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The most commonly observed forms of manganese in aquatic environments are particulate manganese oxides and soluble manganous ions. Although manganese is generally a minor chemical constituent of fresh and marine waters, the remarkable surface area and charge distribution of manganese oxides make them a potentially rich reservoir of metals adsorbed to their surfaces. These adsorbed metals are subsequently released during the reduction of the manganese oxide to manganous ion, and in this way, redox transformations involving manganese may influence the geochemical cycles of numerous metals in aquatic ecosystems.
Microorganisms are known both to oxidize and reduce manganese and are believed to have a major impact on manganese cycles in both terrestrial and aquatic ecosystems (Lovley 1993). The potential role of manganese oxides in the cycling of many metals and the capacity of microorganisms to catalyze the redox transformations of manganese, led us to explore the biogeochemical cycling of manganese in Antarctica's Lake Vanda. This lake's permanent ice cover, calcium chloride (CaCl2) brine in the deep waters, and limited, seasonal surface inflow result in an extremely stable water column amenable to studies of chemical and microbial fine structure.
The water column of Lake Vanda contains both soluble manganous ion (Mn2+) and particulate manganese oxides (Green et al. 1993, pp. 145-163). Two peaks of reduced manganese were detected: one just below the oxic/anoxic interface, as is found in many stratified lakes, and a second peak in oxic waters approximately 5 meters above the interface ( figure 1). The occurrence of two peaks of soluble manganese, one within oxic waters, is uncommon and was used to divide the water column into four vertical zones where microbially mediated manganese reduction or oxidation may be favored (figure 1).
The major peak of soluble manganese was just below the oxic-anoxic interface (zone 4), a possible habitat for anaerobes that derive energy for growth solely from the direct reduction of manganese (Nealson and Myers 1992; Lovley 1993). Although this zone may support the growth of anaerobic manganese-reducing bacteria, experiments conducted during the 1994 field season suggest that manganese is reduced primarily by sulfide (Bratina, Green, and Schmidt 1995).
The second manganese peak was a minor peak approximately 5 meters above the interface (zone 2). Aerobic microbes are more likely to inhabit this zone of the lake and reduce manganese indirectly via extracellular metabolites, although some cases of aerobic enzymatic reduction are known (Gounot 1994). We have isolated, from around this second manganese peak, several bacteria that reduce manganese oxides aerobically. The vertical distribution of aerobic manganese reducers, mechanism of reduction, and potential competitive advantages derived from aerobic manganese reduction are currently under investigation.
Bordering the areas of highly soluble manganese are zones where manganese oxidation is likely to occur (zones 1 and 3). Both of these zones are below pH 7 and 20°C, conditions where chemical oxidation of manganese is known to occur very slowly (Morgan 1967, pp. 561-624). Microbial manganese oxidation has been demonstrated in numerous aquatic environments, particularly just above oxic-anoxic interfaces similar to zone 3 (Ghiorse 1984).
The results for manganese oxidation measurements are summarized in figure 2. Manganese oxides were detected with both the leucocrystal violet (LCV) and leucoberbelin blue (LBB) assays in bins 3 and 5 (zone 1). Manganese oxidation was detected only when samples were amended with nutrients, so whether the activity occurs in situ in this oligotrophic lake is not known. Manganese oxidation in zone 3 (bin 7) was detected with the LCV assay but was not corroborated with the LBB assay. Because all of the assays were performed in the dark, no light-dependent metabolism would have been detected. The major phototroph peak in Lake Vanda is in zone 3 and coincides with a decrease in soluble manganese. Phototrophs, which are known to oxidize manganese (Richardson, Aguilar, and Nealson 1988), could therefore be responsible for manganese oxidation in the area just above the oxic-anoxic interface.
Besides being found in lakes, reduced manganous ions have been detected in the Earth's major ocean basins and numerous groundwater systems. The potential interactions between microbes and metals in Lake Vanda could serve as a useful model for other aquatic systems that are less amenable to study due to the confounding effects of rapid mixing. Special thanks go to field party members Bradley Stevenson and Dave Harris. This research was supported by National Science Foundation grant OPP 93-19708.
References
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