BIOMINERALIZATION BY PROKARYOTES: ROCKS AS MICROBIAL FOSSILS?

A major problem in the understanding of the origin of life and the evolutionary origin of prokaryotes is the general lack of microbial fossils. Because prokaryotes, both from the Domains Bacteria and Archaea, play a major role in the deposition of minerals in the Earth’s crust, minerals synthesized by bacteria might be useful as microbial fossils.

Biomineralization by prokaryotes is split into two fundamental modes: 1) biologically induced mineralization (BIM) and 2) biologically controlled mineralization (BCM). Minerals formed by BIM generally nucleate and grow extracellularly as a result of the metabolic activities of the organism(s) and subsequent chemical reactions involving metabolic products. In some cases, metabolic products diffuse away from cells and minerals form from solution, while in others bacterial surfaces such as cell walls or S-layers or polymeric materials such as slimes, sheaths, or the matrix of biofilms act as sites for the adsorption of ions and mineral nucleation and growth.

Passive and active surface BIM processes have been described. Passive mineralization refers to the simple non-specific binding of cations to a bacterial surface and the recruitment of solution anions resulting in surface nucleation the growth of minerals. Active mineralization occurs by the direct redox transformation of surface-bound metal ions or by the formation of cationic or anionic byproducts of metabolic activities that form minerals on bacterial surfaces.

In BCM, organisms exert great crystallochemical control over nucleation and growth of the mineral. Minerals are generally synthesized at a specific location within or on the cell and only under certain conditions. This location is commonly in an intracellular membrane vesicle. Minerals produced by bacteria through BCM are well-ordered crystals with narrow size distributions, are of high chemical purity, and have consistent, specific crystal morphologies. Thus, BCM processes are under precise chemical/biochemical and genetic control. The best characterized example of BCM in bacteria is magnetosome formation in the magnetotactic bacteria.

The modes of biomineralization described above will be discussed in the context of the biomineralization processes occurring in terrestrial hot springs and how these minerals might serve as microbial fossils.