Nitrogen fixation rates diagnosed from diurnal changes in elemental stoichiometry
Nitrogen fixation rates diagnosed from diurnal changes in elemental stoichiometry
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DOI：
10.1002/lno.10815
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发表时间：
2018-09-01
影响因子：
4.5
通讯作者：
Follows, Michael J.
中科院分区：
文献类型：
--
作者：
Follett, Christopher L.;White, Angelicque E.;Follows, Michael J.
2015年7月，在贫营养的北太平洋副热带环流中，在夏威夷以北遇到的反气旋涡流的表面沃茨中，测量了20-200 μ m粒径部分的碳、氮和磷（分别为C、N和P）组成和元素比。所观察到的颗粒物N：P比在昼夜循环中波动约2倍。我们提出了一个简单的数学论证，将这种变化与生物固氮速率联系起来，并计算出这种大小级别的固氮速率为>= 13 nmol L-1 d（-1）。该值高于用同位素示踪剂测量的同时瓶孵育率，但与该地区的历史率测量结果一致。作为我们的方法的确认，在C：N：P的实验室培养的固氮属束毛藻的昼夜变化进行了测量。在实验室中，我们表明，从化学计量的时间序列固氮的估计是等同于那些直接来自质量平衡。来自示踪剂测量和颗粒化学计量在该领域的固氮率之间的差距表明，大固氮菌可能被低估，在小体积（类似于4 L）瓶孵育的结果，无论是空间异质性或垂直迁移的大细胞。否则，固氮以外的过程必然会引起所观察到的化学计量变化。这种方法代表了一种新的和可扩展的手段，定量原位固氮率从昼夜变化的大小分级化学计量。我们还推断在20-200亩大小的类的碳固定，生长速率和磷的吸收。
The carbon, nitrogen, and phosphorus (C, N, and P, respectively) composition and elemental ratios were measured in the 20-200 mu m size fraction during July 2015 in the surface waters of an anticyclonic eddy encountered north of Hawaii in the oligotrophic North Pacific Subtropical Gyre. The observed particulate N : P ratio fluctuated by approximately a factor of two over the diel cycle. We present a simple mathematical argument connecting this change to a rate of biological nitrogen fixation, and calculate the nitrogen fixation rate to be >= 13 nmol L-1 d(-1) for this size class. This value is higher than simultaneous bottle-incubation based rates measured with isotopic tracers, yet is consistent with historic rate measurements from the region. As confirmation of our methods, diurnal changes in C : N : P of laboratory cultures of the diazotrophic genus Trichodesmium were measured. In the laboratory, we show that estimates of nitrogen fixation from stoichiometric time series are equivalent to those derived directly from mass balance. The disparity between nitrogen fixation rates derived from tracer measurements and particulate stoichiometry in the field suggests that large diazotrophs may be underestimated in small volume (similar to 4 L) bottle incubations as a result of either spatial heterogeneity or vertical migration of large cells. Otherwise, processes other than diazotrophy must cause the observed changes in stoichiometry. This approach represents a novel and scalable means of quantifying in situ nitrogen fixation rates from diurnal changes in size-fractionated stoichiometry. We also infer carbon fixation, growth rates, and phosphorus uptake in the 20-200 mu m size class.