Identification of microbial carbon sources in headwater stream ecosystems

Use of compound specific stable isotopes to identify isotopic composition of microbes inhabiting stream biofilms. Microorganisms serve as the base of the food web in most aquatic ecosystems, and dissolved organic matter (DOM) is the major source of carbon and energy maintaining bacterial metabolism and growth. Determining the source of carbon fueling microbial activity, however, remains one of the key challenges in aquatic biogeochemistry and would provide the mechanistic basis for understanding the regulation of the trophic structure of aquatic ecosystems.
The main objective of our research is to develop and test the use of the δ13C composition of bacterial phospholipid fatty acids (PLFA) for the investigation of microbial carbon sources in streams. The question of how well the isotopic composition of organic matter sources in streams can be resolved over different spatial and temporal scales remains unanswered. Additionally, the δ13C of microbial PLFA is a new method that has yet to be related to DOM, a more challenging task due to the heterogeneous composition of compounds of varying isotopic composition that form DOM. The specific objectives of our research are to determine: (1) The relationship between δ13C of bacterial PLFA and DOM; (2) The separation in the δ13C of allochthonous versus autochthonous materials across spatial and temporal scales within streams; and (3) How the δ13C of organic matter and bacterial PLFA are related in streams.

A coupled investigation of the biogeochemical and hydrological dynamics dictating nitrogen transport in karst watersheds

A collaborative project with Dr. Phil Hays (USGS/Geosciences, University of Arkansas) coupling specific hydrogeological approaches with stable isotopes and microbial assays to further understand how hydrological flowpath influences nutrient cycling in karst groundwater.
Land application of animal manures in karst regions poses a significant threat to water quality, and has been linked to water-quality degradation and eutrophication that range from field- to continental-scale. Long-term sustainability of animal production on karst lands requires that management practices evolve to optimize effective nutrient use while minimizing environmental impacts. To manage animal production in vulnerable karst watersheds effectively, we must thoroughly understand the processes and controls affecting nitrogen transport, transformation, and sequestration.
Our research is focused on the surface-water/ground-water interface and interflow zones in the vadose region overlying karst aquifers. These zones represent one of the few areas of these flow systems where diffuse flow occurs. The extended residence time and exponentially-increased particle surface area for soil-water interaction make the interface and interflow zones optimum zones for biogeochemical processing. In addition, these zones are likely to be the most impacted by agricultural practices at the surface, which can enhance or degrade the potential for nutrient processing in karst. The main goals of the our research are to 1) determine the role of biogeochemical processes occurring at interface and interflow zones, and to assess their role in controlling N transport in karst; and 2) test specific methodologies and verify feasibility of selected approaches for a more comprehensive future study. Objectives will be met using synoptic sampling events that target all hydrologic zones of a shallow flow system within a well-characterized watershed. Concentration and isotopic composition of NO3- will be used to determine the extent of denitrification and immobilization. Controlled experiments will be used to determine the relative bioavailability of dissolved organic carbon, and its role in the cycling of N in each zone. Once the biogeochemical mechanisms proposed are elucidated we will be able to test the impact of agricultural practices on the integrity of these zones, and on how the processes occurring within these zones can be capitalized upon for nutrient management in karst watersheds.

The impact of nutrient enrichment on dissolved organic matter cycling in streams.

