ABSTRACT
ACKNOWLEDGMENTS 30
REFERENCES 30
Manganese, element 25 (atomic weight 54.93805), is the third most abundant transition metal in Earth’s crust (-0.019 mol kg-1), consid erably less abundant than iron (-1.1 mol kg-1), and the eighth most abundant crustal metal overall [1-3]. The manganese abundance of the ocean crust is about 60% greater than that of the continental crust. (In the solar system, the relative abundance of Fe and Mn is about 102 [2].) Manganese is an essential element in living systems, with some 20 identified functions in enzymes and proteins [3]. Major biological roles of manganese are in making 0 2 (photosystem II) and in disposing of superoxide radicals (superoxide dismutase) [3]. The geochemical distri bution of manganese in the hydrosphere, lithosphere, and atmosphere (dust particles) involves oxidation states Mn(II), Mn(III), and Mn(IV) [4], which show a wide range of strengths as Lewis acids, and whose coordination chemistries reflect a strong preference for oxygen donor ligands [3,5,6]. The biota exert a strong influence on the geochemistry
of manganese through bacterial oxidation and reduction [7-9], and through Mn incorporation in new biomass production. Because of the high reduction potentials of the IV and III oxidation states of manga nese in aquatic systems, manganese cycles are linked to a significant degree with the geochemical cycles of carbon, oxygen, iron, sulfur, arse nic, and other redox elements [4,7]. Linkage between the oxygen and manganese cycle is exemplified by the redox reaction
0 2 + 4Mn2+ + 6H20 ^ 4MnOOH(s) + 8H+ (1)
(and similarly for the further oxidation of Mn(III) to M n02). Linkage between the carbon and manganese cycle may be symbolized by the redox reaction:
H2CO(aq) + 2Mn02(s) + 4H+ ^ 2Mn2+ + C 02(aq) + 3H20 (2)
in which H2CO is formaldehyde, the carbohydrate building block. Both reactions (1) and (2) are energetically favorable under the conditions in most natural waters and sediments; both are microbially mediated.
