Low rates of nitrogen fixation in eastern tropical South Pacific surface waters
Low rates of nitrogen fixation in eastern tropical South Pacific surface waters
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DOI：
10.1073/pnas.1515641113
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发表时间：
2016-04-19
影响因子：
11.1
通讯作者：
Capone, Douglas G.
中科院分区：
文献类型：
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作者：
Knapp, Angela N.;Casciotti, Karen L.;Capone, Douglas G.
东热带南太平洋（ETSP）洋的广大地区的地表水硝酸盐含量低，但磷酸盐含量高。有人提出，这种表面营养物质的分布提供了一个有利于氮-2固定的地球化学生态位，氮-2是海洋氮的主要来源。在这里，我们展示了两次巡航到ETSP的结果，其中N-2的固定率及其对出口生产的贡献是通过一套地球化学和生物测量来确定的。仅在6个站点中的2个站点使用氮同位素质量平衡检测到N-2固定，根据沉积物捕集器通量，固定速率为0至23 μ mol N m(-2) d（-1）。虽然N-2固定的分数重要性没有变化，但当缩放到其他生产力指标时，这两个站点的N-2固定率要高出几倍。无论选择何种生产力指标，这些地区的氮素固定率都低于其他低营养地区，氮同位素预算表明，该地区的氮素固定支持的出口产量不超过20%。虽然绿化带综合短期N-2固定率较高，可达100 μ mol N m(-2) d(-1)，并且在所有六个站点都检测到N-2固定，但来自相同巡航的氮酶基因丰度和表达研究与地球化学数据一致，共同表明N-2固定是ETSP地表水新氮的次要来源。这一发现与假设一致，即尽管磷酸盐相对丰富，铁可能限制N-2在ETSP中的固定。
An extensive region of the Eastern Tropical South Pacific (ETSP) Ocean has surface waters that are nitrate-poor yet phosphate-rich. It has been proposed that this distribution of surface nutrients provides a geochemical niche favorable for N-2 fixation, the primary source of nitrogen to the ocean. Here, we present results from two cruises to the ETSP where rates of N-2 fixation and its contribution to export production were determined with a suite of geochemical and biological measurements. N-2 fixation was only detectable using nitrogen isotopic mass balances at two of six stations, and rates ranged from 0 to 23 mu mol N m(-2) d(-1) based on sediment trap fluxes. Whereas the fractional importance of N-2 fixation did not change, the N-2-fixation rates at these two stations were several-fold higher when scaled to other productivity metrics. Regardless of the choice of productivity metric these N-2-fixation rates are low compared with other oligotrophic locations, and the nitrogen isotope budgets indicate that N-2 fixation supports no more than 20% of export production regionally. Although euphotic zone-integrated short-term N-2-fixation rates were higher, up to 100 mu mol N m(-2) d(-1), and detected N-2 fixation at all six stations, studies of nitrogenase gene abundance and expression from the same cruises align with the geochemical data and together indicate that N-2 fixation is a minor source of new nitrogen to surface waters of the ETSP. This finding is consistent with the hypothesis that, despite a relative abundance of phosphate, iron may limit N-2 fixation in the ETSP.