Introduction:

Pollution of the environment by acid, metal-rich solutions is a major environmental problem at sites around the world. Acid solutions form when rocks rich in sulfide minerals are oxidized on exposure to air and water (acid rock drainage), but the rates of sulfide oxidation are typically slow and the sulfuric acid is rapidly neutralized by interaction with surrounding silicate minerals (hydrolysis reactions). However, mining activities associated with extraction of metals such as Au, Ag, Cu, Zn, and U increase the permeability and porosity of rocks and greatly accelerate rates of formation of acid solutions. The acid solutions that form in association with mining activities are referred to as acid mine drainage (AMD). The problem is particularly severe if the concentration of sulfide minerals in the rocks in high, the ability of the surrounding rocks to buffer the pH is low, and the deposit is open to the air. As you will read about below, these conditions can lead to generation of formation of large volumes of pH 0.7 solution (similar to battery acid) that is rich in iron (10's g/L) and toxic metals. In addition to being associated with metal mines, sulfide minerals, especially pyrite (FeS2, also called "fools gold"), are also relatively abundant in coal deposits and uranium, so AMD is an environmental problem that is commonly associated with energy resources.

Perhaps one of the most intriguing aspects of the AMD problem is that the majority of AMD formed is the direct result of microbial activity. We are studying the linkages between the geochemistry of AMD systems and their microbial communities. Much of this work is directed toward understanding the physiology of organisms adapted to live in extreme environments (extreme acidophiles) and the microbial ecology of AMD systems.

More details:

At low pH, the most effective sulfide surface oxidant is ferric iron. Ferric iron ions forms when oxygen oxidizes ferrous iron after it is released by pyrite (FeS2) dissolution. At low pH, the rate of inorganic oxidation of ferrous iron by oxygen is slow. However, microorganisms (bacteria and archaea) are able to catalyze ferrous iron oxidation, thus increase the rate of supply of ferric iron to metal sulfide surfaces. For this reason, iron-oxidizing microorganisms greatly accelerate the rate of production of AMD. We are working to understand the biogeochemical controls on the weathering of sulfide minerals and the generation of acid mine drainage. This project involves work at the interface between geology, chemistry, and biology. Iron Mountain, in northern California, is our primary field site, and most of our work is conducted underground, within the Richmond Mine. Our project has many components, including assesment of the microbial ecology in AMD environments via 16S rRNA gene sequence analysis, culturing, metabolic characterization, and protein analyses. In addition, we are conducting a genomics study of AMD microbial communites. We are also working on genome-based approaches that enable culture-independent understanding microbial activity relevant to geochemical processes that occur in AMD systems. This work is designed to explore the distribution and diversity of metabolic pathways in these communities (e.g., nitrogen fixation, sulfur oxidation, iron oxidation), to understand the mechanisms by which the microbes tolerate the environmental extremes, and to evaluate how these tolerance mechanisms impact the geochemistry of the environment . The Joint Genome Institute (Dept. of Energy) sequenced the genome of one of our isolates and recently we published the first reconstruction of (multiple) genomes from the AMD system (Tyson et al. Nature, Feb. 1 2004). Other projects include determining sulfide mineral dissolution rates, chemical modeling of AMD waters, and evaluating the role of lateral gene transfer in evolution of acidophiles