Structure and function of the microbial community in an in situ reactor to treat an acidic mine pit lake.

Sulfate-reducing bioreactors are a promising option for the treatment of acid mine drainage. We studied the structure and function of a biofilm in a methanol-fed fixed-bed in-lake reactor for the treatment of an acidic pit lake by a combination of laboratory incubations, chemical and molecular analyses and confocal laser scanning microscopy to determine whether competition by different groups of microorganisms as well as the precipitation of minerals affect reactor performance negatively. The biofilm growing on the surface of a synthetic carrier material consisted of dense microbial colonies covered by iron-sulfide precipitates. The microorganisms continuously had to overgrow this mineral coating, resulting in a high biomass turnover. About one third of the added methanol was used by sulfate reduction, and the rest by competing reactions. Sulfate-reducing bacteria as well as methanogens and acetogens were involved in methanol consumption. Six different groups of Deltaproteobacteria, dominated by the genera Desulfomonile, Desulfobacterium and a phylotype related to Geobacter, Gram-positive sulfate reducers of the genus Desulfosporosinus, acetogenic Acetobacteria, different fermenting bacteria as well as methylotrophic methanogens were identified. The versatility of the microbial food web is probably an important factor stabilizing the biofilm function under fluctuating and partly oxidizing conditions in the reactor.