Our lab investigates the combination of electrochemistry with microbial metabolism, currently exploring the capacity of microorganisms to interact with electrodes in both the cathodic and anodic chambers of electrochemical cells, referred to here as electron-donating and electron-accepting processes, respectively.
Electron-donating processes
Microorganisms can make use of electrons donated from the cathode of an electrochemical cell in lieu of more traditional chemical electron donors. Microorganisms can couple oxidation of the cathode to reduction of a suitable electron acceptor such as nitrate or perchlorate for the purpose of gaining energy. When the electron acceptor is a contaminant, bioelectrical stimulation of microbial reduction of that contaminant can be used as a bioremediative strategy. A portion of our perchlorate bioremediation (see below) work makes use of this bioelectrical reactor (BER) technology, taking advantage of the ability of microorganisms to use electrons donated by an electrode for the purpose of perchlorate reduction. Currently we have a patent pending on this technology and another applied for. Researchers: Cameron Thrash, Juan Fernando Villaromero
Electron-accepting processes
Microorganisms can make use of the anode of an electrochemical cell as an electron acceptor similarly to solid-phase iron and other metals. When electrons are passed to the anode they can be harnessed to do work in the form of electrical energy. By stimulating microorganisms to reduce the anode by supplying a suitable oxidizable substrate, the system can produce electricity and is known as a microbial fuel cell (MFC). These systems show great promise in harvesting electricity from substrates that were previously considered waste products and may also play an important part in making current wastewater treatment facilities more efficient and cost-effective. Researchers: Kelly Wrighton, Peter Agbo

Our work includes investigations on the removal of radioactive toxic metals, carcinogenic petroleum-based hydrocarbon contaminants, and toxic munitions byproducts from the environment. Significant public and federal attention has recently focussed on environmental contamination as a result of the Cold War. One common contaminant of drinking water was recently identified to be ammonium perchlorate. This compound is currently used in the manufacture of rocket propellants, explosives, fireworks, matches, and airbags. It affects the thyroid gland and the full extent of its toxicity is still unknown. As such, the US EPA recently set a recommended regulatory limit of 1 part per billion for drinking water. Microbial transformation of this contaminant was identified as the most practical form of remediation. Prior to this, there were only two bacteria known that could remove perchlorate and very little was known about the diversity and ubiquity of this metabolism. Our lab has demonstrated that bacteria with this metabolism are commonly found in the environment. We identified the dominant groups of bacteria found that are responsible for the transformation of perchlorate into innocuous chloride and have isolated and characterized more than 40 of these bacteria in the last 3 years. Additionally, our lab works on the bioremediation of Uranium through adsorption to biogenic iron oxides, as well as studying the implications of microbial Uranium oxidation on existing reductive strategies.

We have identified the common biochemical pathway used by these bacteria for transforming perchlorate and identified the genetic systems involved. We have purified a central enzyme in this pathway and have developed a sensitive probe based on antibodies specific for this enzyme in these bacteria. Current research focuses on the regulation of this system in the presence of other electron acceptors such as oxygen and nitrate. Researchers: Yvonne Sun, Justin Ishida

Our lab does extensive research on organisms that are capable of anaerobic biooxidation of ferrous iron which may be present in rock minerals like pyrite or silicacious minerals such as almandine or staurolite. Anaerobic bio-oxidation of Fe(II) was only recently identified and very little is known regarding the ubiquity and diversity of organisms capable of this metabolism. Previous studies have shown that Fe(II)-oxidation is mediated by anoxygenic phototrophs as well as various nitrate-respiring organisms. Recent studies in our lab have demonstrated that members of the previously described Dechloromonas and Dechlorosoma genera can also oxidize Fe(II) with chlorate or nitrate as alternative electron acceptors. In cell suspension experiments with nitrate as the electron acceptor, the Fe(II) is oxidized to insoluble amorphous Fe(III)-oxide. Amorphous Fe(III)-oxide [Fe2O3.H2O(am)], or ferrihydrite, has often been used for the study of adsorption of trace metals because it is a uniform material with well known surface properties that is easily reproduced. It is also representative of metal oxides in the natural environment and is a precursor to many natural forms of crystalline Fe(III)-oxides such as goethite and hematite. Previous studies have shown that metals such as cobalt, cadmium, lead, uranium, and radium are rapidly adsorbed by this iron form and some of these metals with lower ionic radii (e.g. Co2+, Cd2+) are incorporated into the Fe(III)-oxide structure as the amorphous Fe(III)-oxides begin to recrystallize with age. As such, these trace metals become tightly bound into the Fe(III)-oxide crystal and are thus immobilized.

