Collaborative Research: NSF OCE-BSF: Coupling organic nutrient cycling to methane production in the oligotrophic North Pacific Ocean

开阔的海洋表层水域是大气中甲烷的天然来源。就在十年前，甲烷的来源还是个谜，因为人们只知道甲烷的产生发生在某些没有氧气的环境中。最近，几种代谢途径的发现使微生物能够在氧气存在的情况下将有机物转化为甲烷，这使得甲烷只能在厌氧（无氧）环境中产生的观点发生了转变。研究人员提出，微生物产生甲烷的途径取决于开阔海洋表面水域普遍存在的营养条件。在北大西洋，磷限制了微生物的繁殖，微生物从含磷的有机化合物中获取所需的磷时，会产生甲烷作为副产品。相比之下，氮限制了北太平洋的微生物生产。研究小组提出，在北太平洋，微生物产生甲烷是有机氮降解的副产品。为了验证这一假设，他们提议将之前在北大西洋进行的地球化学和生物测量结果与他们提议在北太平洋进行的一组平行的地球化学测量结果进行比较。该奖项将支持主要本科院校（PUI）的一位早期职业教授与一位资深科学家之间的合作，以及美国和以色列科学家之间的合作。本科生将参与跨学科研究，包括海洋学、同位素生物地球化学和基因组科学，并将在海上进行研究。微生物学和基因组学研究将被整合到普吉特海湾大学基于课程的本科生研究经验中，使不同的学生能够直接参与真实的研究。研究结果也将整合到研究生水平的海洋有机地球化学课程中，可通过麻省理工学院公开课件网站在线获得。这是一个由美国国家科学基金会地球科学理事会（NSF- geo）和以色列两国科学基金会（BSF）根据国家科学基金会和BSF之间的谅解备忘录共同资助的项目。该协议允许美国和以色列研究人员提交一份单一的合作提案，并由NSF进行同行评审。在国家科学基金会的评审结果和认可的国家科学基金会项目的推荐下，每个机构资助预算的比例和与自己国家有关的调查人员。本研究的指导假设是，尽管北大西洋和北太平洋副热带环流的表层海水都是大气甲烷的来源，但在这两个盆地中产生甲烷的潜在微生物过程是根本不同的。马尾藻海的微生物生产长期受到磷的限制。为了减轻这一限制，一些微生物将溶解有机物（HMWDOM）的高分子量部分中的甲基膦酸盐降解为甲烷和磷酸盐。表达碳磷（C-P）裂解酶途径的细菌在马尾藻海微生物群落中占主导地位，并介导这种形式的甲烷产生，使其成为马尾藻海过量甲烷产生的主要途径。相比之下，北太平洋副热带环流（NPSG）的微生物产量长期受到氮限制，该提案假设通过HMWDOM中甲基胺的降解获得氮是产生过量甲烷的主要途径。海洋HMWDOM中甲胺的含量是甲基膦酸盐的20倍，与淡水湖中甲胺转化为甲烷相关的转氨酶基因在海洋细菌基因组中具有密切相关的序列。这些序列在海洋宏基因组中丰富而广泛。虽然贫营养地表水中甲基膦酸盐和甲胺的循环都产生甲烷，但研究假设这两个过程将产生具有不同特征碳同位素值的甲烷。为了验证这一假设，研究小组将测量HMWDOM甲胺和甲基膦酸盐产生的甲烷的稳定碳同位素值。该小组还将进行实验室实验，测试各种寡养和共养海洋细菌分离物将HMWDOM甲基胺转化为甲烷的能力。为了实现这一目标，在夏威夷北部沿着西经158°（ALOHA站经度）到北纬25-28°的NPSG进行了一项实地研究，对每个甲烷生产途径进行了地球化学和生物测量。该团队将获得甲烷和乙烯浓度（C-P裂解酶的两种产物）的水柱剖面、甲烷碳同位素、HMWDOM甲胺和甲基膦酸盐的浓度和碳同位素值。研究人员将利用稳定的碳同位素示踪剂、C-P裂解酶活性、C-P裂解酶与转氨酶基因丰度和表达的比例，量化NPSG中甲胺和甲基膦酸盐的甲烷产量。最后，该团队将使用超转录组学方法比较HMWDOM甲胺和甲基膦酸盐与NPSG中天然微生物群落的生物利用度，以检测HMWDOM添加后微生物代谢功能的变化。总之，这些数据将解决甲胺和甲基膦酸途径对NPSG中好氧甲烷产生的相对贡献，以及甲烷产生背后的微生物群和生态系统特性。通过这种跨学科的方法，这项研究将增强我们对环境中控制好氧甲烷生产过程的理解。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。

Open ocean surface waters are natural sources of methane to the atmosphere. As recently as a decade ago the source of this methane was a mystery, because methane production was only known to occur in certain environments without oxygen. Recently, the discovery of several metabolic pathways that enable microbes to transform organic matter into methane in the presence of oxygen has led to a shift away from the idea that methane can only be produced in anaerobic (oxygen-free) environments. The investigators propose that the pathway microbes use to make methane depends on the nutrient conditions that prevail in open ocean surface waters. In the North Atlantic Ocean, phosphorus limits microbial production, and microbes produce methane as a by-product of getting the phosphorus they need from organic compounds that contain phosphorus. In contrast, nitrogen limits microbial production in the North Pacific Ocean. The team proposes that in the North Pacific Ocean microbes produce methane as a by-product of organic nitrogen degradation. To test this hypothesis, they propose to compare the results of geochemical and biological measurements previously made in the North Atlantic with a parallel set of geochemical measurements they propose to make in the North Pacific Ocean. The award will support collaborations between an early career professor at a primarily undergraduate institution (PUI) and a senior scientist, and between US and Israeli scientists. Undergraduate students will participate in interdisciplinary research spanning oceanography, isotope biogeochemistry, and genome science and will conduct research at sea. The microbiology and genomic research will be integrated into course-based undergraduate research experiences at the University of Puget Sound enabling diverse students to participate directly in authentic research. Results will also be integrated into a graduate level course in marine organic geochemistry available on-line through the MIT Open Courseware website. This is a project jointly funded by the National Science Foundation’s Directorate of Geosciences (NSF-GEO) and the Israel Binational Science Foundation (BSF) in accord with the language in the Memorandum of Understanding between the NSF and the BSF. This Agreement allows a single collaborative proposal, involving US and Israeli investigators, to be submitted and peer-reviewed by NSF. Upon successful results of the NSF merit review and recommendation by the cognizant NSF Program of an award, each Agency funds the proportion of the budget and the investigators associated with its own country.The guiding hypothesis of this study is that although surface seawater in the North Atlantic and North Pacific Subtropical Gyres are both sources of methane to the atmosphere, the underlying microbial processes that produce methane in the two basins are fundamentally different. Microbial production in the Sargasso Sea is chronically phosphorus-limited. To mitigate this limitation, some microbes degrade methylphosphonate that is incorporated into the high molecular weight fraction of dissolved organic matter (HMWDOM) into methane and phosphaote. Bacteria expressing the carbon-phosphorus (C-P) lyase enzyme pathway for phosphonate catabolism dominate the Sargasso Sea microbial community and mediate this form of methane production making it the principal route through which excess methane is produced in the Sargasso Sea. In contrast, microbial production in the North Pacific Subtropical Gyre (NPSG) is chronically nitrogen limited and the proposal postulates that nitrogen acquisition through the degradation of methylamines in HMWDOM is a major route through which excess methane is produced. Methylamines are twenty-fold more abundant than methylphosphonate in marine HMWDOM and the aminotransferase gene linked to the conversion of methylamine into methane in freshwater lakes has closely related sequences in marine bacterial genomes. These sequences are abundant and widespread in marine metagenomes. Although the cycling of methylphosphonate and methylamine in oligotrophic surface waters both produce methane, the study postulates that the two processes will yield methane with distinct and characteristic carbon isotopic values. To test this hypothesis, the team will measure the stable carbon isotope value of the methane produced from HMWDOM methylamine and methylphosphonate. The team will also conduct laboratory experiments that test the capacity of diverse oligotrophic and copiotrophic marine bacterial isolates to convert HMWDOM methylamines to methane. This objective is complemented by a field study in the NPSG northwards from Hawaii along 158°W, the longitude of Station ALOHA, to 25-28°N to conduct geochemical and biological measurements associated with each methane production pathway. The team will obtain water column profiles of methane and ethylene concentration (two products of C-P lyase), methane carbon isotopes, and concentrations and carbon isotope values of HMWDOM methylamine and methylphosphonate. The investigators will quantify the rates of methane production from methylamine and methylphosphonate using stable carbon isotope tracers, C-P lyase activity, and the ratio of C-P lyase to aminotransferase gene abundance and expression in the NPSG. Lastly, the team will compare the bioavailability of HMWDOM methylamine and methylphosphonate to natural microbial communities in the NPSG using a metatranscriptomics approach to examine changes in microbial metabolic functions in response to HMWDOM additions. Together, these data will resolve the relative contribution of the methylamine and methylphosphonate pathways to aerobic methane production in the NPSG and the microbial groups and ecosystem properties underlying methane production. Through this interdisciplinary approach, the study will enhance our understanding of processes controlling aerobic methane production in the 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.