Nitrous oxide and nitrogen gas production in the Arabian Sea - a process and community based study

氮元素(N)是地球上生命的关键，在死亡和腐烂后，它不断在大气、生物量(动物、植物、微生物)和返回大气中循环。在这个氮循环的中心，在陆地和海洋中，是一种被称为细菌的各种各样的微生物。在大气中，氮主要以氮气的形式存在，但也有少量的一氧化二氮(N2O)，这是一种强有力的温室气体。去除氮的过程，如氮气，可以调节植物的生长，间接地调节大气中二氧化碳(CO2)的平衡，从而影响气候。全球海洋的大片区域被氧气(O2)完全氧化或“饱和”，但有些部分没有。例如，黑海完全缺乏90米以下的氧气，其他地区，如非洲西南部的本格拉上升流也缺乏氧气/这两个地区都有氧气最低限区或OMZ。正是这些O2‘饥饿’区域或OMZ对全球海洋中氮的去除和N2O的产生都具有重要意义。我们感兴趣的是阿拉伯海的OMZ，由于它的大小(法国和德国的总和)，在全球N循环中扮演着重要的角色/负责全球海洋20%的N2O产生和30%的N去除。虽然阿拉伯海在全球氮循环中的重要性是已知的，但导致氮气和N2O产生的代谢物过去和现在在一定程度上仍然不清楚。最近，通过更仔细地观察，并结合N示踪剂(15N同位素)，我们第一个实际测量了阿拉伯海中部的N2O产量。此外，我们还证明，大多数(&gt；95%)产生的N2O可以用一个简单的途径来解释，即在没有O2的情况下，亚硝酸盐(NO2-)代谢成N2O。此外，我们还测量了两种已知的氮气生成途径，即反硝化氮气和厌氧氨氧化氮气(Anammox)。然而，很大一部分氮气来自其他地方，我们有证据表明，这种额外的氮气生产新途径直接与腐烂生物质的新陈代谢有关。然而，事情并没有这么简单。一种N2O生成途径需要一些复杂性，才能产生具有阿拉伯海中部OMZ特征的高浓度和低浓度N2O。我们的15N示踪剂再次揭示了这一点，因为它表明，在NO2-(NO2-到NO再到N2O到N2)的代谢过程中，产生的氮气与N2O的比例并不是固定的，似乎是“灵活的”。例如，在水柱N2O浓度较高的地方，我们测量了NO2产生的氮气与N2O的低比率，反之亦然，在水柱N2O浓度较低的地方。尽管这种“灵活”的比例解释了大部分N2O，并帮助我们重新定义了我们对氧气最低限度区域N2O产生的理解/为什么这个比例应该改变，但目前尚不清楚。在这个项目中，我们的目标是根据例如N2O、O2和驱动N循环的细菌来描述阿拉伯海中部选定地点的水柱。我们将在实验中操纵对比水，以测试氮气与N2O产生的比率是“固定”还是“灵活”，筛选与有机物耦合的氮气产生，并利用分子或“遗传”技术分析参与这些气体新陈代谢的活性细菌。更好地了解在阿拉伯海这样重要的区域参与这些复杂新陈代谢的关键过程和细菌，应该有助于科学界建立更好的预测气候模型。

The element nitrogen (N) is key to life on Earth and it is continually being cycled between the atmosphere, biomass (animals, plants, microbes) and back to the atmosphere following death and decay. At the centre of this N cycling, on the land and in the sea, are a wide variety of microscopic organisms known as bacteria. In the atmosphere N exists largely as N2 gas but also in much smaller amounts as nitrous oxide (N2O) which is a potent greenhouse gas. Processes which remove N, as N2, can regulate the growth of plants and, indirectly, the balance of carbon dioxide (CO2) in the atmosphere and, hence, affect climate. Large areas of the global ocean are fully oxygenated or 'saturated' with oxygen (O2) but some parts are not. For example, the Black Sea completely lacks any O2 below 90 m and others such as the Benguela upwelling off south western Africa are also devoid of O2 / both these areas have oxygen minimum zones or OMZ. It is these O2 'starved' regions or OMZ that are significant for both N removal and N2O production in the global ocean. Our interest lies in that of the OMZ of the Arabian Sea which, due to its large size (that of France and Germany combined), plays a significant role in global N cycling / responsible for 20 % of N2O production and 30 % of N removal in the global ocean. While the significance of the Arabian Sea in the global N cycle is known, the metabolisms responsible for N2 and N2O production were, and are still in part, unclear. Recently, by looking a bit closer and in conjunction with N tracers (15N isotopes), we were the first to actually measure N2O production in the central Arabian Sea. Further, we demonstrated that most (>95 %) of the N2O produced could be explained simply by one pathway i.e. the metabolism of nitrite (NO2-) to N2O in the absence of O2. In addition, we measured N removal via two known paths of N2 production e.g. N2 from denitrification and N2 from anaerobic ammonium oxidation (anammox). However, a substantial portion of the N2 is coming from somewhere else and we have evidence that this extra new path of N2 production is directly coupled to the metabolism of decaying biomass. However, it is not as simple as this. One pathway of N2O formation requires some complexity to generate the high and low concentrations of N2O characteristic of the OMZ in the central Arabian Sea. Again, our 15N tracers uncovered some of this by showing that the ratio of N2 to N2O production during the metabolism of NO2- (NO2- to NO to N2O to N2) is not fixed and appears to be 'flexible'. For example, where water column N2O concentration is high, we measured a low ratio of N2 to N2O production from NO2- and vice versa where water column N2O concentration was low. Although this 'flexible' ratio explains the majority of N2O and helps redefine our understanding of N2O production in oxygen minimum zones / why this ratio should change is unknown. In this project we aim to characterise the water column at selected sites in the central Arabian Sea in terms of, for example, N2O, O2 and the bacteria driving the N-cycle. We will experimentally manipulate contrasting waters to test if the ratio of N2 to N2O production is 'fixed' or 'flexible', screen for N2 production coupled to organic matter and analyse the active bacteria involved in the metabolism of these gases by using molecular or 'genetic' techniques. A better understanding of the key processes and bacteria involved in these complex metabolisms in such an important area as the Arabian Sea should help the scientific community build better predictive climate models.