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 mineral-formation 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.