Deciphering the Cryptic Cycling of Methane in Sediments of a Coastal Wetland

这项研究项目调查了南加州沿海湿地无氧区微生物在沉积物中产生甲烷和消耗甲烷之间的密切关系。这两个密切相关的微生物群之间的甲烷直接交换，被称为“隐秘的甲烷循环”，直到最近才被发现，人们对其在减少沿海湿地甲烷排放方面的重要性知之甚少。这项研究将揭示碳是如何在这两种微生物之间移动的，并将确定涉及的微生物。它还将揭示负责的重要代谢反应以及不同环境条件下甲烷生产和消费之间的平衡。对于更广泛的影响，该项目将为两名本科生和一名研究生提供跨学科湿地科学和最先进的实验室方法方面的培训。一门关于全球甲烷排放的新新生课程将把本科生带入该领域，提供有关当地沿海湿地环境的教育。该项目的结果将传播给公众和学术团体，并将使人们更好地了解沿海湿地的甲烷生产和消费。自前工业化时代以来，大气中甲烷的浓度增加了一倍多，因此我们迫切需要了解控制这种强有力的温室气体排放的机制。最近的研究首次证明，在富含有机物的沉积物的硫酸盐还原带中，微生物可以同时生产和消耗甲烷，这一过程被称为“隐蔽甲烷循环”。在这个过程中，甲烷被认为是直接从甲基化甲烷生成转化为甲烷厌氧氧化反应(AOM)。然而，人们对所涉及的有机体的身份、碳从一种新陈代谢到另一种新陈代谢的踪迹，或者这两个过程的环境净结果知之甚少。如果没有这种代谢关系的细节，沉积物中的甲烷收支仍然不完整。滨海湿地对隐蔽甲烷循环的研究特别感兴趣，因为其富含有机和硫酸盐的沉积物促进了甲基化底物的产生，并为AOM提供了电子受体。然而，厌氧微生物从这个生态系统中去除甲烷，特别是沿着海洋和陆地之间的硫酸盐梯度，仍然没有被很好地理解。就智力价值而言，这项研究阐明了沿海湿地中参与隐蔽甲烷循环的产甲烷和营养甲烷古菌的身份，以及在这种代谢关系中推动甲烷去除的电子受体的选择。这项研究为揭开驱动甲烷生成和AOM的酶机制的多功能性提供了新的代谢线索。通过捕捉控制两个过程之间的平衡的环境因素，这项工作的结果进一步加深了对沿海湿地沉积物中甲烷动力学的理解。这些信息可以应用于生物地球化学模型，以改进对该生态系统甲烷排放的预测，该生态系统遍布全球沿海地区。为了产生更广泛的影响，这项研究为两名本科生和一名研究生提供了创新的分析和实验技术培训。该项目每年还会邀请20名本科生参加一个新开发的新生研讨会，名为“甲烷--另一个温室问题”，其中包括对当地湿地的实地考察。研讨会的目标将是教育学生，包括那些没有进入STEM领域的学生，了解全球甲烷的来源和汇及其现在和未来对全球变暖的影响。这项研究的结果将广泛传播到教育和公共推广平台，如高中、社区大学和环境非营利组织，传授科学方法、生物地球化学循环的概念，并加强对这一重要沿海环境的欣赏。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

This research project investigates the close relationship between methane production and consumption in sediments by microbes in the oxygen-free zone of a coastal wetland in Southern California. The direct exchange of methane between those two closely related microbial groups, which has been named "cryptic methane cycling", has only recently been identified and little is known about its importance in reducing methane emissions from coastal wetlands. This research will reveal how carbon moves between the two types of microbes, and will identify the microbes involved. It will also reveal the important metabolic reactions responsible and the balance between methane production and consumption under different environmental conditions. Towards broader impacts, this project will provide training for two undergraduate and one graduate student in interdisciplinary wetland science and state of the art laboratory methods. A new freshmen course on global methane emissions will bring undergraduate students into the field to provide education on local coastal wetland environments. Results of the project will be disseminated to public and academic groups and will provide a better understanding of methane production and consumption in coastal wetlands.Concentrations of atmospheric methane have more than doubled since the pre-industrial era, hence we urgently need to understand the mechanisms that control the emission of this potent greenhouse gas. Recent studies have provided the first evidence for the simultaneous microbial production and consumption of methane in the sulfate reduction zone of organic-rich sediments, a process named the "cryptic methane cycle." In this process, methane is proposed to be passed directly from methylotrophic methanogenesis to anaerobic oxidation of methane (AOM). However, little is known about the identity of the organisms involved, the trail of carbon from one metabolism to the other, or the environmental net result of the two processes. Without the details of this metabolic relationship, methane budgets of sediments remain incomplete. Coastal wetlands are of particular interest for the study of cryptic methane cycling, because their organic and sulfate rich sediments promote the production of methylated substrates for methylotrophic methanogenesis and provide electron acceptors for AOM. Yet, anaerobic microbial removal of methane from this ecosystem, particularly along the sulfate gradient between ocean and land, is still not well understood. Towards intellectual merit, this study elucidates the identity of methanogenic and methanotrophic archaea involved in cryptic methane cycling in a coastal wetland as well as the selection of electron acceptors that fuel methane removal in this metabolic relationship. The research provides new metabolic clues to unravel the versatility of the enzymatic machinery that drives methanogenesis and AOM. By capturing environmental factors that control the balance between the two processes working in close proximity, the results of this work further provide an enhanced understanding of methane dynamics in coastal wetland sediments. This information can be applied to biogeochemical models to improve the prediction of methane emissions from this ecosystem, which is found throughout the global coastal zone. Towards broader impacts, the research provides training in innovative analytical and experimental techniques to two undergraduate and one graduate student. The project further engages 20 undergraduates per year in a newly developed freshman seminar, "Methane - the Other Greenhouse Problem" including a field trip to the local wetland. The goal of the seminar will be to educate students, including those not entering STEM fields, about global sources and sinks of methane and its involvement in global warming now and in the future. Results of this study will be broadly disseminated to educational and public outreach platforms, such as high schools, community colleges, and environmental non-profits to teach scientific methodologies, concepts of biogeochemical cycling, and enhance the appreciation of this vital coastal environment.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.