Enhancing Role of Nitrogen Fixation in Biogeochemical Cycles of the Pacific Arctic.

The Arctic Ocean is warming rapidly, prompting increased attention to the ecosystem's response to sea-ice loss and the influx of organisms from lower latitudes. Recent studies have shown that these exogenous organisms are affecting the biodiversity of Arctic ecosystems, but their impact on biogeochemical cycles remains unclear. This study provides evidence that diazotrophs transported from outside the Arctic can significantly contribute to the Arctic new production. From 2015 to 2020, we conducted broad observations in the Chukchi and Beaufort Seas during late summer and autumn, measuring nitrogen fixation rates, diazotroph community composition, and key processes related to new production (nitrate assimilation and nitrification). In 2017, we observed a marked increase in nitrogen fixation, especially in the off-shelf region undergoing oligotrophication, where it constituted a significant fraction of the new production (interquartile range 10.8%-62.5%, median: 21.5%). UCYN-A2 (Candidatus Atelocyanobacterium thalassa) emerged as the dominant diazotroph across all regions in that year, with a significant positive correlation between its abundance and nitrogen fixation rates, suggesting its key role in the elevated nitrogen fixation. Water mass analyses, sea ice observation, and numerical simulations indicated that UCYN-A2 likely originated in the Bering Sea and was transported to the Arctic off-shelf region, and proliferated there with increasing temperature as a result of unusually early sea ice melt in 2017. These findings indicate that as warming and earlier sea ice retreat continue, the influence of exogenous diazotrophs on Arctic biogeochemical cycles can become increasingly pronounced.