REPEATED EXPOSURE OF A EUKARYOTIC MICROORGANISM TO SHORT WAVE, UV-C RADIATION—IMPLICATION FOR LIFE ON EARLY EARTH AND EXTRATERRESTRIAL PLANETARY BODIES

This study examines the tolerance of Euglena mutabilis—a benthic, acidophilic, photosynthetic, eukaryotic protozoan—to short-wave, UV-C radiation, which is known to damage DNA. Our purpose is to determine the ability of E. mutabilis to reproduce after repeated exposure to UV-C radiation. Euglena mutabilis forms biofilm in acid mine drainage (AMD) and builds Fe-rich biosedimentary structures (i.e., stromatolites) by binding AMD chemical sediments consisting of Fe oxyhydroxides, Fe sulfates, and Al hydroxides. Previous studies show that E. mutabilis cells can survive 110 minutes of UV-C exposure without significant signs of mortality. This experiment builds on the previous study by examining the ability of cells to survive and reproduce after repeated exposure. Cells reared in simulated AMD solution in a petri dish were exposed to radiation for four consecutive hours in a 24 hr period for three consecutive days. Prior to exposure, cells had an elongate morphology with a bright green color; after the experiment, the cells were pale olive green and showed signs of stress by forming resting cysts and remaining motionless. The cells were allowed to recover from exposure before being transferred to a test tube with simulated AMD where they were vigorously shaken to disperse the cells. Aliquots of cells were transferred to eight wells in each of three different growth plates. Cells were counted once every other day for two weeks to build a growth curve. Results show that E. mutabilis is able to reproduce after repeated exposure to UV-C radiation. This discovery has important implications for the evolution of eukaryotic, protozoan life forms on early Earth, as well as on extraterrestrial planetary bodies. Early life on Earth was exposed to harmful levels of UV-C radiation—which is currently blocked on Earth by the presence of the ozone layer—yet survived, reproduced, and evolved. This same pattern may have taken place on Mars early in its history and prior to the near-complete loss of its atmosphere. This study also raises the provocative possibility of using similar UV-C tolerant microorganisms to terraform other planets, such as Mars, where high levels of UV-C are known to occur.