||The purposes of this study were to investigate (1) the seasonal and spatial dynamics of cell densities of unicellular N2-fixing organisms (diazotrophs) in the South China Sea; and (2) environmental factors relating to its dynamics. Four cruises, including one in the late spring, one in summer and two in winter were conducted between August 2011 and December 2013 in the area between 20°-22° N and 116°123° E. Samples were collected from surface to 200m in each station. In addition, nutrient enrichment experiments were carried out, including addition of 0.5μM Glucose + 0.5μM Mannitol (+DOC 1μM)、0.4μM NaNO3 (+N)、0.1μM KH2PO4 (+P)、50nM FeCl3 + 20μM EDTA (+Fe) and Dust(+Dust). Unicellular diazotrophs were identified by the whole cell immunofluorescent method, which detected the existence of nitrogenase in the cells. Number of the cell were counted under a fluorescent microscope. Unicellular diazotrophs were grouped according to cell shapes (coccoid(C) and rod(R)), and cell sizes (0.2-1.5μm, 1.5-2μm, > 2μm). In order to compare with the features of the known groups of unicellular diazotrophic cyanobacteria, the group of 0.2-1.5μm C+R diazotrophs was regarded as UCYN-A alike; group of 1.5-2μm C and >2μm C were combined and regarded as UCYN-B alike; and group of 1.5-2μm R and >2μm R were combined and regarded as UCYN-C alike.|
The results showed that cell densities of UCYN-A alike were higher in winter than other seasons, and higher on the shelf than the basin waters. The dynamics could be associated with the enhanced nutrients, including N, P, Fe and DOC, in winter / shelf. Nutrient enrichment experiments indicated that (conducted only in spring and winter) cell densities of UCYN-A alikes were increased significantly after addition of DOC, P or Fe, respectively. Addition of N also resulted to a significant increase in cell densities in spring and summer, but not in winter. This is likely because the nitrate concentrations in the surface waters in winter were higher than that of other two seasons.
Seasonal distributions of UCYN-B alikes and UCYN-C alikes were similar, both higher in spring and winter than summer. There was no significant pattern on their spatial distributions. Their cell densities were positively related to surface concentration of nitrate and phosphate, respectively. This indicated that high cell densities in winter and spring could be attributed to high ambient nutrient concentrations then. The result of nutrient enrichment experiments also supported this assertion. Addition of P or Fe (only conducted in spring and winter) enhanced the biomass significantly. Addition of N, similar to that of UCYN-A alikes, resulted in significant biomass increase only in summer and spring, not in winter. There was no significant change on cell densities after addition of DOC in spring, summer or winter, suggesting that unicellular diazotrophs in these size ranges did not use DOC, or the in situ DOC concentration was enough for their growth. Based on the results of seasonal/spatial distribution and the nutrient enrichment experiments, nutrient concentrations were found crucial to all sizes of unicellular diazotrophs in term of their biomass fluctuations in the South China Sea.
As for vertical distribution, unicellular diazotrophs were mainly found distributed in the euphotic layer (upper water column with light intensity greater than 0.1% of surface light intensity). UCYN-A alikes seemed distributed deeper than UCYN-B alikes or UCYN-C alikes. The ratios of biomass distributed in the euphotic layer relative to that of the upper 200m water column were found significantly higher for UCYN-B alikes and UCYN-C alikes than that of UCYN-A alikes. Cell densities of UCYN-A alikes at 200m sustained at about 35.5 ± 3.9% that of the surface water.