Manganese Cycling in the Marine Environment

Owings, Shannon
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University of Delaware
In this work the newly developed spectrophotometric method for the simultaneous determination of soluble Mn(II), Mn(III) and total Mn species via a metal substitution reaction (Madison et. al. 2011) was applied in different marine and freshwater environments. At an iron rich freshwater stream in East Boothbay Harbor, ME where an iron oxidizing bacteria, Leptrothrix ochrea, thrives Mn(II) was present (~60μM). Mn(III) was not detected in the freshwater stream indicating the bacteria were not oxidizing Mn(II) for an alternative source of energy. At hydrothermal vents 2,500-3,500 m below the ocean surface along the Mid-Atlantic Ridge high concentrations (~300μM-1.10mM) of Mn(II) were detected, and no Mn(III) was present in the vent fluid samples that were collected. No evidence of microbial or abiotic oxidation of Mn(II) was found due to the absence of Mn(III) and a linear relationship observed from plots of [Mn(II)] vs. temperature, and [Mn(II)] vs. pH plotted for each of the vent sites.. The linear trends indicate a conservative relationship, which explains Mn(II) is diluted with bottom water as it travels away from the vent. Mn(II) does not readily form a MnS solid, and the linear trends are consistent with that fact. Evidence of Mn(III) species was found in the Chesapeake Bay. The water column profiles show Mn(III) in the suboxic zone and Mn(III) makes up about 45-90% of the total Mn pool. The Mn(III) profiles show Mn(III) peaks consistent with the oxidation of Mn(II) to Mn(III) below the oxic zone, and a reduction peak of Mn(IV) to Mn(III) and leaching from the sediments, or possibly produced by microbial reduction. In the Chesapeake Bay there are high amounts of organic matter and biological activity. These factors lead to the production of Mn(III) via biological activity and stabilization of Mn(III) by organic ligands.