LABORATORY EXPERIMENTS DEMONSTRATING TUBE BUILDING AND BINDING AND TRAPPING OF CHEMICAL PRECIPITATES BY EUGLENA MUTABILIS: ARCHITECT OF IRON-RICH STROMATOLITIC STRUCTURES

This study examines the role of Euglena mutabilis in building Fe-rich stromatolitic structures in an acid mine drainage (AMD) environment in Indiana. E. mutabilis is a photosynthesizing, acidophilic, eukaryotic, protozoan that thrives in contaminated, acidic water with pH measuring from 3.0 to 4.0. It forms a thin (<1 mm) biofilm layer over AMD precipitates consisting of Fe oxyhydroxides, Fe sulfates, and Al hydroxides. Microscopic analysis of the biofilm shows that E. mutabilis uses mucilage to trap and bind chemical precipitates, possibly as part of phototactic and aerotactic processes to keep pace with chemical sedimentation to place the cells at the sediment/water interface for light and oxygen. In the laboratory, slurries containing commercial Al oxide powder and powdered Fe-rich stromatolitic material were mixed with E. mutabilis cells reared in simulated AMD. Microscopic analysis of the slurry mixtures showed that E. mutabilis used amoeboid-like motility to trap and bind particles in mucilage secreted along the exterior cell membrane. These particles were then transported along the sides of the cell to the posterior end where they were deposited as a mucilaginous bound bundle. The trapping of Fe-rich particles by E. mutabilis was also observed in experiments where reverse osmosis (RO) water was added to natural AMD to simulate field conditions associated with dilution from increased discharge from rainfall. In response to the addition of RO water, E. mutabilis designed mucilaginous tubes containing numerous cells together with Fe-rich precipitates from the AMD. We hypothesize that the tubes serve as a means of protection from stressful conditions associated with changing pH. Through the processes of amoeboid-like motility to bind sediments and the construction of mucilaginous tubes that trap AMD precipitates, E. mutabilis builds a fine network of bound AMD precipitates similar to that observed in previous scanning electron microscopy studies of Fe-rich stromatolites from the study site. In this manner, E. mutabilis serves as the architect of layered Fe-rich stromatolites. Each layer reflecting changes in chemical conditions associated with seasonal fluctuations in discharge.