Seasonal exometabolites are regulated by essential microbial metabolisms in the oligotrophic ocean

Predictions of how the biogeochemical reservoir of marine dissolved organic matter (DOM) will respond to future ocean changes require an improved understanding of the thousands of individual microbe-molecule interactions which regulate the transformation and fate of DOM. Bulk characterizations of organic matter can mask this complex network of interactions comprised of rich chemical and taxonomic diversity. Here, we present a three-year, depth-resolved time-series of the seasonal dynamics of the exometabolome and the bacterioplankton community at the Bermuda Atlantic Time-series Study (BATS) site. We find both time-series to be highly structured and compositionally distinct across sampling depths. Putative exometabolite identifications (gonyol, glucose 6-sulfate, succinate, and trehalose) indicate that at least a portion of the exometabolome contains rapidly remineralized, labile molecules. We hypothesize that apparent seasonal accumulation of these labile molecules could result from environmental conditions that alter community composition on a seasonal timescale and thus shift the relative proportions of microbial functions that produce and consume the substrates. Critically, we found the composition of seasonal DOM features was more stable interannually than the microbial community structure. By estimating redundancy of metabolic functions responsible for cycling these molecules in BATS metagenomes, we propose a paradigm whereby core microbial metabolisms, either those utilized by all or by a subset of marine microbes, are better predictors of DOM composition than microbial taxonomies. The molecular-level characterization of DOM achieved herein highlights the metabolic imprint of microbial activity in DOM composition and greatly enhances our understanding of the dynamics regulating Earths largest reservoir of organic carbon. Significance statementMarine dissolved organic matter (DOM) is a major carbon reservoir that acts as a critical control on Earths climate. DOM dynamics are largely regulated by a complex web of microbial interactions, but the mechanisms underpinning these processes are not well understood. In a three-year time-series, we found thousands of DOM molecules and microbial taxa exhibited seasonal patterns. Critically, the identity of the microbes was more variable between years than the composition of the DOM molecules. We suggest that shared metabolisms encoded by genes that conduct core microbial functions are responsible for the more stable composition of DOM. This work links DOM molecules with microbial biodiversity, and presents testable predictors of DOM composition in our changing oceans.