We are currently investigating the photochemical and biological reactivity of dissolved organic matter (DOM) in headwater streams. These studies are currently being conducted on a pair of streams differing in inorganic nutrient loading dictated primarily by watershed land use in order to test for the impact of nutrient concentrations on DOM cycling. Future investigations are planned for expanding this research to a number of streams within the Ozark Mountains to further elucidate the relationship between nutrient enrichment and DOM cycling.
Anthropogenic hypoxic zones occurring in coastal zones worldwide are the result of nutrient (inorganic nitrogen and phosphorus) overenrichment derived primarily from associated watersheds. Understanding the biogeochemical processes occurring in small streams is critical to increasing our knowledge of effects of nutrient-enrichment on watersheds and the consequences for the associated coastal ecosystems. Lower order streams are the locations of most nutrient uptake and processing within watersheds, and where the bulk of terrestrial nutrients enter the aquatic environment. Nutrient uptake and processing in streams is often regulated by bioavailable dissolved organic matter (DOM). Elevated nutrient concentrations can increase microbial reworking. Microbial reworking is the early stage of organic matter diagenesis resulting from the synthesis of microbial biomass, and may contribute to larger proportions of refractory DOM in nutrient-rich streams. The increased light availability and water residence time in downstream ecosystems may increase mineralization of refractory forms of DOM, derived from nutrient-enriched headwater streams, through microbial and photochemical processing. Consequently, formation of refractory forms of DOM within small streams could provide a mechanism for the transport of limiting nutrients downstream. Potential changes in DOM composition resulting from nutrient-enrichment may also alter the structure of active microbial communities, thereby impacting the biogeochemical function of streams.
The main goal of our research is to determine the effect of nutrient enrichment on DOM cycling in streams by specifically addressing: (1) the relationship between inorganic nutrient concentration and the biological and photochemical reactivity of stream DOM; (2) Microbial reworking as a mechanism by which refractory DOM can be generated in nutrient-rich streams; (3) How elevated nutrient concentrations influence the utilization of DOM by microbial biofilm communities in streams.

Bioremediation of hydrocontaminated soil

Bioremediation is the use of natural attenuation for the clean up of contaminants in the environment, and it relies upon the activity of microorganisms to breakdown these contaminants. We are involved in a collaborative research program with Drs. Greg Thoma (UA-Chemical Engineering) and Duane Wolf (UA-Crop, Soil and Environmental Science) to study and model phytoremediation of hydrocarbon contaminated soil. Our work has included a combination of field, laboratory and modeling experiments to determine the best management practices for the remediation of hydrocarbons in soil.
Hydrocarbons released in the environment pose a recognized risk to human health due to their toxicity and potential for human exposure. Many hydrocarbons are ranked high on the Superfund Site Priority list due to their prevalence in groundwater and soil. Bioremediation cleanup efforts have met with varied success. It is well known that significant variations in soil microbial community structure are associated with different soil type and ecosystem. The varied success of bioremediation may, therefore, be a direct result of differences in microbial community structure. The current understanding of the microbial ecology of contaminated environments remains limited by a lack of knowledge regarding the link between microbial community structure and its function. Without this mechanistic understanding development of proper management strategies to improve bioremediation efforts remained hindered. Our research aims to assess the importance of microbial community structure to the bioremediation of hydrocarbon contaminated soil. We are interested in understanding how microbial community structure is linked to bioremediation in soils from different ecosystems by tracking which components of these communities are responsible for the degradation.

Impact of elevated CO2 and nutrient enrichment on microbial communities in soil

We are currently collaborating with Sean Schaeffer (Department of Biological Sciences; University of Arkansas) investigating the impact of elevated CO2 on substrate utilization by soil microbes. This work is being conducted as part of a larger effort to assess the impact of elevated CO2 on C and N dynamics of desert ecosystems. Additionally we are working with Dr. Sharon Billings (University of Kansas) to assess the impact of both elevated CO2 and nutrient enrichment on microbial community structure and function in forest soil. This work is part of the Duke University FACTS allowing us to follow 13C-labeled CO2 through the microbial community by measuring the δ13C of soil microbial phospholipid fatty acids (PLFA). The focus of our research is to assess how both microbial community composition, substrate source, and activity have been altered with elevated CO2. It is possible that altered soil C substrate, possibly via root exudates, represent an important change in energy source for the bacterial component of the soil microbial community under elevated CO2. This will have important implications for the rates of soil N-cycling, providing an advantage to one functionally distinct and more N-rich component of the microbial community.