EXTREME ULTRAVIOLET LIGHT TOLERANCE BY THE STROMATOLITE-BUILDING EUKARYOTE EUGLENA MUTABILIS

Eukaryotic biofilms dominated by the phototrophic protozoa Euglena mutabilis build Fe-rich stromatolites in an acid mine drainage (AMD) environment associated with the abandoned Green Valley coalmine site in Indiana. E. mutabilis is a true acidophile thriving in acidic waters from pH 3.0 to 3.5, with high concentrations of total dissolved solids up to 18 g/L, and tolerating pH as low as 1.2. This preliminary study examines the response of E. mutabilis to a germicidal ultraviolet (UV) light source with a wavelength of 254 nm, simulating > 200 nm UV exposure anticipated on early Earth and on such extraterrestrial bodies as Mars. Aquatic pond water species––amoebas, ciliates, and such metazoans as rotifers––considered to be sensitive to UV radiation were also tested as a control population. For all analyses, the UV light source was held at a constant position of 48 mm above the microbial samples. A monolayer of microbial cells from pond water and from Euglena­-dominated biofilm was pipetted onto glass slides for exposure of 15-second intervals or longer, depending on response. The effect of UV radiation on the mortality rate of the microorganisms was monitored using bright-field microscopy and digitally recorded. The pond water microorganisms showed 98% mortality within six minutes of exposure. In almost all cases, the cell structure was compromised. In contrast, Euglena mutabilis showed no changes in cell morphology and no pronounced mortality after 110 minutes of exposure. The study was terminated after 110 minutes because E. mutabilis cells were showing signs of stress due to dehydration of the cells on the microscopic slide. This study, along with previous studies on the oxygen-saturation of water by E. mutabilis biofilms, has important implications for the evolution of life and oxygenation of the atmosphere in environments exposed to elevated levels of harmful UV radiation, such as that on early Earth and other planets without UV-absorbing atmospheres. Perhaps the ancestral Euglenozoa––positioned near the base of the eukaryotic evolutionary tree and among the first to possess a mitochondrion––evolved the ability to tolerate UV exposure while conducting oxygenic photosynthesis that contributed to Fe precipitation and the release of oxygen to the early atmosphere in the late Archean–early Proterozoic.