Oxygen minimum zones (OMZs), characterized by depleted dissolved oxygen concentration in the intermediate depth of the water column, are predicted to expand under the influence of global warming. Recent studies in the Eastern Tropical South Pacific Ocean and Arabian Sea have reported that heterotrophic nitrogen fixation is active in the OMZs. In this study, we investigated the community structure of diazotrophs in the OMZ of the Costa Rica Dome (CRD) upwelling region in the Eastern Tropical North Pacific Ocean, using 454-pyrosequencing of nifH gene amplicons. Comparing diazotroph assemblages in different depth strata of the OMZ (200-1000 m in depth), we found a unique diazotroph community in the OMZ core, which was mainly dominated by methanotroph-like diazotrophs, suggesting a potential coupling of nitrogen cycle and methane assimilation. In addition, some OTUs revealed in this study, especially those belonging to the large sub-cluster Vibrio diazotrophicus, were reported to be abundant and expressing the nifH gene in other OMZs. Our results suggest that the unique hydrographic conditions in OMZs may support similar assemblages of diazotrophs, and heterotrophic nitrogen fixation could also be occurring in our studied region. Our study provides the first insight into the composition and distribution of putative diazotrophs in the CRD OMZ.

中文翻译：

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哥斯达黎加圆顶深氧最低区固氮菌群落结构

预计在全球变暖的影响下，以水体中间深度溶解氧浓度耗尽为特征的最低含氧区 (OMZ) 将扩大。最近在东热带南太平洋和阿拉伯海的研究报告称，异养固氮在 OMZ 中很活跃。在这项研究中，我们使用 nifH 基因扩增子的 454 焦磷酸测序研究了东热带北太平洋哥斯达黎加圆顶 (CRD) 上涌区 OMZ 中固氮菌的群落结构。比较 OMZ 不同深度地层（200-1000 m）的固氮菌组合，我们在 OMZ 核心发现了一个独特的固氮菌群落，主要以类甲烷氧化菌为主，表明存在氮循环和甲烷同化的潜在耦合. 此外，本研究中发现的一些 OTU，特别是那些属于大型亚群固氮弧菌的 OTU，据报道在其他 OMZ 中含量丰富并表达 nifH 基因。我们的研究结果表明，OMZ 中独特的水文条件可能支持类似的固氮菌组合，并且我们研究的地区也可能发生异养固氮。我们的研究首次了解了 CRD OMZ 中假定的固氮菌的组成和分布。异养固氮也可能发生在我们研究的地区。我们的研究首次了解了 CRD OMZ 中假定的固氮菌的组成和分布。异养固氮也可能发生在我们研究的地区。我们的研究首次了解了 CRD OMZ 中假定的固氮菌的组成和分布。