Example Nanoparticles formed by microbes in anoxic regions of AMD systems

Example Nanoparticles formed by microbes in anoxic regions of AMD systems 

SRB are essentially ubiquitous in aqueous environments that contain organic carbon and sulfate (e.g., subsurface aquifers and lake sediments). Moreover, analysis of a key gene associated with sulfate reduction (dissimilatory sulfite reductase) indicates that microbial sulfate reduction is an ancient trait, suggesting that organisms may have contributed to sulfide mineral formation throughout much of Earth history (Wagner et al. 1998). SRB are tolerant to environmental extremes of heat (some are hyperthermophiles) and salinity (some are halophiles). 

The SRB imports both the organic compounds and sulfate into the cytoplasm. The organic is oxidized to CO2 through a modified citric acid cycle and reducing molecules such as NADH and FADH2 are generated. These coenzymes transport 2 electrons and 1 H and 2 electrons and 2 H, respectively, to the cytoplasmic membrane where deydro-genases separate electrons and H. Protons are transported outside the membrane to 

It is interesting to note that sulfide, the waste product of anaerobic respiration by SRB, is relatively toxic to microorganisms in high concentrations due to its ability to denature certain proteins and bind to metal centered enzymes (Brock and Madigan 1991 Postgate 1965 Trudinger et al. 1972). Although an incidental process that occurs exterior to the cell, precipitation of very insoluble metal sulfides serves to make the environment more hospitable for microbial communities in these habitats. The process also may contribute to the formation of ore bodies (Druschel et al. 2002). 

In anaerobic (or microaerophilic) environments, sulfide accumulates until the solution becomes saturated with a mineral phase for which precipitation kinetics are favorable. Labrenz et al. (2000) and Druschel et al. (2002) present a geochemical model that predicts the sequence of precipitation based on the solution composition and some kinetic considerations. They show that formation of monomineralic deposits of nanoparticulate metal sulfide phases can occur if metals are resupplied by fluid flow at 

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