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，它不断在大气，生物质（动物，植物，微生物）之间循环，并在死亡和腐烂后返回大气。在这种氮循环的中心，在陆地和海洋中，是各种各样的微生物，称为细菌。在大气中，氮主要以N2气体存在，但也以一氧化二氮（N2 O）的形式存在，这是一种强有力的温室气体。以N2形式去除氮的过程可以调节植物的生长，并间接调节大气中二氧化碳（CO2）的平衡，从而影响气候。全球海洋的大部分区域都是完全含氧的或氧（O2）“饱和”的，但有些部分不是。例如，黑海在90米以下完全缺乏任何氧气，而其他地区，如非洲西南部的本格拉涌升流也缺乏氧气/这两个地区都有氧气最低区或OMZ。正是这些O2“饥饿”区域或OMZ对全球海洋中的N去除和N2 O产生都很重要。我们的兴趣在于阿拉伯海的OMZ，由于其面积大（法国和德国的总和），在全球N循环中发挥着重要作用/负责全球海洋中20%的N2 O生产和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，但负责N2和N2 O生产的代谢过程过去和现在仍然部分不清楚。最近，通过仔细观察并结合N示踪剂（15 N同位素），我们是第一个实际测量阿拉伯海中部N2 O产量的人。此外，我们证明，大部分（> 95%）的N2 O产生可以简单地解释为一个途径，即在没有O2的情况下，亚硝酸盐（NO2-）代谢为N2 O。此外，我们通过两种已知的N2生产途径，例如来自反硝化的N2和来自厌氧氨氧化（anammox）的N2，测量了N的去除。然而，相当大一部分的N2来自其他地方，我们有证据表明，这一额外的N2生产新途径直接与腐烂生物质的代谢有关。然而，事情并非如此简单。N2 O形成的一个途径需要一些复杂性，以产生阿拉伯海中部OMZ特征的高浓度和低浓度N2 O。同样，我们的15 N示踪剂通过显示在NO2-（NO2-至NO至N2 O至N2）代谢期间N2至N2 O产生的比率不固定并且似乎是“灵活的”来揭示其中的一些。例如，在水柱N2 O浓度高的情况下，我们测量到从NO2-产生的N2与N2 O的低比率，反之亦然，在水柱N2 O浓度低的情况下。虽然这种“灵活”的比例解释了大部分N2 O，并有助于重新定义我们对氧气最小区N2 O生产的理解/为什么这个比例应该改变是未知的。在这个项目中，我们的目标是在选定的地点在阿拉伯海中部的水柱，例如，N2 O，O2和细菌驱动的N-循环。我们将通过实验操作对比沃茨来测试N2与N2 O生产的比率是“固定的”还是“灵活的”，筛选与有机物结合的N2生产，并通过使用分子或“遗传”技术分析参与这些气体代谢的活性细菌。更好地了解在阿拉伯海这样一个重要地区参与这些复杂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.