MICROMORPHOLOGIC CHARACTERISTICS AND ORIGIN OF THE INTERBEDDED POROUS, SPONGE-LIKE LAYERS OF FE-RICH STROMATOLITES FROM AN ACID MINE DRAINAGE SYSTEM IN WESTERN INDIANA

This study is part of ongoing research to understand the variability of macro- and micromorphologies in Fe-rich stromatolites and their tracemakers from an acid mine drainage (AMD) system in western Indiana. The acidity of the system limits diversity to a few species of biofilm-forming microorganisms. The purpose of this research is to characterize the textural morphologies of highly porous, sponge-like layers and to identify microorganisms responsible for their formation. Stromatolite samples were collected from the abandoned Green Valley coal mine site. In general, the stromatolites consist of thinly laminated layers alternating with thicker (up to 50 mm), porous, sponge-like layers. Previous studies show that the thinly laminated layers are constructed by a eukaryotic biofilm community dominated by Euglena mutabilis, an acidophilic microorganism. This study specifically examines the characteristics and origin of the interbedded porous, sponge-like layers. Samples of the porous layer were analyzed microscopically and via scanning electron microscopy (SEM). Microscopically, the layer consists of an intricate lacy network of Fe-rich, threadlike filaments. SEM analysis of this lacy network shows that it is mainly composed of Fe-encrusted linked chains of diatoms, with lesser amounts of Fe-encrusted filamentous algae, Chlamydomonas, and filamentous bacteria. We propose that the porous layer forms from the encrustation of AMD precipitates on the surface of diatom-dominated biofilm, which is present during warmer summer months in the AMD system at the mine site. The overall stromatolitic layering (thinly laminated layers alternating with thicker porous layers) likely represents periodic changes in microbial populations that occupy the AMD environment through time.