Molecular response diversity of dissolved organic matter under ocean warming

How dissolved organic matter (DOM) composition varies along ocean temperature gradients is critical for understanding its role in future ocean carbon cycling. Here, we use a highly resolved dataset of over 800 DOM samples covering the surface waters to the deep Atlantic, Southern, and Pacific oceans to examine DOM thermal responses, defined as temperature-associated changes in DOM molecular relative abundance. Towards deeper waters, both response strength (i.e., net response) and diversity (i.e., variation among molecules) decline. However, these two dimensions show opposite trends with apparent concentrations of more recalcitrant molecules: the net response becomes more negative, while response diversity increases. Consistent with this divergence, response diversity is more strongly associated with photochemical molecular signatures than is the response strength, suggesting a possible photochemical imprint on persistent DOM. By projecting global ocean thermal responses from 1950 to 2020 using environmental temperature, salinity, and radiation, we predict that increases in response diversity are most apparent below 1000 m, which could potentially increase the recalcitrant deep-ocean carbon sink. These findings identify molecular response diversity as a distinct dimension of DOM responses to ocean warming and highlight the potential role of photochemical imprints in shaping marine DOM persistence under climate change.