Manganese has a multi-faceted role in biological systems. Manganese can exist in 11 oxidation states (+7 to -3), more than any other element, but the aqueous chemistry of manganese is largely restricted to Mn²⁺ complexes. Both Mn²⁺ and Mn³⁺ are found in enzymes. The ionic radius of 0.9Å places Mn²⁺ midway between that of Mg²⁺ and Ca²⁺ so it is not surprising that there is overlap in function with this group in providing structural charge stabilization of enzymes and, in some cases, substrates such as Mn ATP. In this regard, manganese has been useful in substitutions for Mg²⁺ and Ca²⁺ as a handle for NMR and EPR spectroscopic probes of active site architecture and catalysis (reviewed by Reed and Poyner, 2000). Manganese also acts as a superacid catalyst in several hydrolytic enzyme-catalyzed reactions (reviewed by Crowley et al, 2000).

In general, the relevant redox functions of manganese in enzymes come into play largely with oxygen as the substrate or product. Two manganese enzymes that protect bacterial cells from active oxygen species, superoxide dismutase and pseudocatalase, have been described and some bacteria also use low molecular weight Mn(II) complexes to scavenge superoxide radicals and hydrogen peroxide (reviewed by Horsburgh et al, 20002). Manganese is an essential component of the oxygen-evolving complex in photosystem II in plants and cyanobacteria (Keren et al, 2002). The first known manganese-containing aromatic ring cleavage dioxygenase was discovered in Bacillus brevis. Additional examples of these enzymes, which were previously known in only iron-containing forms, have since been found in other bacteria (reviewed by Que and Reynolds, 2000).

Manganese is known to be required for both endospore formation and germination in Bacillus spp. (reviewed by Jakubovics and Jenkinson, 2001). All of the metalloproteins in Borrelia burgdorferi, the causative agent of Lyme disease, use manganese and iron is not required (Posey and Gherardini, 2000). Lactobacillus spp. also have an absolute requirement for manganese but do not require iron for growth (Imbert and Blondeau, 1998). Relative levels of manganese and iron in growth medium were found to influence the carbohydrate degradation pathway used by Deinococcus radiodurans (Zhang et al, 2000). The mechanisms and regulation of manganese transport in bacteria and yeast have been characterized. Bacteria import manganese via ABC transporters and P-type ATPases, and yeast are known to use NRAMP family metal transporters for manganese uptake (reviewed by Culotta, 2000). Several prokaryotes can reduce Mn(IV) to Mn(II), and some organisms can derive sufficient energy from manganese reduction to support growth (reviewed by Nealson AND Saffarini, 1994). Oxidation of Mn(II) to insoluble Mn(III,VI) oxides by prokaryotes has also been described (Francis et al, 2002).

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