Manganese cycle

The manganese cycle is the biogeochemical cycle of manganese through the atmosphere, hydrosphere, biosphere and lithosphere. There are bacteria that oxidise manganese to insoluble oxides, and others that reduce it to Mn²⁺ in order to use it.

Manganese is a heavy metal that comprises about 0.1% of the Earth's crust and a necessary element for biological processes. It is cycled through the Earth in similar ways to iron, but with distinct redox pathways. Human activities have impacted the fluxes of manganese among the different spheres of the Earth.

Manganese is a necessary element for biological functions such as photosynthesis, and some manganese oxidizing bacteria utilize this element in anoxic environments. Movement of manganese (Mn) among the global "spheres" (described below) is mediated by both physical and biological processes. Manganese in the lithosphere enters the hydrosphere from erosion and dissolution of bedrock in rivers, in solution it then makes its way into the ocean. Once in the ocean, Mn can form minerals and sink to the ocean floor where the solid phase is buried. The global manganese cycle is being altered by anthropogenic influences, such as mining and mineral processing for industrial use, as well as through the burning of fossil fuels.

Manganese is the tenth most abundant metal in the Earth's crust, making up approximately 0.1% of the total composition, or about 0.019 mol kg⁻¹, which is found mostly in the oceanic crust.

Manganese (Mn) commonly precipitates in igneous rocks in the form of early-stage crystalline minerals, which, once exposed to water and/or oxygen, are highly soluble and easily oxidized to form Mn oxides on the surfaces of rocks. Dendritic crystals rich in Mn form when microbes reprecipitate the Mn from the rocks on which they develop onto the surface after utilizing the Mn for their metabolism. For certain cyanobacteria found on desert varnish samples, for example, it has been found that manganese is used as a catalytic antioxidant to facilitate survival in the harsh sunlight and water conditions they face on desert rock surfaces.

Manganese is an important soil micronutrient for plant growth, playing an essential role as a catalyst in the oxygen-evolving complex of photosystem II, a photosynthetic pathway. Soil fungi in particular have been found to oxidize the reduced, soluble form of manganese (Mn²⁺) under anaerobic conditions, and may reprecipitate it as manganese oxides (Mn⁺³ to Mn⁺⁷) under aerobic conditions, where the preferred metabolic pathway typically involves the utilization of oxygen. Although not all iron-reducing bacteria have the capability of reducing manganese, there is overlap in the taxa that can perform both metabolisms; these organisms are very common in a range of environmental conditions. Challenges however persist in isolating these microbes in cultures.

Depending on the pH, organic substrate availability, and oxygen concentration, Mn can either behave as an oxidation catalyst or an electron receptor. Though much of the total Mn that is cycled in soil is biologically-mediated, some inorganic reactions also contribute to Mn oxidation or precipitation of Mn oxides. The reduction potential (pe) and pH are two known constraints on the solubility of Mn in soils. As pH increases, Mn speciation becomes less sensitive to variations in pe. In acidic (pH = 5) soils with high reduction potentials (pe > 8), the forms of Mn are mostly reducible, with exchangeable and soluble Mn decreasing dramatically in concentration with increases in pe. Mn is also found in inorganic chelation complexes, where Mn forms coordinate bonds with SO₄^2-, HCO₃⁻, and Cl⁻ ions. These complexes are important for organic matter stabilization in soils, as they have high surface areas and interact with organic matter through adsorption.

Iron (Fe) and Manganese (Mn) similarities in their respective cycles and are often studied together. Both have similar sources in the hydrosphere, which are hydrothermal vent fluxes, dust inputs, and weathering of rocks. The major removal of Mn from the ocean involves similar processes to Fe as well, with the most abundant removal from the hydrosphere via biological uptake, oxidative precipitation, and scavenging. Microorganisms oxidize the bioavailable Mn(II) to form Mn(IV), an insoluble manganese oxide that aggregates to form particulate matter that can then sink to the ocean floor. Manganese is important in aquatic ecosystems for photosynthesis and other biological functions.