Nitrogen Fixation in Denitrified Marine Waters

Nitrogen fixation is an essential process that biologically transforms atmospheric dinitrogen gas to ammonia, therefore compensating for nitrogen losses occurring via denitrification and anammox. Currently, inputs and losses of nitrogen to the ocean resulting from these processes are thought to be spatially separated: nitrogen fixation takes place primarily in open ocean environments (mainly through diazotrophic cyanobacteria), whereas nitrogen losses occur in oxygen-depleted intermediate waters and sediments (mostly via denitrifying and anammox bacteria). Here we report on rates of nitrogen fixation obtained during two oceanographic cruises in 2005 and 2007 in the eastern tropical South Pacific (ETSP), a region characterized by the presence of coastal upwelling and a major permanent oxygen minimum zone (OMZ). Our results show significant rates of nitrogen fixation in the water column; however, integrated rates from the surface down to 120 m varied by ∼30 fold between cruises (7.5±4.6 versus 190±82.3 µmol m−2 d−1). Moreover, rates were measured down to 400 m depth in 2007, indicating that the contribution to the integrated rates of the subsurface oxygen-deficient layer was ∼5 times higher (574±294 µmol m−2 d−1) than the oxic euphotic layer (48±68 µmol m−2 d−1). Concurrent molecular measurements detected the dinitrogenase reductase gene nifH in surface and subsurface waters. Phylogenetic analysis of the nifH sequences showed the presence of a diverse diazotrophic community at the time of the highest measured nitrogen fixation rates. Our results thus demonstrate the occurrence of nitrogen fixation in nutrient-rich coastal upwelling systems and, importantly, within the underlying OMZ. They also suggest that nitrogen fixation is a widespread process that can sporadically provide a supplementary source of fixed nitrogen in these regions.固氮作用（nitrogen fixation）是一类关键的生物地球化学过程，可将大气中的二氮气体（dinitrogen gas）生物转化为氨（ammonia），从而弥补反硝化作用（denitrification）与厌氧氨氧化作用（anammox）所造成的氮损失。目前学界普遍认为，海洋中上述过程所引发的氮输入与氮流失在空间上相互分离：固氮作用主要发生在开阔大洋环境中，主要由固氮蓝细菌（diazotrophic cyanobacteria）介导；而氮流失则发生在缺氧中层水与沉积物环境中，主要通过反硝化细菌与厌氧氨氧化细菌完成。本研究报道了2005年与2007年两次南太平洋东部热带海域（eastern tropical South Pacific, ETSP）海洋考察航次（oceanographic cruises）中测得的固氮速率数据。该海域以沿岸上升流（coastal upwelling）发育与永久性缺氧最小值区（oxygen minimum zone, OMZ）的存在为典型特征。研究结果显示，水柱（water column）中存在显著的固氮速率；不过，两次航次中从表层至120米水深的垂直累计速率（integrated rates）差异可达约30倍（分别为7.5±4.6与190±82.3 μmol m⁻² d⁻¹）。此外，2007年航次的速率测定深度可达400米，结果表明次表层缺氧层对累计固氮速率的贡献约为有氧真光层（euphotic layer）的5倍（次表层缺氧层为574±294 μmol m⁻² d⁻¹，真光层为48±68 μmol m⁻² d⁻¹）。同步开展的分子生物学检测在表层与次表层水体中检出了固氮酶还原酶基因nifH（dinitrogenase reductase gene nifH）。对nifH序列的系统发育分析（phylogenetic analysis）显示，在固氮速率测得最高的采样时段，海域内存在多样性丰富的固氮微生物群落（diazotrophic community）。综上，本研究结果证实了营养盐富集的沿岸上升流系统及其下方缺氧最小值区内均存在固氮作用；同时表明固氮作用是一类广泛分布的过程，可在上述海域间歇性地为其提供固定态氮的补充来源。