Heterotrophic Diazotrophs in a Eutrophic Temperate Bay (jiaozhou Bay) Broadens the Domain of N2 Fixation in China's Coastal Waters

N-2 fixation in coastal waters represents an overlooked process in the global marine nitrogen cycle, yet the mechanisms and controls are still poorly understood. This study first conducted simultaneous measurements of N-2 fixation rates, primary production, chlorophyll a (Chl a) concentration, organic carbon concentration, and the composition of nitrogen-fixing genes (nifH) in a eutrophic temperate bay (Jiaozhou Bay, the western Yellow Sea) from 2015 to 2017. The activity of N-2 fixation (not detected-1.53 nmol N L-1 h(-1)) were observed at most sampling stations, suggesting that the N-replete conditions in Jiaozhou Bay did not prevent N-2 fixation. The major N-2 fixers in JB were heterotrophic, while N-2 fixation were mainly contributed by > 10 mu m fraction, indicating the possible active N-2 fixation of heterotrophic diazotrophs on large particles or planktonic aggregates. Positive correlations between N-2 fixation and organic carbon concentration were observed in spring. Further estimates of the level of phytoplankton-derived organic carbon and its potential relationship with N-2 fixation suggest that phytoplankton processes may stimulate N-2 fixation in Jiaozhou Bay by increasing the bioavailability of organic carbon or/and providing suitable environmental conditions for N-2 fixation. Our results first infer the co-regulation of physical factors and biological processes on N-2 fixation. As Jiaozhou Bay has been subject to anthropogenic pollution of inorganic nutrients, our results provide insights into the dominant controls on N-2 fixation and relevant biogeochemical processes in coastal waters under anthropogenic disturbance.