COMPLEX STROMATOLITIC STRUCTURES IN AN ACID MINE DRAINAGE ENVIRONMENT: A CONSEQUENCE OF EUKARYOTE-DOMINATED BIOFILMS

Scanning electron microscopy (SEM) was used to study eukaryote-dominated biofilms and associated Fe-rich stromatolites in an acid mine drainage (AMD) system in Indiana. AMD biofilms consist of three types, each dominated by a single eukaryote comprising >90% of the biofilm: Euglena mutabilis-dominated, diatom-dominated, and filamentous algae-dominated. Also present and comprising <1% of the biofilm biomass are Chlamydomonas sp., filamentous, rod-shaped, and cocci bacteria, and fungi. SEM examination of the biofilms shows Fe precipitates encrusted on diatoms, algae, Chlamydomonas, and filamentous bacteria. Entwining E. mutabilis cells also trap precipitates; however, cell surfaces are generally free of precipitates, possibly because their flexible cell membrane precludes attachment. On the megascopic scale, Fe-rich stromatolites consist of alternating thinly laminated, dense wavy layers alternating with spongy porous layers. SEM analysis of the porous layers reveals that they are derived from the accumulation of mineralized casts of linked diatoms and or filamentous algae, with a lesser amount of filamentous bacteria, that form a porous network. The wavy laminated layers are composed of intricate micron-scale layering, consisting of a highly porous horizon grading downward into more massive horizon having <5% porosity. This sequence is repeated multiple times to form the macrosopic texture of the wavy laminated layers. Within the porous horizon, decaying cells of E. mutabilis are located within individual pore spaces, suggesting that the porosity results from precipitates trapped by a dense biofilm of predominantly E. mutabilis cells. These cells are also present in the few pore spaces within the more massive horizon. The layers in the stromatolite likely reflect seasonal variations in microbial biofilm communities and changes in water chemistry. The more highly porous layers represent precipitation on microbial biofilms during peak growing season under typical AMD conditions. Denser layers likely represent precipitation during colder months when biofilm communities are less active or during periods of rapid precipitation from increased pH associated with increased discharge.