Although all forms of life are of critical importance biogeochemicalfy, bacteria are especially significant. This uniqueness arises not only from the great diversity of environments populated by bacteria, but also because they perform many biogeochemical transformations that are carried out poorly or not at all by higher organisms. In addition, bacteria exhibit a profound ability to bind substantial quantities of metallic ions. This retention of metals is facilitated by electrostatic interactions with anionic carboxyl or phosphoryl groups in the structural polymers of the cells. The macromolecular constituents in cell walls and external sheaths of bacteria are particularly reactive, so metals tend to concentrate at the cell surface. These cellular structures also tenaciously bind metallic ions during diagenesis and serve as distinct nucleation sites for the formation of authigenic minerals. Evidence of micro-fossils in ancient sedimentary rocks suggests further that bacteria contributed to analogous processes in the past.