DIATOMS IN COAL-RELATED ACID MINE DRAINAGE

Biofilm composed of a number of eukaryotic microorganisms forms in acid mine drainage systems at the abandoned Green Valley coal mine site, western Indiana. Under normal site conditions (pH 3.0-3.5), the biofilm is dominated by an acidophilic euglenoid, Euglena mutabilis. Scattered throughout and making up <1% of the biofilm are diatoms consisting primarily of one species of Nitzschia. Nitzschia are also present in small (<20 cm diameter), isolated communities containing lesser amounts of fungal hyphae, filamentous algae, and E. mutabilis. The patchy nature of these isolated communities reflects restriction to areas more conducive for diatom growth. Site observations over a three-year period show that the distribution and composition of the eukaryotic biofilm varies significantly over relatively short periods of time due to rapid changes in the physicochemical condition of the acidic water in response to increased hydrologic input from rainfall and snowmelt. Diatom communities, in particular, were observed to expand their distribution across a channel following a catastrophic rainfall event (up to 12 cm) that temporarily diluted the acidic water. After several weeks when site conditions returned to pre-existing levels, diatom communities retreated to isolated patches, and E. mutabilis flourished.

The biofilm overlies layered iron-rich biolaminates (stromatolites) that record the temporal oscillations in microbial activity. The stromatolites are composed of thin, wavy layers often containing partially decayed E. mutabilis-dominated biofilm. These layers represent the biological activity and iron precipitation under normal acid mine drainage conditions. Alternating with the thin wavy layers are thicker (up to 2 cm) sponge-like layers consisting, of iron precipitated on and incorporated into radiating colonies of diatoms. We believe the spongy layers reflect periodic changes in the physicochemical conditions of the environment, which allowed diatoms to temporarily dominate the system.