Silicate minerals

There has been much interest in the weathering of silicate minerals (Bennett et al., 2001; Rogers and Bennett, 2004), the result of which is the formation of new phases (e.g., clays and oxyhydroxide minerals) and altered aqueous and atmospheric geochemistry. Microbes mediate silicate mineral weathering in a number of ways: they may have a direct effect by producing acids, bases, and ligands which differently promote mineral weathering by catalyzing the release of ions to solution, or they may effect a more indirect response by producing compounds such as extracellular polysaccharides that bind particles together, increasing water retention at mineral surfaces and thereby increase the time available for hydrolysis (Bennett et al., 2001). Microbial involvement in surface processes has been considered at a microenvironmental level, with microgeochemical environments being very different from those of bulk solutions, often resulting in localized etching (Fisk et al., 1998; Thorseth et al., 1995). While bacteria have been implicated in the accelerated weathering of minerals, it is not clear if this is simply the coincidental result of microbial metabolism, or if it represents a specific strategy offering the colonizing bacteria a competitive advantage. All microorganisms require elements such as K, Fe, Mg, and so on that can be derived from silicate mineral weathering; however, there are cases whereby colonizing microorganisms do not prefer any particular naturally favorable site (e.g., a pore or fissure), but rather colonize a substrate on the basis of their own inherent growth patterns (Brehm et al., 2005). Mineral dissolution may also be inhibited by the metabolic activities of microorganisms, for example the formation of extracellular polysaccharides that may irreversibly bind to mineral surfaces thus preventing dissolution (Welch and Vandevivere, 1994; Welch et al., 1999).

Laboratory studies of silicate weathering have demonstrated that microbes limited by Mg and K produce organic ligands that accelerate dissolution of biotite (Paris et al., 1995), while field experiments have shown bacteria preferentially colonizing potassium-rich mineral phases, as well as preferential colonization of apatite grain inclusions (phosphorus-rich) within K-feldspars (Bennett et al., 1996). Gleeson et al. (2005,2006) have demonstrated that both bacteria and fungi preferentially colonize different minerals in response to the elemental content of that mineral. It is well understood that quartz is one of the rock forming minerals that is most resistant to weathering (White and Brantley, 2003), and in granitic rocks, quartz grains have a much higher resistance to chemical weathering than many coexisting minerals such as feldspar. However, despite the great chemical and physical resistance of quartz, as well as the lack of obvious nutrition to be derived through its breakdown, some microorganisms have been shown to preferentially colonize quartz (Brehm et al., 2005; Gleeson et al., 2005). Although the surface distribution of microorganisms may be controlled by mineralogy and the ability of an organism to take advantage of nutrients within mineral structures, surface attachment processes are also likely to be important and may be different for different mineral types. For example, microtopography, surface composition, surface charge, and hydrophobicity may play an integral role in microbial attachment and detachment processes and biofilm formation (Bennett et al., 1996).

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