Euglena mutabilis, an acidophilic protozoan, forms bright green benthic mats in acid mine drainage (AMD) channels at the Green Valley coal mine in western Indiana. Benthic communities reside in AMD pH<4.2 with high concentrations of dissolved constituents (up to 22 g/l), including Fe²⁺ (up to 2.3 g/l). E. mutabilis contributes to the attenuation of Fe, and possibly other metals, via intracellular sequestration and oxygenic photosynthesis, the latter driving oxidation and precipitation of reduced metal species, particularly Fe²⁺.

Physicochemical parameters of pH, Eh, dissolved oxygen, salinity, water temperature, specific conductivity, and total dissolved solids were monitored every 2 to 3 weeks for one year. Microbial distribution in the channels was characterized by percent coverage and described as 1) “thick microbial mats” composed of a dense community of E. mutabilis >1 mm thick that blocked exposure of the underlying Fe-rich precipitates, or 2) “thin microbial mats” measuring <1 mm thick with a patchy distribution. The distribution of E. mutabilis appears to be most strongly influenced by water temperature. Microbial communities were thickest and most extensive, covering up to 100% of the substrate, during spring and fall when water temperatures ranged from 13 to 28°C. During winter and summer, water temperatures were outside this range, and microbial distribution was extremely patchy, with communities restricted to deeper pools and to areas where AMD discharged at the surface. Both of these locations may have served as refugia for E. mutabilis, protecting the microbe from extreme temperature variations until more conducive conditions prevailed.

E. mutabilis also responded to rapid changes in pH caused by increased hydrologic input associated with rainfall events. When AMD increased above pH 4.0, Al- and Fe-rich hydroxide precipitates formed and covered the microbial mats. In <12 hours, E. mutabilis moved through the chemical sediments and reestablished a community at the sediment-water interface. When microbial mats were eroded away by catastrophic rainfall events, the microbe reproduced at a rapid rate to establish patchy communities within 3 to 5 days.