EUKARYOTE-DOMINATED BIOFILMS IN ACID MINE DRAINAGE

Benthic microbial biofilms occur at the sediment-water interface in acid mine drainage (AMD) channels constructed at the abandoned Green Valley coal mine site, western Indiana. Community structure, biofilm morphology, and their distribution were monitored from 1999 to 2004. The acidic effluent contains high concentrations of dissolved solids up to 28.7 g/L and pH of 3.0 to 3.5, with pH as low as 2.2 during extended dry periods and pH 4.0 or greater during increased hydrologic input from rainfall. Eukaryotic microorganisms form the bulk of biofilm biomass, creating three major biofilms: Euglena mutabilis-, diatom-, and filamentous algae-dominated layers. Also present in very minor amounts (<1%) in each biofilm are cells of the other dominant biofilms, as well as the acidophilic, photosynthetic protozoan Chlamydomonas and five groups of aerobic or microaerophilic gram-negative bacilli. Biofilm morphology is influenced by characteristics of the dominant eukaryotes along with the amount of mucilage secreted by the consortium. E. mutabilis biofilm is bright green and thin, <5 mm, with minor amounts of mucilage coating cells. The olive-green diatom-dominated biofilm rises above the substrate as much 50 mm because of copious production of mucilage. Filamentous algae biofilm is distinctive because it forms 2 to 20 cm long tassels of entwined filaments attached to the substrate. Biofilm distribution is influenced, in part, by seasonal variations in water temperature. E. mutabilis-dominated biofilm is the most common, occurring year-round, and forms mats covering up to 100% of the substrate in spring and fall. The community retreats to the upper channel in summer and winter where groundwater seeps buffer extreme water temperatures. Conversely, the diatom biofilm is absent in the winter and forms isolated patches in spring and fall. The community expands from the patches in summer when water temperatures are optimal for growth and reproduction, creating diatom blooms that branch upward from the substrate. Filamentous algae biofilm only occurs in isolated patches, covering <1% of the substrate. All communities are passive recipients of AMD precipitates, and thus far, we have observed diatom frustules, algal filaments, and bacteria encrusted in iron-rich precipitates.