Low nitrate availability promotes diatom diazotroph associations in the marginal seas of the western Pacific

Heterocystous cyanobacteria that form diatom diazotroph associations (DDAs) are important N-2-fixers in tropical and subtropical oceans, but factors affecting their symbioses and abundances are not well understood. We investigated the seasonal dynamics of diazotrophic cyanobacteria Richelia and Calothrix (cyanobionts) and their DDAs in the South China Sea (SCS) and the Kuroshio. Cyanobiont abundance and the proportions of diatoms forming DDAs, measured by symbiotic percentages (SPs), were determined, and a nitrate addition experiment was carried out to discern the effects of nitrate availability on DDAs. Vertically-integrated cyanobiont abundance was higher in summer than winter, higher in the Kuroshio than the SCS, and positively correlated with nitracline depth. On average, 83% of the cyanobionts were distributed above the nitracline. SPs were high when the nitracline was deep. Hemiauloid diatoms Hemiaulus membranaceus and H. sinensis were more likely to form DDAs than the rhizosolenoid diatom, Rhizo solenia clevei. Hemiauloids were abundant in the warm seasons when the SPs were high as the nitracline deepened; rhizosolenoids were abundant in winter when the SPs were low as the nitracline shoaled. Although enrichment with nitrate dramatically reduced the SP and increased the abundance of R. clevei, the abundances of H. membranaceus and H. sinensis were not affected, suggesting that hemiauloids were more dependent than rhizosolenoids on the nitrogen fixed by Richelia. Low nitrate availability increased the abundances of symbiotic cyanobionts and promoted their association with host diatoms. Nitrate depletion might thus drive the growth of hemiauloid or rhizosolenoid diatoms when they form DDAs with N-2-fixing cyanobacteria.