Modes of Biomineralization of Magnetite by Microbes

Biomineralization processes have traditionally been grouped into two distinct modes; biologically induced mineralization (BIM) and biologically controlled mineralization (BCM). In BIM, mi­ crobes cause mineral formation by sorbing solutes onto their cell surfaces or extruded organic polymers and/or releasing reactive metabolites which alter the saturation state of the solution proxi­ mal to the cell or polymer surface. Such mineral products appear to have no specific recognized functions. On the other hand, in BCM microbes exert a great degree of chemical and genetic control over the nucleation and growth of mineral particles, presumably because the biominerals produced serve some physiological func­ tion. Interestingly, there are examples where the same biomineral is produced by both modes in the same sedimentary environment. For example, the magnetic mineral magnetite (Fe3O4) is generated extracellularly in the bulk pore waters of sediments by various Fe(III)-reducing bacteria under anaerobic conditions, while some other anaerobic and microaerophilic bacteria and possibly protists form magnetite intracellularly within preformed vesicles. Differ­ ences in precipitation mechanisms might be caused by enzymatic activity at specific sites on the surface of the cell. Whereas one type of microbe might facilitate the transport of dissolved Fe(III) into the cell, another type will express its reductive enzymes and cause the reduction of Fe(III) external to the cell. Still other mi­ crobes might induce magnetite formation indirectly through the oxidation of Fe(II), followed by the reaction of dissolved Fe(II) with