Microbially mediated redox transformations of manganese (II) along with some other trace elements: a study from Antarctic lakes

The significance of freshwater systems in global manganesecycles is well appreciated. Yet, the polar systems, which encompass the largest freshwater repository in the world, have been least studied for their role in manganesecycling. Here, we present results from a study that was conducted in the brackish waterlakes in the Larsemann Hills region (east Antarctica). The rate of in situ manganeseoxidation ranged from 0.04 to 3.96 ppb day−1. These lakes harbor numerous manganese-oxidizing bacteria (105 to 106 CFU l−1), predominantly belonging to genera Shewanella, Pseudomonas and an unclassified genus in the family Oxalobacteriaceae. Experiments were conducted with representatives of predominant genera to understand their contribution to Mn cycling and also to assess their metabolic capabilities in the presence of this metal. In general, the total and respiring cell counts were stimulated to a maximum when the growth medium was amended with 10 mM manganese. The addition of manganesepromoted the use of d-mannitol, maltose, etc., but inhibited the use of maltotriose, l-serine and glycyl l-glutamic acid. The bacterial isolates were able to catalyze both the redox reactions in manganesecycling. In vitro manganeseoxidation rates ranged from 3 to 147 ppb day−1, while manganesereduction rates ranged from 35 to 213 ppb day−1. It was also observed that the maximum stimulation of manganeseoxidation occurred in the presence of cobalt (81 ± 57 ppb day−1), rather than iron (37 ± 16 ppb day−1) and nickel (40 ± 47 ppb day−1). Our studies suggest that cobalt could have a more profound role in manganeseoxidation, while nickel promoted manganesereduction in polar aquatic systems.