Department of Biology
UNCG Biologist Dr. Anne Hershey has received a National Science Foundation grant to fund her research project entitled Controls on methanogenesis, methane oxidation, and consumer utilization of methane-derived carbon in arctic lakes.
This project follows up on some of Dr. Hershey's previous work on controls of arctic lake productivity and arctic lake food webs by focusing on the production of methane and the role of methane as an energy source for organisms in the lake. Methane is produced by a group of bacteria in lake sediments, then utilized by another group of bacteria. These bacteria are consumed by benthic invertebrates which are, in turn, eaten by fish. In this way, methane is integral to lake food web dynamics.
Project summary: The overall aim of the proposed research is to assess the current importance of microbial processes involved with CH4 cycling dynamics in arctic lakes, to assess transfer of CH4-derived C (MDC) to arctic lake food webs, and to evaluate the rate of change in importance of MDC in these food webs. Lakes in the Arctic Foothills region, near Toolik Lake, Alaska, are characterized by high connectivity to the landscape, high inputs of terrestrial dissolved organic carbon (DOC), and low algal production. This DOC supports microbial metabolism in lakes, including methanogenic archaebacteria or methanogens, which live in subsurface anaerobic sediments, but export methane to overlying oxic sediments and the oxic water column. A high proportion of the methane produced by methanogens is oxidized by aerobic methane oxidizing bacteria (MOB) within the surficial sediments. It has recently been recognized that MOB are utilized by benthic consumers, especially midge larvae in the Tribe Chironomini (usually dominated by Chironomus spp). This is also true in arctic lakes in the Toolik Lake region, and our previous studies have shown that Chironomini production overwhelming dominates secondary production in these lakes. Furthermore, our recent work has shown little utilization of algal productivity by this dominant group of consumers. Thus, study of the controls on utilization of MDC by Chironomus, which is very suitable for experimental work, will provide us with a good assessment of the role of MDC in arctic lake food webs, and will enhance our understanding of overall arctic lake productivity. Furthermore, stable isotope studies of Chironomini in 3 different lakes for which we have long-term data strongly indicate that the importance of MDC in the diet of this dominant consumer is increasing fairly rapidly.
Although methane biogeochemistry is relatively well studied in wetlands, lake studies have focused largely on emissions of methane to the atmosphere, which are very significant in northern latitudes. There has been little investigation of the role of methane cycling processes in overall lake productivity. There are two major groups of methanogens that produce methane by two very different pathways, respectively: fermentation of acetate (acetoclastic methanogenesis) and the H2 - CO2 pathway (hydrogenotrophic methanogenesis). Limited studies in lakes suggest that acetoclastic methanogenesis, which depends on high quality DOC, is generally dominant, but the hydrogenotrophic pathway may become increasingly important as DOC decreases in quality. Little is known about the relative importance of these pathways in lake sediments, but it is reasonable to expect that the balance between the two may change under different conditions of DOC loading from the landscape. Such changes will affect total methane produced and oxidized, and transfer of MDC through the food web. Our overarching hypothesis is that the relative importance of methanogenesis, methane oxidation, and food-web incorporation of methane-derived carbon versus the importance of photoautotrophic production will be proportionally greater in small lakes which have greater allochthonous loading than large lakes. This hypothesis will be examined through three objectives: (1) Determine the relative importance of methanogenesis, methane oxidation, and the role of MDC compared to algal productivity in arctic lakes as a function of lake size. (2) Determine the pathways for and the controls on methanogenesis and transfer of MDC to higher trophic levels in arctic lakes. (3) Determine the rate of change in importance of MDC in arctic lake food webs as a function of lake size. Mechanistic studies described in the proposal will be used in combination with field studies on rates of change in importance of MDC to Chironomini between lakes to enhance our understanding of the changing trophic role of MDC in arctic lake food webs. Overall, we will develop a first ever, integrated understanding of the energy and C linkages among methanogens, MOB and higher trophic levels for arctic lake ecosystems, which are nested in rapidly changing landscape.
September, 2008