a higher proportion of molecular formulae containing CHOS and CHONS in summer. Proxies of DOM recalcitrance, such as aromaticity, unsaturation, and molecular size, were higher in summer, confirming our hypothesis of an accumulation of recalcitrant DOM in summer. In parallel, an increase in DOM diversity (as number of total molecular formulae, their relative intensity, their average distance (Dist) in the van Krevelen space and their functional diversity) was observed, suggesting that the accumulated DOM in summer is likely the result of the interplay between different processes including dissolved primary production, photodegradation and prokaryotic activity. Our results may have significant implications for carbon sequestration through the microbial carbon pump in the Mediterranean Sea as this DOM accumulated in the surface is likely to be stored once exported into deep layers.