Nitrous Oxide Consumption in Surface Waters

一氧化二氮（N2O）是地球大气的微量成分。它是一种强温室气体，其浓度自工业革命以来一直在增加，约占温室效应总量的6%。N2O也与平流层的臭氧消耗有关。海洋是大气中N2O的净来源。作为氮循环的一部分，一氧化二氮在海洋中由微生物产生和消耗。将N2O转化为N2（二氮气体）是唯一已知的N2O生物汇，并由一种酶催化，该酶被氧强烈抑制，因此N2O的消耗被认为仅限于无氧环境。因此，令人惊讶的是，在氧气丰富的海洋表面，消耗N2O的遗传能力既存在又活跃。实验表明，当氧气被去除时，海洋表面微生物迅速消耗N2O。微生物似乎可以利用一氧化二氮作为氧气的替代品进行呼吸。为什么在完全含氧的地表水中保持这种能力是有利的？这种潜在的N2O消耗是否构成N2O的实际汇，从而可能减少N2O从海洋到大气的净转移？氧化亚氮（N2O）还原为N2被认为是一个专性厌氧过程，通常仅限于水和沉积物中的缺氧环境。先前的研究表明，编码N2O还原酶的基因（nosZ）在海洋表面存在并表达，当氧气被去除时，海洋表面样品迅速将N2O还原为N2。地表水中的nosZ基因似乎几乎完全属于不执行反硝化上游步骤的微生物-它们是兼性N2O呼吸器。本文提出的研究将调查可能控制或刺激地表水N2O减少的因素，并描述负责这一过程的微生物。实验将验证一种假设，即N2O呼吸器是多功能异养生物，其活性与有机物供应有关，并可能与颗粒物质有关或与原位初级生产有关。研究人员将进行15N示踪培养，以测量N2O的还原速率及其对各种有机基质的响应，包括原位颗粒物质和新鲜浮游植物渗出物。含有nosZ的微生物的数量和群落组成将使用一套分子生物学方法来确定——这是必要的，因为nosZ基因是如此多样化，以至于以前的方法可能低估了它的丰度和多样性，并且可能没有确定负责该过程的主要微生物。含nosz的微生物基因组草图将被表征，以研究它们的碳代谢，并将它们的生活方式与有机物供应或浮游植物联系起来。计划进行一次实地考察，以调查北大西洋副热带环流中N2O减少的意义。如果N2O在海洋表面的广大地区以低速率消耗，其对海洋和大气的总体N2O收支的影响可能很大。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 trace component of Earth’s atmosphere. It is a strong greenhouse gas, whose concentration has been increasing since the industrial revolution, and contributes about 6% to the total greenhouse effect. N2O is also implicated in ozone depletion in the stratosphere. The ocean is a net source of N2O to the atmosphere. N2O is produced and consumed in the ocean by microbes as part of the nitrogen cycle. The conversion of N2O to N2 (dinitrogen gas) is the only known biological sink for N2O and is catalyzed by an enzyme that is strongly inhibited by oxygen, so N2O consumption has been thought to be confined to oxygen-free environments. Thus, it was surprising to find that the genetic capability for N2O consumption was both present and active in the surface ocean, where oxygen is abundant. Experiments showed that surface ocean microbes rapidly consume N2O when oxygen is removed. It appears that the microbes can use N2O as an alternative to oxygen for respiration. Why would it be advantageous to retain this capacity in fully oxygenated surface water? Does this potential N2O consumption constitute an actual sink for N2O, which might reduce the net transfer of N2O from the ocean to the atmosphere? The reduction of nitrous oxide (N2O) to N2 is considered to be an obligately anaerobic process, usually restricted to anoxic environments in water and sediments. Previous work showed that the genes (nosZ) encoding the N2O reductase enzyme were both present and expressed in the surface ocean, and that surface ocean samples rapidly reduce N2O to N2 when oxygen is removed. The nosZ genes in surface waters appear to belong almost exclusively to microbes that do not perform the upstream steps in denitrification – they are facultative N2O respirers. The research proposed here will investigate the factors that might control or stimulate N2O reduction in surface waters and characterize the microbes responsible for the process. Experiments will test the hypothesis that N2O respirers are versatile heterotrophs whose activity is related to organic matter supply and possibly associated with particulate material or linked to in situ primary production. Researchers will perform 15N tracer incubations to measure the rate of N2O reduction and its response to various kinds of organic substrates, including in situ particulate material and fresh phytoplankton exudates. The quantity and community composition of nosZ-containing microbes will be determined using a suite of molecular biological methods – necessary because the nosZ gene is so diverse that previous methods have likely underestimated both its abundance and diversity and may not have identified the main microbes responsible for the process. Draft genomes of nosZ-containing microbes will be characterized to investigate their carbon metabolism and link their lifestyles to organic matter supply or phytoplankton. One field expedition is planned to investigate the significance of N2O reduction in the North Atlantic subtropical gyre. If N2O consumption occurs even at low rates over vast regions of the surface ocean, its impact on the overall N2O budget of the ocean and atmosphere could be large.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.