OPEN-OCEAN OLIGOTROPHIC MICROBIAL COMMUNITIES AND THEIR INTERCONNECTIONS WITH CYANOBACTERIA-DERIVED DISSOLVED ORGANIC MATTER

Abstract

Diverse and complex microbial communities in the ocean play important roles in remineralizing photosynthate materials and transforming labile dissolved organic matter (DOM) to a refractory state within the marine biogeochemical cycle. Picocyanobacteria are one of the most abundant groups contributing to global primary production and their released organic materials are a crucial flux for carbon sequestration in the ocean. To understand the interactions between microorganisms and DOM produced by picocyanobacteria in the ocean, water samples collected from the Gulf Stream in the Atlantic Ocean were amended with Synechococcus-derived DOM and incubated for 1.5 years. The addition of Synechococcus-derived DOM resulted in a dramatic change of the oligotrophic microbial community, resulting in a succession of community composition. Thaumarchaeota became the most dominating phylum after 60 days in response to ammonium accumulation since day 30 and remained in high relative abundance throughout the rest of the 540-day incubation. This surprising phenomenon suggests that Thaumarchaeota could be a significant driver of carbon cycling in the deep ocean. Network analysis showed that 203 nitrogen-containing molecular formulae exhibited strong positive correlations with diverse heterotrophic bacteria, indicating a sequential pattern of microbial production and degradation of such nitrogen-containing compounds mediated by SAR202 bacteria during the 1.5-year incubation. This supports the postulation that SAR202 bacteria play important roles as secondary producers and contribute to the refractory DOM storage in the deep ocean. Contrary to inferences from previous incubation studies, the microbial community in our microcosm did not revert to the initial composition, and several microbial groups that were rare in the initial community became prevalent. DOM fluctuated as dissolved organic nitrogen and phosphorus increased and dissolved organic carbon decreased from day 364 to 540. The oligotrophic microbial community can continuously utilize DOM in the incubation months after the initial amendment of labile Synechococcus-derived DOM, suggesting that diverse generalist microorganisms play long-term roles in the transformation of DOM to a refractory state. This study untangles the intricate connections between the open-ocean oligotrophic microbial community and the DOM derived from open-ocean strain Synechococcus, which provides insights into long-term microbes-DOM dynamics within the marine biogeochemical cycle.