Sources and transformation of dissolved and particulate organic nitrogen in the North Pacific Subtropical Gyre indicated by compound-specific δ 15 N analysis of amino acids
Sources and transformation of dissolved and particulate organic nitrogen in the North Pacific Subtropical Gyre indicated by compound-specific δ 15 N analysis of amino acids
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DOI：
10.1016/j.gca.2017.07.036
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发表时间：
2018
影响因子：
5
通讯作者：
Yasuhiko T. Yamaguchi;M. McCarthy
中科院分区：
文献类型：
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作者：
Yasuhiko T. Yamaguchi;M. McCarthy
本研究探索利用溶解有机氮和颗粒有机氮(DON、PON)耦合样品的化合物特异性氨基酸氮同位素(δ15NAA)作为一种新的方法来研究相对来源、转化过程以及这两种主要形式的N-循环在海洋水柱中的潜在耦合。我们测量了北太平洋副热带环流从表层到中层深处高分子有机氮和悬浮有机氮中δ-15NAA的分布。一种新的分析方法实现了比以前工作更高的δ15NAA DON测量精度，使我们能够解决以前在δ15NAA签名中模糊的差异，无论是深度还是水池之间的差异。我们认为在DON和PON中，总可水解性氨基酸(TEA)的δ15N值可以代替蛋白质类的δ15N值。结合散装的δ15N值，可以直接评估δ15N的值以及散装、蛋白质和“其他-N”的变化。这些新的测量提出了三个主要结论。首先，表层和中层HMW DON的δ15NAA特征表明主要是异养细菌来源，与表面材料相比，中层HMW DON具有更多降解物质的特征。这些结果与之前的一种说法形成了鲜明对比，即HMW donδ15NA模式本质上是由表层海洋中的蓝藻“预先形成”的，随着深度的变化很小。其次，核电站表层HMW DON和悬浮δ的15NAA值和模式不同，表明这两个N库的来源和循环是非常不耦合的。基于分子δ15N信号，我们提出了一个新的假设，即表层HMW DON的产生最终来自次表层硝酸盐，而混合层中的PON与N2固定和N循环密切相关。相比之下，中层海水HMW DON与悬浮PON的δ15NAA特征比较也表明，中层可能存在一些HMW DON的PON来源。综上所述，这些结果表明，异养细菌(微生物氮泵)将相对不稳定的DON转化为较不稳定的DON，可能是在表层和中层贫营养海洋中DON产生和改变的关键途径。最后，与THA相比，Other-N的δ15N值受到异养变化的影响较小，这可能与氨基糖或其他不太稳定的含氮有机分子的贡献大于预期一致。
This study explores the use of compound-specific nitrogen isotopes of amino acids (δ15NAA) of coupled dissolved and particulate organic nitrogen (DON, PON) samples as a new approach to examine relative sources, transformation processes, and the potential coupling of these two major forms of N cycle in the ocean water column. We measured δ15NAAdistributions in high-molecular-weight dissolved organic nitrogen (HMW DON) and suspended PON in the North Pacific Subtropical Gyre (NPSG) from surface to mesopelagic depths. A new analytical approach achieved far greater δ15NAAmeasurement precision for DON than earlier work, allowing us to resolve previously obscured differences in δ15NAAsignatures, both with depth and between ON pools. We propose that δ15N values of total hydrolysable amino acids (THAA) represents a proxy for proteinaceous ON δ15N values in DON and PON. Together with bulk δ15N values, this allows δ15N values and changes in bulk, proteinaceous, and “other-N” to be directly evaluated.These novel measurements suggest three main conclusions. First, the δ15NAAsignatures of both surface and mesopelagic HMW DON suggest mainly heterotrophic bacterial sources, with mesopelagic HMW DON bearing signatures of far more degraded material compared to surface material. These results contrast with a previous proposal that HMW DON δ15NAApatterns are essentially “pre-formed” by cyanobacteria in the surface ocean, undergo little change with depth. Second, different δ15NAAvalues and patterns of HMW DON vs. suspended PON in the surface NPSG suggest that sources and cycling of these two N reservoirs are surpisingly decoupled. Based on molecular δ15N signatures, we propose a new hypothesis that production of surface HMW DON is ultimately derived from subsurface nitrate, while PON in the mixed layer is strongly linked to N2fixation and N recycling. In contrast, the comparative δ15NAAsignatures of HMW DON vs. suspended PON in the mesopelagic also suggest a possible PON source for some HMW DON in the mid-water column. Together, these results suggest that conversion of relatively labile ON to less labile DON by heterotrophic bacteria (a “microbial nitrogen pump”) may be the key pathway for production and alteration of DON in both the surface and the mesopelagic oligotrophic ocean. Finally, in contrast to THAA, δ15N values of the other-N were substantially less affected by heterotrophic alteration, which may be consistent with a larger than expected contribution of amino sugars, or other less labile nitrogenous organic molecules.