Thematic Articles

A synergistic reaction pathway has been identified between semiconducting minerals and bacteria. Such reactions sustain electron and energy flow from light to nonphototrophic bacteria via semiconducting minerals, which act as a catalytic shuttle. Understanding this pathway may shed light on a unique ecosystem that potentially carries out phototrophic metabolism without the involvement of phototrophic organisms. Four key natural elements of this system are sunlight, semiconducting minerals, nonphototrophic bacteria, and water. This pathway also suggests a “selfcleansing” mechanism that may exist in nature, whereby both oxidative and reductive degradation of contaminants can occur.

Mining activities have created great wealth, but they have also produced colossal quantities of tailings. An important source of heavy metal contamination, sulfide tailings are usually disposed of in open-air impoundments and thus are exposed to microbial oxidation. Microbial activities greatly enhance sulfide oxidation and result in the release of heavy metals and the precipitation of iron (oxy) hydroxides and sulfates. These secondary minerals in turn influence the mobility of dissolved metals and play important roles in the natural attenuation of heavy metals. Elucidating the microbe–mineral interactions in tailings will help us mitigate the environmental impacts of mining activities.

Clay minerals are ubiquitous in soils, sediments, and sedimentary rocks, and they play important roles in environmental processes. Microbes are also abundant in these geological media, and they interact with clays via a variety of mechanisms, such as reduction and oxidation of structural iron and mineral dissolution and precipitation through the production of siderophores and organic acids. These interactions greatly accelerate clay mineral reaction rates. While it is certain that microbes play important roles in clay mineral transformations, quantitative assessment of these roles is limited. This paper reviews some active areas of research on clay–microbe interactions and provides perspectives for future work.

Anthropogenic sources of the toxic metals chromium and uranium have contaminated the ecosystem and become major public and political concerns. Biomineralization, a process by which microorganisms transform aqueous metal ions into amorphous or crystalline precipitates, is regarded as a promising and cost-effective strategy for remediating chromium and uranium contamination. This review describes the potential and limitations of bioremediation for these two toxic metals and highlights the importance of biologically mediated transformation, immobilization, and mineralization of toxic metals during the course of remediation. It also provides nonspecialists with an introduction to several of the main approaches to remediation and acknowledges some questions about this technology that remain to be answered.

Conceptually, minerals represent challenging “substrates” (sources of nutrients and/or energy) for prokaryotes because they can transfer only soluble compounds into or out of their cells. Yet, prokaryotes are able to use a wide array of minerals as sources of energy, trace nutrients, electron acceptors and, remarkably, for positioning themselves using the Earth’s magnetic field. Mineral dissolution exposes microorganisms to a wide range of soluble and potentially toxic metals. Conversely, microbial mineralformation processes can entrap cells, producing microfossils. Intuitively, mineral dissolution and mineral precipitation must provide a benefit for the organism, that is, they must supply the cell with the energy and materials needed to maintain cell structure and function.

Minerals and microbes have coevolved throughout much of Earth history. They interact at the microscopic scale, but their effects are manifested macroscopically. Minerals support microbial growth by providing essential nutrients, and microbial activity alters mineral solubility and the oxidation state of certain constituent elements. Microbially mediated dissolution, precipitation, and transformation of minerals are either directly controlled by microorganisms or induced by biochemical reactions that usually take place outside the cell. All these reactions alter metal mobility, leading to the release or sequestration of heavy metals and radionuclides. These processes therefore have implications for ore formation and the bioremediation of contaminated sites.