SEMIQUANTITATIVE ANALYSIS OF PRECIPITATES IN AN IRON-RICH STROMATOLITE DERIVED FROM THE BIOLOGICAL ACTIVITY OF EUKARYOTE-DOMINATED BIOFILMS IN AN ACID MINE DRAINAGE ENVIRONMENT

Scanning electron microscopy (SEM) with semiquantitative X-ray microanalysis was used to characterize precipitates in a Fe-rich stromatolite in an acid mine drainage (AMD) system in Indiana. Stromatolites result from biological activity of eukaryote-dominated biofilms and fluctuations in AMD geochemistry. Acidic water at the site ranges from pH 3.0 to 3.5, increasing to pH 5.0 during periods of increased discharge. Stromatolite-building biofilms are composed of three microbial communities, each dominated by a single eukaryote comprising >90% of the biofilm: Euglena mutabilis-dominated, filamentous algae-dominated, and diatom-dominated. Chlamydomonas sp., fungi, and rod, cocci, and filamentous bacteria are also present, comprising <1% of the biomass. SEM examination shows that porous textures are derived (1) from microcrystalline Fe minerals precipitated or encrusted on diatoms, filamentous algae, filamentous bacteria, and Chlamydomonas and (2) by E. mutabilis motility through microcrystalline aggregates of Fe-minerals, some of which may have been trapped by mucilage layers to form a mesh-like framework. The more massive layers of the stromatolite lack crystallinity, except for occasional pore spaces lined with microcrystalline Fe minerals, and may represent compacted chemical sediments. Semiquantitative analysis of precipitates encrusted on microorganisms and forming the mesh-like framework around E. mutabilis reveals Fe, S, and O, indicative of schwertmannite (Fe₁₆O₁₆(SO₄)₂(OH)₁₂). Also present is a tabular microcrystalline mineral containing K, Fe, S, and O, indicative of jarosite (KFe₃(SO₄)₂(OH)₆. Some stromatolitic samples also have elevated concentrations of Al, Mg, and Cl. Aluminum hydroxides form in the AMD system when pH increases from increased discharge. The occurrence of Mg along with S may be suggestive of epsomite (MgSO4·7H₂0), which occurs at some AMD sites. Additional work is necessary to target the mineral sources of the Mg and Cl occurrences.