MICROSTRUCTURES AND MICROORGANISMS IN THE MACROFABRIC OF IRON STROMATOLITES, GREEN VALLEY MINE, INDIANA

The microstructures and microorganisms that make up the macrofabric of iron-rich stromatolites in acid mine drainage (AMD) at the Green Valley Mine, Indiana, were evaluated using petrographic and scanning electron microscopic (SEM) techniques. The stromatolites are composed of thin wavy laminae and thicker spongy, porous layers produced by several types of mat-building microbial consortia that grew under varying physicochemical conditions. Previous microscopic evaluation of the biofilm shows that main mat-forming community is composed of E. mutabilis with rare Chlamydomonas, diatoms, fungi, algae, and bacteria, present under normal AMD conditions. Very patchy diatom-dominated communities are also present and become temporarily more prevalent when increased discharge to the channel dilutes the acidic effluent.

SEM and petrographic analyses reveal different aspects of the stromatolitic microstructures that otherwise would not be visible if only one technique was used. The SEM analysis of the stromatolites shows preferential preservation of diatom and bacterial species, along with mineralized EPS (extracellular polymeric substances) in the form of sheaths surrounding diatoms and as bacterial strands interwoven within the iron precipitates. The thin wavy laminae are composed of iron precipitates containing mineralized casts of cocci bacteria with lesser amounts of iron-coated diatom tests that are interlayered with lamina of monospecific diatoms. The porous sponge-like layers also contain casts of cocci bacteria and iron-coated diatoms. Mineralized casts of rod-shaped bacteria are also present in both layers, but in considerably lesser amounts. Thin section analysis of the stromatolites shows that the porous layers differ from the wavy layers in that some of the diatoms are preserved as upward radiating colonies that contribute to the resulting porosity.