A proposed strategy for the remediation of uranium (U) contaminated sites is based on immobilizing U by reducing the oxidized soluble U, U(VI), to form a reduced insoluble end product, U(IV), uraninite. Our previous NABIR funded studies have demonstrated that radionuclides such as uranium and cobalt are rapidly removed from solution during the biogenic formation of Fe(III)-oxides. In the case of uranium, X-ray spectroscopy analysis indicated that the uranium was in the hexavalent form (normally soluble) and was bound to the precipitated Fe(III)-oxides thus demonstrating the bioremediative potential of this process.

However, it is now recognized that a diversity of microorganisms cansubsequently catalyze the oxidation of U(IV) coupled to the reduction of NO3. Our recent studies have indicated that microorganisms capable of nitrate-dependent bio-oxidation of U(IV) inhabit uranium-contaminated and uncontaminated DOE sites. These organisms, including strain TPSY and "C. millennium", can utilize U(IV) as the sole electron donor which has the potential to produce mobile U in anoxic environments. In order to predictably model remediation efforts based on U(VI) reduction, it is essential to understand the microorganisms and the physiology of anaerobic, U(IV) bio-oxidation and the impact of this microbial metabolism on the long term sequestration of U in the environment. Researchers: Karrie Weber, Kamal Gandhi, Saumyaditya Bose, Ian Van Trump, Cameron Thrash, Caroline Chow

Our lab also has isolated the first organism of any type capable of the anaerobic degradation of benzene. Benzene is an important industrial chemical with an annual worldwide production in excess of 25 million tonnes. Benzene is currently recognized as one of the most important contaminants in the US and is among the most prevalent organic contaminants in ground waters. It is of major concern due to its toxicity (as a potent carcinogen) and relatively high solubility. It is currently ranked fifth on the US National Priorities List (NPL),and has been found in at least 816 of the 1,428 current or former NPL sites. Our organism, Dechloromonas aromatica strain RCB, oxidizes benzene with nitrate, chlorate, or perchlorate as alternative electron acceptors (surrogates for oxygen). We have recently identified that this organism uses a novel biomethylation reaction for destabilizing the benzene molecular structure prior to degradation. The complete genome sequence of RCB is now available and current research takes advantage of this combining microarray data with transposon libraries to uncover the genetic systems behind these novel metabolic pathways. Researchers: Kathy Byrne-Bailey, Mark Heinnickel, Forest Kaser, Lacey Westphal

Redox chemistry, especially that of ferric iron [Fe(III)], is one of the most geochemically significant processes that occurs in soils, paddy fields, wetlands, and aquatic sediments. Iron redox chemistry in soils can be altered both abiotically and biologically through the activity of indigenous microorganisms. Previous studies have indicated that the biological iron redox cycle is preferentially mediated through microbial oxidation and reduction of the quinone moieties of humic substance (HS) that abiotically interact with the mineral iron content of soils and sediments. Such redox cycling of iron can affect important soil characteristics such as pH, fate and transport of trace metals, sulfates, and phosphates adsorbed to the surface of the Fe(III) oxides, and cause an increase in the ionic strength of the soil solution as well as displacement of sodium, potassium, calcium, and magnesium into solution. These effects are compounded by the chelation and absorptive characteristics of humic substances which further control the complexation, transportation, and bioavailability of these ionic species.

Furthermore, reduction of soil iron through redox reactive reduced HS can result in soil gleying effects in which the Fe(III) oxides in the soil are converted to Fe(II) compounds resulting in a gray bleached appearance. Such reductive mechanisms can greatly influence many soil characteristics including aggregate stability, permeability, friability, porosity, and hydraulic conductivity, as well as clay swelling and mobility. In addition, the geochemistry and binding capacity of HS for cations and hydrophobic compounds is directly dependent upon the redox state of the HS quinone moieties. Reduced HS have a significantly higher affinity for cations and a lower affinity for hydrophobic compounds than oxidized HS. Thus, the solubility, transportation, and bioavailability of both hydrophilic and hydrophobic compounds including many pesticides and herbicides in any soil environment is partially a function of the redox state of the HS. As such, the mineralogy, morphology, friability, porosity, and fertility of soil is partially a function of the concentration and redox state of the HS content of soil. Researchers: Ian Van Trump