Collaborative Research: Nitrous oxide reduction in oxygen minimum zones: an understudied but critical loss term in ocean greenhouse gas cycling

一氧化二氮（N2O）是水生和陆地环境中微生物产生的一种气体，与其他温室气体一样，它会导致全球变暖。此外，一氧化二氮会破坏臭氧，而臭氧是保护地球免受危险紫外线辐射的一种气体。在海洋中，N2O的产生在很大程度上受可用溶解氧的数量控制，在低氧浓度下产生更多的N2O；然而，当没有氧气可用时，一种被称为缺氧的情况下，海洋中的微生物从产生一氧化二氮转变为消耗一氧化二氮。近年来，很明显，在海洋的某些地区，低氧区正在扩大，这引起了人们对将产生更多一氧化二氮的担忧。如果发生这种情况，更多的一氧化二氮将被排放到大气中，并将导致进一步的全球变暖和臭氧破坏。正因为如此，研究主要集中在了解低氧条件下海洋中产生了多少一氧化二氮。然而，如果缺氧区也扩大了，这至少可以在一定程度上平衡由于存在氧气但浓度较低的区域扩大而导致的N2O产量的预测增加。本研究旨在同时测量低氧区和缺氧区N2O的产生和消耗，并确定负责N2O产生和消耗的微生物。我们的研究结果将：1)使我们更好地了解缺氧区N2O消耗如何在海洋低氧区继续扩大的情况下帮助平衡N2O产量的增加，2)有助于为旨在预测未来海洋条件下海洋N2O生产和向大气排放的模型提供信息，3)使我们更好地了解参与N2O生产和消费的微生物。我们的研究将支持一名博士后和本科生，他们将在海洋化学和社区基因组学的界面工作。私立学校计划特别考虑来自代表性不足的少数族裔和机会有限的院校学生的申请。pi还计划了许多其他教育/推广项目，包括教师培训讲习班、教师实习、学术和公开讲座系列。随着溶解氧（DO）浓度从含氧过渡到低氧，海洋产生的强效温室和臭氧破坏气体一氧化二氮（N2O）增加。随着气候变化，全球海洋DO浓度下降，海洋缺氧区已经扩大，预计将继续扩大。这种增加引起了人们的关注，即未来海洋中一氧化二氮的产生将会增加，这将导致向大气中排放更多的气体。因此，许多研究都集中在量化与N2O产量大幅增加相对应的氧阈值上。相比之下，相对较少的研究旨在量化在缺氧条件下微生物将N2O还原为N2的净N2O消耗能力，以缓冲缺氧区与缺氧区同时扩大时预测的N2O产量增加。为此，本研究旨在同时量化含氧-低氧-缺氧水柱区域的N2O产量和消耗，以确定N2O消耗的潜力，以抵消N2O产量的预测增长。我们的实地工作将在不列颠哥伦比亚省的萨尼奇湾进行，这是一个理想的自然实验室，因为它的特点是一个完善的氧跃层和缺氧区。具体来说，我们的目标是1)测量大量N2O浓度，并使用15N示踪技术，量化DO浓度从缺氧到缺氧条件下N2O的产生和消耗率；2)量化缺氧区N2O消耗与上盖缺氧区N2O产量增加的平衡程度；3)明确将观察到的N2O产生和消耗率与介导这一过程的微生物联系起来。特别关注通过反硝化（NO3-到N2）和非反硝化（N2O到N2）分类群来区分N2O消耗。最终，我们的研究结果将提供波动海洋条件下N2O消耗率的定量信息，从而帮助限制氧气对N2O净产量和海洋到大气温室气体通量的影响模型。此外，这项工作将确定能够还原N2O的微生物的分类宽度及其与实际N2O还原速率的联系，从而为检测特定生物特征是否预测海洋N2O动态提供定量理解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Nitrous oxide (N2O) is a gas produced by microbes in both aquatic and terrestrial environments, and, like other greenhouse gases, it contributes to global warming. Furthermore, N2O can destroy ozone, a gas responsible for protecting the earth from dangerous ultraviolet radiation. In the ocean, N2O production is largely controlled by the amount of available dissolved oxygen, with more N2O being produced under low oxygen concentrations; however, when no oxygen is available, a scenario referred to as anoxia, microbes in the ocean switch from producing N2O to consuming N2O. In recent years, it has become evident that zones of low oxygen are expanding in some areas of the oceans, and this has raised concern that more N2O will be produced. If this occurs, more N2O will be emitted to the atmosphere, and will lead to further global warming and ozone destruction. Because of this, research has largely focused on understanding how much N2O is produced in the ocean under low oxygen conditions. If, however, anoxic zones also increase in size, this could act to balance out, at least to some degree, the predicted increase in N2O production caused by the expansion of zones where oxygen is present but in low concentrations. This study aims to simultaneously measure N2O production and consumption, in both low oxygen and anoxic zones and identify the microbes responsible for N2O production and consumption. Our results will: 1) lead to a much better understanding of how N2O consumption in anoxic zones could help to balance out an increase in N2O production if low oxygen zones in the ocean continue to expand, 2) help to inform models aimed at predicting oceanic N2O production and emissions to the atmosphere under future ocean conditions, and 3) allow us to better understand the microbes involved in N2O production and consumption. Our study will support a postdoc and undergraduate students who will work at the interface of marine chemistry and community genomics. The PIs plan to specifically consider applications from underrepresented minorities and students at institutions with limited opportunities. The PIs also plan a number of other educational/outreach programs ranging from teacher-training workshops, teacher internships, and academic and public lecture series. The oceanic production of the potent greenhouse and ozone destroying gas nitrous oxide (N2O) increases as dissolved oxygen (DO) concentrations transition from oxic to hypoxic. Marine DO concentrations have decreased globally with climate change and oceanic hypoxic zones have expanded and predicted to continue expanding. This increase is cause for concern that N2O production in the ocean will increase in the future which would lead to higher emissions to the atmosphere. As a result, much research has focused on quantifying the oxygen thresholds that correspond to large increases in N2O production. In contrast, relatively few studies have aimed to quantify the capacity for net N2O consumption, resulting from microbial N2O reduction to N2 under anoxic conditions, to buffer against predicted N2O production increases if anoxic zones expand in conjunction with hypoxic zones. To this end, this study aims to simultaneously quantify N2O production and consumption from oxic-hypoxic-anoxic water column zones, in order to determine the potential for N2O consumption to counteract predicted increases in N2O production. Our field work be conducted in Saanich Inlet, a British Columbian fjord which is an ideal natural laboratory for our study, as it is characterized by a well-established oxycline and anoxic zone. Specifically, we aim to 1) measure bulk N2O concentrations, and, using 15N tracer techniques, quantify N2O production and consumption rates as DO concentrations decrease from oxic to anoxic conditions, 2) quantify the magnitude by which N2O consumption in the anoxic zone balances increased N2O production in the overlying hypoxic region, and 3) definitively link observed N2O production and consumption rates to the microorganisms mediating this process, focusing specifically on distinguishing N2O consumption via denitrifier (NO3- to N2) versus non-denitrifier (N2O to N2 only) taxa. Ultimately, our results will provide quantitative information on N2O consumption rates over fluctuating ocean conditions, thereby helping constrain models of oxygen effects on net N2O production and ocean-to-atmosphere greenhouse gas fluxes. Furthermore, this work will identify the taxonomic breadth of microbes capable of N2O reduction and their linkage to actual N2O reduction rates, thereby providing a quantitative understanding of whether or not the detection of specific bio-signatures is predictive of marine N2O dynamics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.