MANGANESE OXIDES IN CAVES - IN SITU BIOINDICATOR EXPERIMENTS CAN REVEAL ENZYMATIC PATHWAYS OF MN(II) OXIDATION

Identifying the factors that affect biogeochemical cycling has important implications for maintaining oligotrophic ecosystems that are susceptible to anthropogenic input, such as caves. Biogenic Mn(III/IV) oxides are common in caves and their growth can be stimulated by exogenous carbon input; making them useful as bioindicators of exogenous carbon input. The type, quality and quantity of carbon that can modulate the pathways and extent of Mn cycling in caves is largely unknown. Bacterial, archaeal, and fungal communities associated with Mn(III/IV) oxide deposits were assessed in both pristine and anthropogenically impacted caves in the southern Appalachians. Differences observed in the bacterial and archaeal community structure appeared to be driven by long-term exogenous carbon loading from either anthropogenic sources or natural carbon sources. In contrast, fungal community composition at these sites suggests that fungal assemblages are independent of carbon infiltration.

In order to determine whether exogenous input such as lint, food crumbs, wood and/or sewage would stimulate biotic Mn(II) oxidation in situ, cave sites were amended with various carbon sources that are chemically similar to those associated with anthropogenic input. Carbon treatments that stimulated Mn(II) oxidation resulted in significant changes to the microbial communities, indicating that anthropogenic input can both enhance biotic Mn(II) oxidation and potentially shape community structure and diversity. Geochemical analyses of sediment substrates suggest that some enzymatic Mn(II) oxidation pathways (such as MnP oxidation) can be limited by carbon and low C:N ratios. Bacterially selective Mn(II)-oxidizer media supplemented with 100 µM Cu(II) inhibited Mn(II) oxidation, suggesting that the bacteria in these cave systems were using superoxide to oxidize Mn(II), since this pathway is inhibited by 100 µM Cu(II) in vitro. In contrast, the fungally selective media with 100 µM Cu(II) stimulated Mn(II) oxidation, suggesting the use of multicopper oxidase enzymes for fungal Mn(II) oxidation.