San Diego State University
Lars T. Angenent Washington University
Theodore K. Raab Stanford University
Proposal # 0808604
Investigators from San Diego State University, Washington University, and Stanford University will collaborate to study the role of iron and humic substances as electron acceptors in anaerobic respiration in peat soils of drained thaw lake basins in the Arctic. The role of microbial physiology in carbon flux from Arctic soils has substantial implications of climate change, and the PIs will apply novel methods that promise new insights. The study will further our understanding of controls over and seasonal and spatial variability in of carbon fluxes from Arctic soils. The goals are to: (1) determine the specific mechanisms of exocellular electron transfer in Alaskan peat soils, and the microorganisms that mediate these processes, (2) quantify the importance of exocellular electron transfer to soil respiration in Arctic drained thaw lake basins, (3) determine the effects of polygon-induced microtopography on exocellular electron transfer, and (4) determine how age and complexity of soil humic materials along a soil age gradient affect rates of exo-electron transfer, and the conditions under which it occurs.
Our goal for this project was to learn more about a special type of anaerobic respiration in which iron oxides and complex organic compounds in soils (humic substances, HS) are used instead of oxygen by microorganisms in the soil. This type of process, where iron and HS are used as electron acceptors for anaerobic respiration, has been studied in some wetlands but never before in Arctic soils. This process is tied directly to the carbon cycle and the production of greenhouse gases, carbon dioxide (CO2) and methane (CH4), and so understanding this process in the Arctic is important for predicting what will happen to the large amount of carbon stored in frozen arctic soils as the climate warms. We studied soils in the Arctic Coastal Plain near Barrow, in northern Alaska. These soils are permanently frozen below about one foot deep, and this permafrost layer prevents water from draining away. Therefore these soils stay wet most of the summer, blocking the diffusion of oxygen into the soil, leading to anoxic conditions over much of the landscape. Under anoxic conditions, soil microbes switch to different types of anaerobic metabolism depending on what alternative electron acceptors are available. This ecosystem in unique in that the more commonly studied alternatives to oxygen, such as nitrate and sulfate, are available in very small amounts, but large amounts of oxidized iron is present, in both soluble and insoluble forms. The large amount of organic matter that develops in these soils over time from the partial decomposition of dead plants also contributes to the production of HS. Respiration of iron oxides and HS are different from respiration using oxygen or small, simple, soluble compounds like nitrate or sulfate, in that both require the microbes to transfer electrons outside of their cells to these large, complex extracellular electron acceptors. In fact, the same microbes that breathe with iron and HS can also generate electricity in devices called microbial fuel cells, in which they use an electrode as an electron acceptor. Therefore, in addition to studying how iron and HS were used in respiration, we also built biochemical electrode systems to measure current generation by soil microbes. We found that respiration using iron oxides was responsible for about half of the CO2 release from soils during the second half of the summer, and that respiration using HS also contributed around 10-20% of the CO2 during the first half of the summer. The amount of iron available for anaerobic respiration varied across the landscape: younger soils with thinner organic layers had more iron due to a shallower mineral layer, whereas older soils had less iron because the mineral layer was buried by a thicker organic layer. Despite the variations in iron across the landscape, there was enough iron to allow for high rates of anaerobic respiration. However, CH4 production decreased in areas with more iron. This is because producing CH4 is a hard way for microbes to make a living, and so it usually only happens when there are no other alternative electron acceptors available. In other words, when iron oxides are around it allows the microbial community to produce more CO2 than CH4. We now have a much better grasp on what controls the release of these greenhouse gases from the Arctic Coastal Plain of northern Alaska. Besides learning more about how iron and HS control the carbon cycle, we also learned a lot about the microbiology of this system. We randomly sequenced DNA from the soil microbial community to find out what types of genes were common in a system where iron and HS respiration are so important. We found many decaheme cytochrome proteins, like hemoglobin in our blood expect with ten heme groups instead of just one. The presence of all these heme groups allows for "long distance" electron transport from inside the cell where food is being broken down, to the outside of the cell where the iron oxides, HS or electrodes are. We discovered that a mysterious group of bacteria called Acidobacteria are important for iron and/or HS respiration in these soils. This project also had impacts on outreach, education and human resource development. This project has involved nine undergraduates, ten graduate students, two high school teachers, and several other post-docs and collaborators. Most of these participants were exposed to research in the Arctic for the first time. It has formed the core for four Ph.D. dissertations. So far publications from this project include at least eight different undergraduate and graduate student coauthors. The project contributed material to university classes and to high school curricula. Of the personnel working on this project, ten were female and eight represented ethnic minorities. The work also resulted in the development of a portable potentiostat-controlled electrode system capable of monitoring microbial activity under harsh arctic conditions.
