Seasonal patterns of DOM molecules are linked to microbial functions in the oligotrophic ocean.

Hundreds of thousands of individual microbe-molecule interactions regulate the flux, transformation, and fate of carbon stored in the climatically important reservoir of marine dissolved organic matter (DOM). While marine microbial communities have been characterized at high resolution for over a decade, observations of the molecules cycled by the microbial-chemical network at similar resolution are limited. In addition, bulk characterizations of DOM can mask the complex network of interactions comprised of rich chemical diversities. Here, we present a three-year, depth-resolved, molecular time-series of DOM and prokaryoplankton at the Bermuda Atlantic Time-series Study (BATS) site. Both time-series exhibited seasonality that was compositionally distinct and primarily endemic to one sampling depth. We also putatively identified four exometabolites (gonyol, glucose-6-sulfate, succinate, and trehalose) that exhibit seasonal accumulation. We hypothesize these patterns 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 observed the interannual composition of seasonal DOM molecules to be more stable than the taxonomy of the microbial community. This points to an important role of functional redundancy in regulating DOM composition. We tested this observation by querying metagenomes for pathways that utilize metabolic by-products putatively identified in the DOM time-series. We find that core microbial metabolisms, either those required by all or by a subset of marine microbes, are important predictors of DOM composition. The molecular-level characterization of DOM herein highlights the potential imprint of microbial activity on seasonal DOM composition.IMPORTANCEMarine dissolved organic matter (DOM) is a major carbon reservoir that acts as a critical control on the Earth's climate. DOM dynamics are largely regulated by a complex web of chemical-microbial interactions, but the mechanisms underpinning these processes are not well understood. In a three-year time-series, we found that the identity of the microbes is more likely to change between years than the composition of the DOM molecules. The taxonomic variability suggests that metabolisms shared across taxa, encoded by genes that conduct core microbial functions, are responsible for the more stable composition of DOM. While more than three decades of marine prokaryoplankton time-series are available, a similar reference for DOM molecules was missing. This time-series provides an improved understanding of the different responses of DOM molecules and microbes to seasonal environmental changes.