The response of diazotrophs to nutrient amendment in the South China Sea and western North Pacific

[摘要] The availability of iron (Fe) and phosphorus (P) has been shownto be a key factor regulating rates of nitrogen fixation in the westernsubtropical Pacific. However, the relative importance of Fe and P at finer spatialscales between the northern South China Sea (NSCS) and the western boundaryof the North Pacific is poorly constrained. Furthermore, nutrient limitationof specific diazotroph types has not yet been assessed. Here we investigatedthese unknowns by (i) carrying out measurements of finer-scale spatialvariabilities in N 2 fixation rates and diazotroph nifH gene abundancesthroughout these regions and (ii) conducting eight additional Fe andphosphate addition bioassay experiments where both changes in N 2 fixation rates and the nifH gene abundances of specific diazotrophs weremeasured. Overall, nitrogen fixation rates and nifH gene abundances were lowerin the NSCS than around the Luzon Strait and the western North Pacific. Thenutrient addition bioassay experiments demonstrated that N 2 fixationrates in the central NSCS were co-limited by Fe and P, whereas at thewestern boundary of the North Pacific they were P-limited. Changes in theabundances of nifH in response to nutrient addition varied in how well theycorrelated with changes in N 2 fixation rates, and in six out of eightexperiments the largest responses in nifH gene abundances were dominated byeither Trichodesmium or UCYN-B (unicellular diazotrophic cyanobacteria group B). In general, nutrient addition had a relatively restrictedimpact on the composition of the six phylotypes that we surveyed apart fromon UCYN-B. This unicellular cyanobacterium group showed increasedcontribution to the total nifH gene abundance following P addition at siteswhere N 2 fixation rates were P-limited. Our study providescomprehensive evidence of nutrient controls on N 2 fixation biogeographyin the margin of the western North Pacific. Future research that moreaccurately constrains nutrient supply rates to this region would bebeneficial for resolving what controls diazotroph community structure.

[学科分类] 大气科学