| Summary: | The significance of freshwater systems in global manganese cycles is well appreciated. Yet, the polar systems, which encompass the largest freshwater repository in the world, have been least studied for their role in manganese cycling. Results from a study that was conducted in the brackish water lakes in the Larsemann Hills region (east Antarctica) is presented. The rate of in situ manganese oxidation ranged from 0.04 to 3.96 ppb day sup(-1). These lakes harbor numerous manganese-oxidizing bacteria (10 sup(5) to 10 sup(6) CFU l sup(-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 manganese promoted 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 manganese cycling. In vitro manganese oxidation rates ranged from 3 to 147 ppb day sup(-1), while manganese reduction rates ranged from 35 to 213 ppb day sup(-1). It was also observed that the maximum stimulation of manganese oxidation occurred in the presence of cobalt (81 plus or minus 57 ppb day sup(-1)), rather than iron (37 plus or minus 16 ppb day sup(-1)) and nickel (40 plus or minus 47 ppb day sup(-1)). Our studies suggest that cobalt could have a more profound role in manganese oxidation, while nickel promoted manganese reduction in polar aquatic systems
|
|---|