Evidence for dissolved organic matter diagenetic transformation and associated changes in the microbial community at Ocean Station Papa

Brandon M Stephens1, Craig A Carlson1, Daniel Petras2, Keri Opalk1, Lihini Aluwihare3, Pieter Dorrestein4, Dennis A Hansell5 and Shuting Liu1, (1)University of California Santa Barbara, Marine Science Institute/Department of Ecology, Evolution and Marine Biology, Santa Barbara, CA, United States, (2)University of California San Diego, Scripps Institution of Oceanography, La Jolla, United States, (3)Scripps Institution of Oceanography, La Jolla, United States, (4)University of California San Diego, Collaborative Mass Spectrometry Innovation Center, La Jolla, CA, United States, (5)University of Miami, Miami, United States
Abstract:
Marine dissolved organic matter (DOM) plays an integral role in the biogeochemical cycling of carbon and nutrients, where the activity of DOM is bound by a combination of its chemical composition and the composition of surrounding microbial communities. The ability for microbes to access DOM (i.e., ‘DOM bioavailability’) is estimated either across natural temporal/spatial gradients or over time in dark incubations by monitoring changes to DOM concentrations and the microbial community. For 24 days in August 2018, as part of NASA’s Export Processes in the Ocean from RemoTe Sensing (EXPORTS) program, we sampled over natural gradients and in dark incubations to monitor for DOM composition and 16S microbial community composition changes near Ocean Station Papa (OSP). At surface depths (~5m), we observed greater total dissolved amino acid (TDAA) concentrations (3.0 vs. 1.5 µmol TDAA as carbon) during times when both bacterial production rates doubled and chlorophyll-a doubled (0.15 to 0.30 µg L‑1). When TDAA concentrations were elevated there was an associated detectable removal of DOC after ~6-10 days in dark incubations, providing evidence for enhanced DOM bioavailability as initial TDAA increased. DOM diagenetic transformation was also observed over time in dark incubations as indicated by amino acid-based indicators (e.g., Mol% GABA+B-Ala) and by estimations of oxidation state (by LC-MSxMS), where such DOM trends mimicked those of samples collected across the surface to mesopelagic (~500m) depth trajectory. Additionally, as surface TDAA increased and DOM became more bioavailable, unique initial responding bacteria were of the SAR11 and SAR86 taxa and shifted to increased abundances of Vibrio, Rhodobacter and Oceanospiralles taxa over time in dark incubations. Together, results provide evidence for a tightly coupled DOM-bacteria production-response relationship at OSP where DOM can be quickly removed when more bioavailable. This coupling could help to sustain low levels of production via nutrient recycling at a relatively quiescent nutrient-limited site.