THE EUKARYOTIC MICROORGANISM EUGLENA MUTABILIS BINDS CHEMICAL SEDIMENTS TO BUILD IRON-RICH STROMATOLITES

Eukaryotic biofilm communities occur in association with Fe-rich stromatolites in an acid mine drainage (AMD) environment at the abandoned Green Valley coalmine site in western Indiana. This study examines the formation of stromatolitic structures by Euglena mutabilis, an acidophilic, eukaryotic microorganism. E. mutabilis is the dominant microorganism that forms biofilm communities along with lesser amounts of diatoms, green algae, and Chlamydomonas sp. in AMD effluent measuring from 3.0 to 3.5 pH, with high concentrations of total dissolved solids, including Fe. The study used E. mutabilis cells collected from the study site and reared in the laboratory in synthetic AMD with nutrients to isolate the microorganisms from AMD precipitates. A monolayer of E. mutabilis cells was placed on a glass slide and covered with a very thin dusting of 12-µm Al oxide powder, which was used as a proxy for Fe precipitates that dominate the chemical sediments in the effluent of the AMD environment. The glass slide was examined using bright-field microscopy, and cell behavior in the Al powder was recorded using time-lapse photography. The microorganisms moved through the powder with amoeboid-like motility while particles collected along the sides of each cell. With continued locomotion, the Al particles were transferred along the cell with the aid of mucilage and were accumulated in a bundle at the posterior end of the cell. Each cell dragged the bundle of mucilage-bound particles until the bundle became too large to transport, which was then released. This repeated behavior resulted in a trail of deposited bundles. Analysis of the Fe-rich stromatolites collected from the coalmine site via scanning electron microscopy shows that a web like network of bound AMD precipitates, in part, forms the microstructural framework of the stromatolite. We suggest that the microstructural texture common in Fe-rich AMD stromatolites results from the trapping and binding of AMD precipitates by E. mutabilis cells in the manner observed in the laboratory experiment. In the AMD environment, E. mutabilis-dominated biofilms would trap, bind, and deposit bundles of Fe precipitants as they moved upward through the precipitants by phototaxis and aerotaxis.