Differential responses of bacteria to diatom-derived dissolved organic matter in the Arctic Ocean

The Arctic sea ice cover is undergoing an unprecedented decline due to climate change. This loss may result in the earlier start of ice algae blooms and more intense phyto-plankton blooms, leading to higher concentrations of dissolved organic matter (DOM) derived from primary production. We investigated the response of early summer Arctic bacterial communities to the addition of Arctic diatom-derived DOM through biodegradation experiments in Baffin Bay. DOM produced by the planktonic diatom Chaetoceros neogracilis and the sea ice diatom Fragilariopsis cylindrus was added to seawater from 3 stations with different ice cover (2 ice zones and 1 open water zone) for 12 d. At the 3 stations, the addition of inorganic nutrients (PO4 and NO3) was not sufficient to stimulate bacterial growth compared to the controls, suggesting that bacteria were mainly limited by organic carbon. The addition of DOM from C. neogracilis stimulated bacterial abundance and production, with a more pronounced response in the ice zone compared to the open water zone. The enhanced bacterial metabolism was ac companied by changes in the bacterial community composition determined by 16S rRNA sequencing, driven by operational taxonomic units (OTUs) related to Pseudoalteromonas and Polaribacter that increased in relative abundance with DOM addition. Moreover, in the ice zone, DOM from C. neogracilis induced a priming effect on the bacterial utilization of ambient DOM. Our findings suggest that phytoplankton blooms, through the production of labile organic matter, will strongly affect bacterial heterotrophic activity, composition and dissolved organic carbon cycling in the Arctic Ocean.