SEM ANALYSES REVEAL COMPLEX STROMATOLITIC FABRIC BUILT BY EUKARYOTIC MICROORGANISMS IN AN ACID MINE DRAINAGE SYSTEM

Scanning electron microscopic (SEM) examinations of microbial biofilms and associated iron-rich stromatolites from an acid mine drainage (AMD) system in Indiana reveal a diverse and complex eukaryotic microbial consortium that forms the fabric of layered stromatolitic deposits. Biofilms are composed of three microbial communities, each dominated by a single eukaryote making up more than 90% of the biofilm: Euglena mutabilis-dominated, diatom-dominated, and filamentous algae-dominated. Also present and making up less than 1% of the biofilms are Chlamydonomas sp., fungi, rod-shaped bacteria, and filamentous bacteria. SEM analysis of the living biofilms shows iron encrusted on cell membranes and walls of diatoms, filamentous algae, and filamentous bacteria. The megascopic fabric of the iron-rich stromatolites consists of spongy, porous layers alternating with thinly laminated, denser wavy layers. SEM analysis of these layers reveals that the porous layers are derived by either (1) iron precipitation around E. mutabilis cells within a dense E. mutabilis-dominated biofilm resulting in a network of voids when E. mutabilis cells decay upon burial or (2) iron encrusted on linked diatoms, filamentous algae, and to a lesser extent filamentous bacteria that form a web-like network. The laminated wavy layers consist of dense iron-rich chemical sediments with less than 10% void spaces, with the voids being formed from entrapment of E. mutabilis cells. The layers in the stromatolites likely reflect seasonal variations in microbial biofilm communities and changes in water chemistry. The porous layers represent precipitation on microbial biofilms during peak growing season under normal AMD conditions. The denser wavy layers likely represent precipitation during colder months when biofilm communities are at a minimum or during periods of rapid precipitation from increased pH associated with increase discharge. The latter process results in burial of the biofilms under a layer of precipitates that prevents phototactic and aerotactic responses of the eukaryotic microorganisms.