A EUKARYOTIC STROMATOLITE ENGINEER: FROM CHEMICAL SEDIMENTS TO BOUND BIOSTRUCTURES

This study examines the role of Euglena mutabilis as an ecosystem engineer, which is most responsible for the construction of Fe-rich stromatolitic structures in an acid mine drainage (AMD) environment. E. mutabilis is a benthic, acidophilic, photosynthetic, eukaryotic protozoan without a flagellum that thrives in contaminated, acidic water. E. mutabilis-dominated consortia form a <1 mm biofilm layer over AMD precipitates composed of Fe oxyhydroxides, Fe sulfates, and Al hydroxides. Mucilage secreted from cells assists with adhesion of cells to one another and to the precipitate surface and may serve as a defense against adverse environmental conditions. This study shows that E. mutabilis also uses mucilage to trap and bind AMD precipitates, likely as a phototactic and aerotactic response to keep pace with chemical sedimentation. This activity places cells at the sediment-water interface for light and oxygen. In laboratory experiments, E. mutabilis cells were placed in slurries of Al oxide powder and powdered Fe-rich stromatolitic material. Microscopic observations of the slurry mixtures showed that E. mutabilis uses mucilage and gliding motility to trap particles to its cell membrane. These particles were transported along the membrane to the posterior end where they were deposited as a strand of mucilaginous bound sediment. AMD precipitates were also trapped by E. mutabilis in experiments where reverse osmosis water was added to natural AMD to simulate field conditions associated with dilution from increased discharge from rainfall. The increase in pH elicited a tube-building response where numerous cells were grouped together with AMD precipitates in tubular, mucilaginous structures. We hypothesize that E. mutabilis constructs tubes for motility and protection under stressful conditions. Through these processes, E. mutabilis engineers an intricate network of bound AMD precipitates similar to those observed in scanning electron microscopic studies of Fe-rich stromatolites. In this manner, E. mutabilis serves as an ecosystem engineer to build layered Fe-rich stromatolites in which other acidophilic protozoans, algae, and bacteria are part of the microbial consortium. Each layer reflects changes in chemical conditions associated with seasonal fluctuations in discharge.