The importance of microbial Mn oxidation in the upper ocean: a comparison of the Sargasso Sea and equatorial Pacific

The processes responsible for Mn uptake onto suspended particles in the upper water column were studied in the equatorial Pacific and in the Sargasso Sea at the Bermuda Atlantic Time Series (BATS) Station. Oxidation was followed by incubating freshly collected seawater with 54 Mn and using a combination of procedures developed by others to distinguish oxidation from adsorption and biological from non-biological processes. Results in the Sargasso Sea indicate that oxidation is the principal pathway for the formation of particulate Mn throughout the year, in good agreement with the findings of earlier workers. These data, combined with earlier studies from diverse environments, suggest that microbial Mn oxidation is a ubiquitous process in oxygenated marine environments. However, in the equatorial Pacific we detected no microbial oxidation above 175 m. Formation of particulate 54Mn occurred and was inhibited in the presence of azide, indicating a biological process, but the particulate Mn could not be dissolved by the addition of ascorbate, an important criterion, as Mn oxides are readily reduced at seawater pH by ascorbate. In the Pacific, the non-oxidative, biologically mediated uptake was enhanced by light, possibly because of uptake by phytoplankton, as Mn is an essential micronutrient. By contrast, oxidative biological uptake in the Sargasso Sea was inhibited by light, in agreement with earlier work. The results indicate that the geochemical cycling of Mn is different in the two environments, presumably reflecting aspects of the ecology of Mn oxidizing bacteria that are not understood. These findings suggest that it is premature to generalize about the relative importance of oxidative vs. non-oxidative scavenging of Mn from the euphotic zone in the open ocean without additional data.