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 mortality rate of Gamma A using a combination of dilution experiments and quantitative polymerase chain reaction (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 the efficient transfer of fixed nitrogen by Gamma A to higher trophic levels. Although the growth rates of Gamma A did not show clear responses to nutrient amendments, Gamma A abundance had a significant relationship with nutrient concentration and microzooplankton grazing mortality rate. This suggests that microzooplankton grazing, as well as nutrient concentration, plays a vital role in constraining the Gamma A distribution in the Kuroshio region. Our findings highlight the importance of the further in situ quantification of microzooplankton grazing rates to understand the distribution of diazotrophs and the associated nitrogen transfer into the food web.