Microbial Manganese Oxidation in the Lower Mississippi River: Methods and Evidence

Abstract

Recent work has led to the suggestion that biologically-mediated redox processes might be important in the regulation of dissolved trace element concentrations in rivers, especially with regard to manganese. Here, we focus on the removal of dissolved Mn from lower Mississippi River water. Experiments indicate that dissolved Mn can be rapidly removed from lower Mississippi River water on a timescale of days or less and that Mn oxides are formed. However, demonstrating a biological origin for this removal is problematic. Experiments reveal that commonly used microbial controls, including NaN₃ mixtures, HgCl₂, heat sterilization, and sonification all affect fluvial particulate Mn through dissolution, disaggregation, interference with adsorption, or particle ageing. Thus, these microbial controls may affect abiotic as well as biological processes. Evidence supporting microbial removal of dissolved Mn from lower Mississippi River water includes a temperature optimum for the process (～30°C), a lower activation energy than reported for heterogeneous inorganic Mn oxidation, and a faster rate than reported for autocatalytic inorganic Mn oxidation. This rapid Mn oxidation process occurs at essentially the same rate in the dark as well as the light. Observation of Mn removal at similar rates in a blackwater river in addition to the lower Mississippi, suggests that this is a common phenomenon in fluvial systems. If, as has been shown by other lab studies, the freshly biologically precipitated Mn oxides have a high specific surface area, then our observations provide a potential link between microbial activity, Mn cycling, and the cycling of other particle-reactive trace elements in rivers. Our results also indicate that unfiltered river water samples for dissolved Mn analysis should be filtered as soon as possible or at least stored cold if immediate filtration is not possible.