N2 fixation rate and diazotroph community structure in the western tropical North Pacific Ocean  

作　　者：Run Zhang Dongsheng Zhang Min Chen Zhibing Jiang Chunsheng Wang Minfang Zheng Yusheng Qiu Jie Huang 

机构地区：[1]College of Ocean and Earth Sciences,Xiamen University,Xiamen 361102,China [2]Key Laboratory of Marine Ecosystem and Biogeochemistry,State Oceanic Administration/Second Institute of Oceanography,Ministry of Natural Resources,Hangzhou 310012,China [3]Ministry of Education Key Laboratory for Earth System Modeling/Department of Earth System Science,Tsinghua University,Beijing 100084,China

出　　处：《Acta Oceanologica Sinica》2019年第12期26-34,共9页海洋学报（英文版）

基　　金：The National Basic Research Program of China under contract No.2015CB452903;the Foundation of China Ocean Mineral Resources R&D Association under contract No.DY135-E2-2-03;the Science and Technology Basic Resources Investigation Program of China under contract No.2017FY201403;the National Natural Science Foundation of China under contract Nos 41676174,41206104 and 41876198

摘　　要：In the present study, we report N2 fixation rate(15N isotope tracer assay) and the diazotroph community structure(using the molecular method) in the western tropical North Pacific Ocean(WTNP)(13°–20°N, 120°–160°E). Our independent evidence on the basis of both in situ N2 fixation activity and diazotroph community structure showed the dominance of unicellular N2 fixation over majority of the WTNP surface waters during the sampling periods.Moreover, a shift in the diazotrophic composition from unicellular cyanobacteria group B-dominated to Trichodesmium spp.-dominated toward the western boundary current(Kuroshio) was also observed in 2013. We hypothesize that nutrient availability may have played a major role in regulating the biogeography of N2 fixation.In surface waters, volumetric N2 fixation rate(calculated by nitrogen) ranged between 0.6 and 2.6 nmol/(L·d) and averaged(1.2±0.5) nmol/(L·d), with <10 μm size fraction contributed predominantly(88%±6%) to the total rate between 135°E and 160°E. Depth-integrated N2 fixation rate over the upper 200 m ranged between 150 μmol/(m^2·d)and 480 μmol/(m^2·d)average(225±105) μmol/(m^2·d). N2 fixation can account for 6.2%±3.7% of the depthintegrated primary production, suggesting that N2 fixation is a significant N source sustaining new and export production in the WTNP. The role of N2 fixation in biogeochemical cycling in this climate change-vulnerable region calls for further investigations.In the present study, we report N2 fixation rate(15N isotope tracer assay) and the diazotroph community structure（using the molecular method） in the western tropical North Pacific Ocean（WTNP）（13°–20°N, 120°–160°E）. Our independent evidence on the basis of both in situ N2 fixation activity and diazotroph community structure showed the dominance of unicellular N2 fixation over majority of the WTNP surface waters during the sampling periods.Moreover, a shift in the diazotrophic composition from unicellular cyanobacteria group B-dominated to Trichodesmium spp.-dominated toward the western boundary current（Kuroshio） was also observed in 2013. We hypothesize that nutrient availability may have played a major role in regulating the biogeography of N2 fixation.In surface waters, volumetric N2 fixation rate（calculated by nitrogen） ranged between 0.6 and 2.6 nmol/（L·d） and averaged（1.2±0.5） nmol/（L·d）, with <10 μm size fraction contributed predominantly（88%±6%） to the total rate between 135°E and 160°E. Depth-integrated N2 fixation rate over the upper 200 m ranged between 150 μmol/（m2·d）and 480 μmol/（m2·d）(average（225±105） μmol/（m2·d）. N2 fixation can account for 6.2%±3.7% of the depthintegrated primary production, suggesting that N2 fixation is a significant N source sustaining new and export production in the WTNP. The role of N2 fixation in biogeochemical cycling in this climate change-vulnerable region calls for further investigations.

关 键 词：western tropical North Pacific Ocean(WTNP) N2 fixation 15N isotope tracer assay unicellular diazotroph 

分 类 号：P734[天文地球—海洋化学]