Grazing mortality as a controlling factor in the uncultured non-cyanobacterial diazotroph (Gamma A) around the Kuroshio region

Abstract. Nitrogen-fixing microorganisms (diazotrophs) significantly influence marine productivity by reducing nitrogen gas into bioavailable nitrogen. Recently, non-cyanobacterial diazotrophs (NCDs) have been identified as important contributors to marine nitrogen fixation. Among them, Gamma A is one of the best-studied marine NCDs because of its ubiquitous occurrence; however, the factors controlling its distribution remain unknown. In particular, the importance of microzooplankton grazing as a top-down control has not yet been examined. In this study, we investigated the diazotroph community structure using nifH amplicon sequencing, and quantified the growth and microzooplankton grazing rate on Gamma A using a combination of dilution experiments and quantitative PCR in well-lit waters at the northern edge of the Kuroshio Current off the southern coast of Japan. In the study region, Gamma A was ubiquitous and dominant in the diazotroph communities, whereas cyanobacterial diazotrophs had lower relative abundances. The microzooplankton grazing rate of Gamma A was significantly higher than that of the whole phytoplankton community and was generally balanced with its growth rate, suggesting efficient transfer of fixed nitrogen by Gamma A to higher trophic levels. Although the in situ growth rates of Gamma A did not show clear responses to nutrient amendments, Gamma A abundance had a significant negative relationship with microzooplankton grazing. This suggests that microzooplankton grazing, rather than nutrient concentration, plays a vital role in constraining Gamma A distribution in the Kuroshio region. Our findings highlight the importance of further in situ quantification of microzooplankton grazing rates to understand the distribution of diazotrophs and its associated nitrogen transfer into the food web.