Low Nitrogen Fixation Related to Shallow Nitracline Across the Eastern Indian Ocean
Low Nitrogen Fixation Related to Shallow Nitracline Across the Eastern Indian Ocean
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DOI：
10.1029/2022jg007104
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发表时间：
2022-10
期刊：
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通讯作者：
Takuya Sato;T. Shiozaki;F. Hashihama;Mitsuhide Sato;A. Murata;K. Sasaoka;Shin‐ichiro Umeda;
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作者：
Takuya Sato;T. Shiozaki;F. Hashihama;Mitsuhide Sato;A. Murata;K. Sasaoka;Shin‐ichiro Umeda;
东印度洋（EIO）的特点是贫营养的地表水与丰富的灰尘供应，因此，这种环境被认为是提供了一个有利的生态位蓝藻固氮生物。然而，直接测量报告低固氮异养固氮生物占主导地位，这表明EIO具有独特的地球物理特征和营养供应控制固氮，在大西洋和太平洋。在这项研究中，我们调查了整个EIO从孟加拉湾到澳大利亚西部的固氮生物的活动和社区。除澳大利亚西部（82-286 μ mol N m−2 d−1）外，在EIO广泛观察到低固氮（26.1-75.6 μmol N m− 2 d −1）。虽然在EIO的表面硝酸盐浓度低，向上的硝酸盐通量较高，因为较浅的硝态氮比其他贫营养海洋。这造成了广泛的低aeFe：N（气溶胶铁与氮的供应比）条件，导致低固氮率。NifH扩增子测序表明，异养固氮生物在EIO中广泛占主导地位，与蓝藻固氮生物（束毛藻，UCYN-A1）空间分离。在一般情况下，蓝藻固氮生物丰富的孟加拉湾和澳大利亚西部的表面沃茨与高aeFe：N供应，而异养固氮生物占主导地位，在其他广泛的地区。这一发现表明，浅硝态氮和相关的高向上硝酸盐供应的独特的固氮在EIO的关键因素。
The eastern Indian Ocean (EIO) is characterized by oligotrophic surface water with abundant dust supply, and thus, this environment is considered to provide a favorable niche for cyanobacterial diazotrophs. However, direct measurements have reported low nitrogen fixation with the dominance of heterotrophic diazotrophs, suggesting that the EIO has unique geophysical features and nutrient supplies controlling diazotrophy, in contrast to those in the Atlantic and Pacific Oceans. In this study, we investigated the activity and community of diazotrophs across the EIO from the Bay of Bengal to the west of Australia. Low nitrogen fixation was observed broadly in the EIO (26.1–75.6 μmol N m−2 d−1), except in the west of Australia (82–286 μmol N m−2 d−1). Although the surface nitrate concentration was low in the EIO, the upward nitrate flux was higher because of the shallower nitracline than that in other oligotrophic oceans. This created widespread conditions with low aeFe:N (supply ratio of aerosol iron to nitrogen), resulting in low nitrogen fixation rates. NifH amplicon sequencing demonstrated the dominance of heterotrophic diazotrophs widely in the EIO, with spatial segregation from cyanobacterial diazotrophs (Trichodesmium, UCYN‐A1). In general, cyanobacterial diazotrophs were abundant in the surface waters of the Bay of Bengal and west of Australia with a high aeFe:N supply, whereas heterotrophic diazotrophs were dominant in other broad regions. This finding demonstrates that the shallow nitracline and associated high upward nitrate supply are key factors for the unique diazotrophy in the EIO.