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，它在大气、生物（动物、植物、微生物）之间不断循环，并在死亡和腐烂后回到大气中。在这个氮循环的中心，在陆地和海洋中，是各种各样的微生物，被称为细菌。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N和N2的过程可以调节植物的生长，并间接调节大气中二氧化碳的平衡，从而影响气候。全球海洋的大部分区域都是完全含氧的或“饱和”的氧气（O2），但有些部分不是。例如，黑海在90米以下完全没有氧气，其他地区，如非洲西南部的本格拉上升流也没有氧气/这些地区都有氧气最低带或OMZ。正是这些缺氧区域（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循环中起着重要作用/负责全球海洋中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，但负责产生N2和N2O的代谢在过去和现在仍然部分不清楚。最近，通过与N示踪剂（15N同位素）的结合，我们第一次实际测量了阿拉伯海中部的N2O产量。此外，我们证明了大多数（bbb95 %）的N2O产生可以简单地解释为一个途径，即亚硝酸盐（NO2-）代谢成N2O在缺乏O2。此外，我们还通过两种已知的氮气生产途径，即反硝化和厌氧氨氧化（anammox）的氮气去除量进行了测量。然而，相当一部分氮气来自其他地方，我们有证据表明，这一额外的新的氮气生产途径与腐烂生物量的代谢直接相关。然而，事情并没有这么简单。N2O形成的一个途径需要一定的复杂性来产生阿拉伯海中部OMZ特征的高浓度和低浓度N2O。同样，我们的15N示踪剂通过显示NO2-代谢过程中N2与N2O的生产比例（NO2-与NO / N2O / N2）不是固定的，并且似乎是“灵活的”，揭示了其中的一些问题。例如，在水柱N2O浓度高的地方，我们测量到NO2-产生的N2与N2O的比例很低，反之亦然，水柱N2O浓度低。虽然这个“灵活”的比例解释了大部分的N2O，并帮助重新定义了我们对氧最低区N2O产量的理解，但为什么这个比例应该改变是未知的。在这个项目中，我们的目标是描述阿拉伯海中部选定地点的水柱，例如，N2O， O2和驱动n循环的细菌。我们将通过实验操作对比水，以测试N2与N2O的生产比例是“固定”还是“灵活”，筛选与有机物质耦合的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。在阿拉伯海这样一个重要的地区，更好地了解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.