Research

Microbial induced metal redox cycling and biomineralization

Listed below are brief descriptions of some of the research avenues that are currently of interest to the Hansel lab.

Factors Dictating the Microbial Reduction of Fe

Within natural surface environments, Fe exists within two dominant oxidation states:  Fe(II) and Fe(III).  At circumneutral pH, Fe(III) exists as solid-phase (hydr)oxides, which serve as strong sorbents for contaminants within the environment.  A phylogenetically and physiologically diverse group of organisms, referred to as dissimilatory iron-reducing bacteria (DIRB), catalyze the reduction of Fe(III) to Fe(II) to gain energy for growth and function.  Upon respiration, the strong reductant Fe(II) is produced, which may reactant with oxidants within the system, rendering contaminants such as uranium or chromium immobile.  Iron(II) may also react with Fe and Mn (hydr)oxide surfaces inducing mineralogical transformations, which consequently changes the reactivity and bioavailability of the substrates.  Understanding these reactions are crucial for predicting the long-term capacity of sediments for microbial respiration and contaminant transformation and/or sequestration.  Yet, little is know regarding the controls on microbial respiration of Fe(III) (hydr)oxides, especially those more closely mimicking natural Fe(III) (hydr)oxides.  We are investigating the impact of mineral structure and metal impurities within the hydroxide structure on (1) the rate and extent of microbial Fe(III) reduction, (2) the mechanism adopted for reduction, and (3) the resultant Fe phases formed and their reactivity.  This information will elucidate the factors that need to be considered when making predictions of the microbial reduction capacity of sediments and their potential for bioremediation strategies.  

This research involves aspects of microbe-mineral interactions, mineralogy, geochemistry, and environmental microbiology.  Students will be introduced to a variety of experimental (flow through mineral beds, bacterial monitoring), microscopic (scanning electron and transmission electron microscopy – SEM/TEM), and spectroscopic (X-ray absorption spectroscopy – XAFS/EXAFS) techniques. 

The Role of Microbial Metabolites on Mn and Metal Redox Chemistry

Manganese oxides are strong contaminant sorbents and are considered one of most powerful oxidants within natural systems.  The oxidation of soluble Mn(II) to insoluble Mn(III)/(IV) oxides is kinetically slow in the absence of surface catalysts or Mn(II)-oxidizing bacteria/fungi.  Recently, we have discovered that Mn can be oxidized by redox-active metabolites formed during growth of a diverse range of bacteria.  Due in part to enhanced oxidation of filtrates in light, we hypothesize that an organic metabolite produced during heterotrophic growth is photo-excited, producing radicals, which then oxidize Mn(II) – thus, Mn(II) oxidation is a coupled biotic-abiotic process.  We are particularly interested in the role of these indirect microbial processes on metal redox cycling and are investigating the identification, ubiquity, reactivity, and metal specificity of metabolites formed by metal oxidizing/reducing bacteria.  We are also interested in how redox mechanisms are related to the physiological and phylogenetic diversity of Bacteria and Archaea.  These metabolites formed during microbial growth and respiration may represent an unrecognized control on metal cycling.  Furthermore, the link between light and microbial organic exudates may lend further insight into the reactions controlling metal redox transformations within surface environments having potential implications for modern (surface primary productivity) and ancient (banded iron formations) systems. 

This research involves aspects of redox chemistry, mineralogy, microbial ecology, and molecular microbiology.  Students will be introduced to microbial, photo-, redox-, and organic chemistry using a variety of experimental (chemical assays), chromatographic (high performance liquid chromatography – HPLC), microbiological (culturing/characterization) and molecular biological (PCR based analysis, DNA sequencing/analysis) techniques.

The Impact of Antibiotics and Antibiotic Resistance on Metal Redox Cycling

Antibiotics are routinely used in livestock production as both a prophylactic and growth promoter.  Due to low absorption rates, it has been estimated that as much as 75% of antibiotics administered to feedlot animals could be excreted unaltered in feces.  Consequently, soils and waters adjacent to animal feedlots have been found to contain high concentrations of antibiotics, promoting the evolution of antibiotic resistance genes.  A strong correlation between antibiotic resistance and metal resistance is frequently observed, prompting concern that high metal concentrations may select for antibiotic resistance factors.  We are interested in how the presence of antibiotics and antibiotic resistance impacts the cycling of metals within the soils, sediments, and waters.  Antibiotics may form strong metal complexes and many are redox- and photo-active, which may greatly contribute to the fate and transport of metals.  Also, contaminants such as the metalloid arsenic, may undergo redox transformations via microbial detoxification mechanisms;  enhanced metal toxicity associated with antibiotic resistance may alter the As cycle.  We are investigating the complexation and redox properties of antibiotics found within soils and waters produced from natural indigenous bacterial communities and through agricultural practices.  We are also interested in whether high cell densities produced during biostimulation (bioremediation) strategies may stimulate antibiotic production, introducing antibiotic and metal resistance evolution and consequently impacting remediation success.

This research involves aspects of redox chemistry, mineralogy, environmental microbiology, and organic geochemistry.  Students will be introduced to microbial, photo-, redox-, and organic chemistry using a variety of experimental (chemical assays), chromatographic (high performance liquid chromatography – HPLC), microbiological (culturing/characterization) and spectroscopic (Fourier transform infrared (FTIR) spectroscopy) techniques.