Heterotrophic bacteria as major nitrogen fixers in the euphotic zone of the Indian Ocean
Heterotrophic bacteria as major nitrogen fixers in the euphotic zone of the Indian Ocean
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DOI：
10.1002/2014gb004886
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发表时间：
2014-10
影响因子：
5.2
通讯作者：
T. Shiozaki;M. Ijichi;T. Kodama;S. Takeda;K. Furuya
中科院分区：
文献类型：
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作者：
T. Shiozaki;M. Ijichi;T. Kodama;S. Takeda;K. Furuya
与大西洋和太平洋相比，人们对印度洋的重氮营养状况知之甚少。我们首先研究了东北季风期间沿69°E(17°N到20°S)的贫营养印度洋真光带内重氮菌群落结构及其固氮活性，该海域普遍存在一条浅硝化线(49-59m)，海表面温度在25°C以上。阿拉伯海表层的溶解铁浓度和铁/硝酸盐+亚硝酸盐的比值明显高于赤道和南印度洋。阿拉伯海的固氮量(24.6-47.1μMolm−2 d−1)也显著大于赤道和南印度洋的固氮量(6.27-16.6μMolm−2 d−1)，表明铁可以控制印度洋的重氮营养。NifH的系统发育分析表明，除阿拉伯海外，研究区大部分重氮菌属于变形杆菌，蓝藻重氮菌不存在。此外，固氮与整个研究区域的光照强度无关。这些结果与印度洋的固氮一致，主要是由异养细菌完成的，而不是蓝藻。蓝藻的低重氮营养归因于在太平洋和大西洋少营养海洋中很少观察到的浅水硝化物线。由于较浅的硝酸线有利于增加向上的硝酸盐通量，蓝藻重氮藻相对于非重氮浮游植物的竞争优势并不像在其他少营养海洋中那样显著。
Diazotrophy in the Indian Ocean is poorly understood compared to that in the Atlantic and Pacific Oceans. We first examined the basin‐scale community structure of diazotrophs and their nitrogen fixation activity within the euphotic zone during the northeast monsoon period along about 69°E from 17°N to 20°S in the oligotrophic Indian Ocean, where a shallow nitracline (49–59 m) prevailed widely and the sea surface temperature (SST) was above 25°C. Phosphate was detectable at the surface throughout the study area. The dissolved iron concentration and the ratio of iron to nitrate + nitrite at the surface were significantly higher in the Arabian Sea than in the equatorial and southern Indian Ocean. Nitrogen fixation in the Arabian Sea (24.6–47.1 μmolN m−2 d−1) was also significantly greater than that in the equatorial and southern Indian Ocean (6.27–16.6 μmolN m−2 d−1), indicating that iron could control diazotrophy in the Indian Ocean. Phylogenetic analysis of nifH showed that most diazotrophs belonged to the Proteobacteria and that cyanobacterial diazotrophs were absent in the study area except in the Arabian Sea. Furthermore, nitrogen fixation was not associated with light intensity throughout the study area. These results are consistent with nitrogen fixation in the Indian Ocean, being largely performed by heterotrophic bacteria and not by cyanobacteria. The low cyanobacterial diazotrophy was attributed to the shallow nitracline, which is rarely observed in the Pacific and Atlantic oligotrophic oceans. Because the shallower nitracline favored enhanced upward nitrate flux, the competitive advantage of cyanobacterial diazotrophs over nondiazotrophic phytoplankton was not as significant as it is in other oligotrophic oceans.