Controls on nitrite oxidation in the upper Southern Ocean: insights from winter kinetics experiments in the Indian sector
Controls on nitrite oxidation in the upper Southern Ocean: insights from winter kinetics experiments in the Indian sector
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DOI：
10.5194/bg-19-3425-2022
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发表时间：
2022-07-20
期刊：
影响因子：
4.9
通讯作者：
Fawcett, Sarah E.
中科院分区：
文献类型：
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作者：
Mdutyana, Mhlangabezi;Marshall, Tanya;Fawcett, Sarah E.
在整个南大洋的冬季，硝化作用是占主导地位的混合层氮循环过程，其中一些硝酸盐在随后的生长季节持续燃料生产力。由于这种硝酸盐构成了浮游植物的再生而不是新的营养源，因此它不会支持大气CO2的净去除。为了更好地了解南大洋硝化作用的控制，我们进行了亚硝酸盐氧化动力学实验，在整个西印度群岛的表面沃茨在冬季。虽然所有实验（总共七个）都产生了与底物浓度的米氏关系，但亚硝酸盐氧化速率仅在亚硝酸盐浓度超过115 +/- 2：3至245 +/- 18 nM时才大幅增加，这表明亚硝酸盐氧化细菌（NOB）需要最低限度（即，“阈值”）亚硝酸盐浓度以产生硝酸盐。亚硝酸盐氧化的半饱和常数范围从134 +/- 8到403 +/- 24 nM，表明南大洋NOB对亚硝酸盐的亲和力相对较高，与培养实验的结果相反。尽管NOB对亚硝酸盐有很高的亲和力，但在南大洋的混合层中，无论季节如何，NOB的浓度很少下降到150 nM以下。在上混合层，我们测量的铵氧化率是2 - 7倍高于亚硝酸盐氧化的一致率，表明亚硝酸盐氧化是在冬季南大洋硝化的限速步骤。铵和亚硝酸盐氧化的脱钩，结合NOB可能的亚硝酸盐浓度阈值，可以解释整个南大洋的混合层全年持续存在的非零亚硝酸盐。此外，亚硝酸盐氧化可能受到溶解铁的限制，在南大洋上部，溶解铁的可用性很低。我们的研究结果对理解南大洋和其他地方的硝化作用、铵和亚硝酸盐分布的控制具有重要意义。
Across the Southern Ocean in winter, nitrification is the dominant mixed-layer nitrogen cycle process, with some of the nitrate produced therefrom persisting to fuel productivity during the subsequent growing season. Because this nitrate constitutes a regenerated rather than a new nutrient source to phytoplankton, it will not support the net removal of atmospheric CO2. To better understand the controls on Southern Ocean nitrification, we conducted nitrite oxidation kinetics experiments in surface waters across the western Indian sector in winter. While all experiments (seven in total) yielded a Michaelis-Menten relationship with substrate concentration, the nitrite oxidation rates only increased substantially once the nitrite concentration exceeded 115 +/- 2:3 to 245 +/- 18 nM, suggesting that nitrite-oxidizing bacteria (NOB) require a minimum (i.e., "threshold") nitrite concentration to produce nitrate. The half-saturation constant for nitrite oxidation ranged from 134 +/- 8 to 403 +/- 24 nM, indicating a relatively high affinity of Southern Ocean NOB for nitrite, in contrast to results from culture experiments. Despite the high affinity of NOB for nitrite, its concentration rarely declines below 150nM in the Southern Ocean's mixed layer, regardless of season. In the upper mixed layer, we measured ammonium oxidation rates that were two- to seven-fold higher than the coincident rates of nitrite oxidation, indicating that nitrite oxidation is the rate-limiting step for nitrification in the winter Southern Ocean. The decoupling of ammonium and nitrite oxidation, combined with a possible nitrite concentration threshold for NOB, may explain the non-zero nitrite that persists throughout the Southern Ocean's mixed layer year-round. Additionally, nitrite oxidation may be limited by dissolved iron, the availability of which is low across the upper Southern Ocean. Our findings have implications for understanding the controls on nitrification and ammonium and nitrite distributions, both in the Southern Ocean and elsewhere.