COLLABORATIVE RESEARCH: Acetate Biogeochemistry In Northern Wetlands: Implications for Methane Formation During Climate Change

摘要北方湿地储存着大量的碳，是大气甲烷的重要来源。迫在眉睫的气候变化可能会极大地影响这些环境在碳循环中的作用。有机物的分解在这些湿地中以厌氧方式进行。然而，最近的发现表明，相对于更南方的系统，北部湿地的最终产甲烷步骤发生了变化，尽管甲烷的生产仍在通过氢气利用，但乙酸盐仍是最终产物。如果这种现象无处不在，那么醋酸盐在北部湿地中并不是重要的中间体，而是分解的最终产物；它最终会在有氧条件下降解为二氧化碳，而不是甲烷。醋酸盐在北部湿地的积累代表了我们对厌氧分解的理解的范式转变。北方的气候变暖可能会使这一过程转向发生在更南部湿地的情况。如果是这样的话，那么北方的甲烷产量将大大超过仅靠变暖可能产生的甲烷产量。这项工作将检验这样一种假设，即北方湿地不支持显著的乙酸盐甲烷生成，以及乙酸盐的形成和积累是有机物厌氧降解过程中的重要终端步骤。一个多学科小组正在研究这一现象，他们将利用实地和实验室调查以及微生物学、分子生物学、地球化学、土壤学和稳定同位素方法。该项目的目标是：1)通过检查醋酸盐浓度和生产率、地球化学条件，以及通过对代表北极、亚北极和北方主要环境的各种北方湿地类型应用同位素和分子技术，评估醋酸盐积累现象的普遍性；2)通过进行季节性研究，为确定甲烷产生途径和醋酸盐循环控制的变化提供一个时间框架；3)通过实验室对培养条件的操纵，确定物理和化学参数对终端分解的影响；4)确定细菌种群是否与其他湿地和其他厌氧栖息地不同。将使用培养分析、放射性示踪剂分析、稳定同位素和自然放射性碳丰度来研究野外和实验室实验中的分解途径。包括PCR-DGGE和杂交探测在内的分子方法将被用来确定细菌多样性和种群结构。与更南端的湿地相比，这些湿地中甲烷形成的生化途径是独特的，这一证明对于我们预测这些湿地将在向全球大气贡献甲烷方面扮演什么角色以及它们降解储存的碳的能力具有巨大的意义。醋酸盐可以作为主要的有机最终产物，从而构成与产甲烷同时发生的单独的终端分解途径。这里提出的多学科研究将勾勒出这些过程的重要细节，并将有助于展示其空间意义。

AbstractNorthern wetlands store large quantities of carbon and are significant sources of atmospheric methane. Impending climatic changes may greatly affect the role of these environments in the carbon cycle. Decomposition of organic matter occurs anaerobically in these wetlands. However, recent findings suggest that the terminal methanogenic step in northern wetlands is altered relative to more southerly systems, with acetate acting as a terminal product even though methane production continues via hydrogen utilization. If this phenomenon is ubiquitous, then acetate is not a significant intermediate in northern wetlands, but serves as a terminal product of decomposition; one that is ultimately degraded aerobically to CO2 rather than methane. Acetate accumulation in northern wetlands represents a paradigm shift in our understanding of anaerobic decomposition. It is possible that warming in the north will shift this process toward what occurs in more southerly wetlands. If so, then methane production in the north will increase greatly over what might occur from warming alone.This work will test the hypothesis that northern wetlands do not support significant methanogenesis from acetate and that acetate formation and accumulation is an important terminal step during anaerobic degradation of organic matter. This phenomenon is being investigated by a multidisciplinary team that will utilize field and laboratory investigations and microbiological, molecular biology, geochemical, pedological, and stable isotope approaches.The objectives of the project are: 1) assess the ubiquity of the acetate-accumulating phenomenon by examining acetate concentrations and production rates, geochemical conditions, and by applying isotopic and molecular techniques to a variety of northern wetland types representative of the major circumpolar arctic, subarctic and northern boreal environments; 2) Provide a temporal framework for determining variations in controls on methanogenic pathways and acetate cycling by conducting seasonal studies; 3) determine the effects of physical and chemical parameters on terminal decomposition using laboratory manipulations of incubation conditions; 4) determine if the bacterial populations are distinct compared to other wetlands and other anaerobic habitats.Incubation assays, radiotracer analyses, stable isotopes and natural radiocarbon abundances will be used to address pathways of decomposition in field and laboratory experiments. Molecular approaches including PCR-DGGE and hybridization probing will be used to determine bacterial diversity and population structure.A demonstration that biochemical pathways of methane formation in these wetlands are unique compared to their more southerly counterparts, has tremendous implications for our ability to predict the what the role of these wetlands will be in contributing methane to the global atmosphere and in their ability to degrade stored carbon. Acetate may serve as a primary organic end product, which would thus constitute a separate terminal decomposition pathway occurring simultaneously with methanogenesis. The multi-disciplinary study proposed here will delineate the important details of these processes and will serve to demonstrate their spatial significance.