Northern 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.
