Role of manganese in suboxic biogeochemical cycles

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Manganese is an important component of marine oxidation-reduction cycles because it exists in multiple redox states and is rapidly cycled between these states by bacterially mediated processes. The ease with which electrons are transferred between manganese species allows the manganese cycle to be readily coupled to other redox cycles. In nature, manganese exists in three oxidation states: Mn(II), Mn(III), and Mn(IV). Of these, Mn(III) has mostly been ignored. Different approaches to measuring manganese give contradictory information about the distribution of manganese dissolved in sediment pore waters that suggests that the suboxic part of the sediment contains not one but two soluble species, Mn(II) and Mn(III). The presence of Mn(III) in suboxic water and pore water can resolved several important issues. It would provide a mechanism for transferring dissolved Mn from sediment to water column; and it would provide evidence for a novel pathway in the nitrogen cycle whereby ammonia is oxidized directly to nitrogen gas by manganese. We propose to conduct a biogeochemical and microbiological study, in collaboration with researchers from two US universities, of the nature, stoichiometry, and rate of reactions that respectively produce and consume dissolved Mn(III). The study will be carried out in the St. Lawrence Estuary - Saguenay fjord environment for these reasons: 1)We have nearly 30 years worth of data on manganese in sediment and water column. 2) The sediments contain high concentrations of manganese, and manganese cycling is a dominant diagenetic process. 3) The solid phase manganese distribution is in steady state over decadal time scales, whereas the distribution of dissolved manganese is not. 4) The system provides excellent examples of low oxygen and high oxygen bottom waters that can help us compare and contrast end-member environments. 5) Many of the scientific questions we will investigate stem from previous work carried out in this system.