||Heterocystous cyanobacteria (cyanobionts), including Richelia intracellularis and Calothrix rhizosoleniae, that form diatom diazotroph associations (DDAs) are important N2 fixers in the nitrate-limited tropical and subtropical oceans. Richelia are usually symbiotic (SYM) inside the diatoms Rhizosolenia, Guinardia, and Hemiaulus, while Calothrix epiphytically on the diatoms Chaetoceros and Bacteriastrum. Little is known about how rare the distributions of free-living cyanobionts (FL). The factors and mechanisms controlling their symbioses and abundances are not well understood.|
Cyanobiont abundance and the proportions of diatoms forming DDAs, measured by symbiotic percentages (SPs), were determined in the South China Sea (SCS) and the Kuroshio in 29 cruises conducted between March 2001 and May 2010 and an onboard nitrate addition experiment was carried out to discern the effects of nitrate availability on DDAs. On average, the population of cyanobionts was consisted of 90% of SYM and 10% of FL. The abundance of all cyanobiont (SYM plus FL), between 0.2-2.6×106 heterocysts m-2, was higher in summer than winter, higher in the Kuroshio than the SCS, and positively correlated with surface seawater temperature and nitracline depth, the proxy of nitrate availability. Over 82% of the cyanobionts were distributed above the nitracline. SPs were high when the nitracline was deep. Hemiaulus membranaceus and H. sinensis were more likely to form DDAs than Rhizosolenia clevei. Hemiaulus spp. were abundant in the warm seasons when the SPs were high as the nitracline deepened; Rhizosolenia clevei were abundant in winter when the SPs were low as the nitracline shoaled. Although nitrate enrichment reduced the SP and increased the abundance of R. clevei, the abundances of H. membranaceus and H. sinensis were not affected, suggesting that Hemiaulus spp. were more dependent than Rhizosolenia on the nitrogen fixed by Richelia. Low nitrate availability increased the abundances of SYM and promoted their association with host diatoms. The effects of other factors such like temperature, phosphate, and iron were need to be proved in the further experiments.
The abundance of FL was rare (<0.3×106 heterocysts m-2) in the SCS and the Kuroshio. FL rareness was hypothesized of being sinking fast to the deep, with longer filaments sank faster. The hypothesis was examined using a mono-cultured FL to measure the sinking rates with various filament lengths, using three dominant species of diatoms in a mix-culture as comparisons. Laboratory experiments indicated that FL with Long (12-21 cells) filament sank (1.16 ± 0.11 m d-1) significantly faster than that of the diatoms (0.45 ± 0.04 m d-1), but not for Short (<12 cells) FL (0.50 ± 0.05 m d-1). This with the results of over 97% of FL with Short filament in the SCS and the Kuroshio suggested that Long filaments sank rapidly and left Short FL being observed commonly in the euphotic oceans. But the FL rareness cannot be explained by its fast sinking because Short FL had sinking rates not significantly different from the diatoms. The sinking hypothesis should be rejected. The growth rate of FL (0.25 ± 0.03 d-1) measured in this study was lower than that of SYM recorded in the reference, implying another possible cause of FL rareness.