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  1. null (Ed.)
  2. Manganese (Mn) is an essential element for life. Although its concentration is at (sub)nanomolar levels throughout the ocean, it affects the oxygen concentration of the ocean because it is central to the photosynthetic formation of dioxygen, O2, in photosystem center II. Mn inputs into the ocean are from atmospheric transport of particles and their dissolution to form dissolved Mn, and from the flux of dissolved Mn from rivers, sediments and hydrothermal vents. The main removal mechanism is transport of particulate Mn from dust and organic matter to the sediments. The environmental chemistry of manganese centers on its +2, +3 and +4 oxidation states. Most recent data show that Mn(II) is dissolved, that Mn(IV) is particulate MnO2, and that Mn(III) can be particulate or dissolved when bound to organic complexes [denoted as Mn(III)-L]. Mn(II) is oxidized primarily by microbial processes whereas MnO2 is reduced by abiotic and biotic processes. Photochemical processing aids redox cycling in surface waters. In suboxic zones, which are defined as areas with dissolved O2 concentrations below 3 M, both oxidation and reduction processes can occur but usually at different depths. In suboxic zones, dissolved Mn is also released from organic matter during its decomposition and from MnO2 reduction. 
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