Heterotrophic diazotrophs in a eutrophic temperate bay (Jiaozhou Bay) broadens the domain of N2 fixation in China's coastal waters

Abstract

N2 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 N2 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 N2 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 N2 fixation. The major N2 fixers in JB were heterotrophic, while N2 fixation were mainly contributed by > 10 μm fraction, indicating the possible active N2 fixation of heterotrophic diazotrophs on large particles or planktonic aggregates. Positive correlations between N2 fixation and organic carbon concentration were observed in spring. Further estimates of the level of phytoplankton-derived organic carbon and its potential relationship with N2 fixation suggest that phytoplankton processes may stimulate N2 fixation in Jiaozhou Bay by increasing the bioavailability of organic carbon or/and providing suitable environmental conditions for N2 fixation. Our results first infer the co-regulation of physical factors and biological processes on N2 fixation. As Jiaozhou Bay has been subject to anthropogenic pollution of inorganic nutrients, our results provide insights into the dominant controls on N2 fixation and relevant biogeochemical processes in coastal waters under anthropogenic disturbance.