The Evolution of Aerobic Metabolism
Research Staff: Lynn J. Rothschild and Friedemann Freund
Our objective is to test the hypothesis that, in spite of overwhelmingly anaerobic conditions on the early Earth, early organisms living in contact with rocks, were under evolutionary stress to develop the enzymatic defense mechanisms that helped them survive the threat from reactive oxygen species (ROS). Gaining insight into these complex relationships will open a new approach to understanding one of the key steps in the evolution of early life: the evolution of aerobic metabolism. This is central to the Astrobiology mission. It will open new avenues toward understanding the evolution of life on earth, and the potential for aerobic life elsewhere.
A paradox marks the evolution of early Life: Though oxygen is toxic through production of ROS, life evolved to detoxify such ROS, utilize oxygen, and even produce it. On an anaerobic early Earth, why would organisms be under evolutionary pressure to evolve ROS defenses?
Minerals in igneous rocks contain a reactive form of oxygen, peroxy, O3Si/OO
\SiO3. When minerals weather, peroxy hydrolyzes to H2O2. We propose to test the hypotheses that 1) organisms living on the early Earth in intimate contact with rock surfaces were subjected to a constant trickle of H2O2 and that 2)organisms were under environmental stress to develop strategies to either detoxify the ROS or repair the molecular damage that they cause.One way to activate the peroxy and generate highly oxidizing ROS is to crush rocks. The necessary hardware to perform experiments under controlled atmosphere has been built. A first set of experiments has been performed. An analytical procedure to measure the total peroxy content in rocks has been successfully tested. On the basis of the positive outcome, rock crushing experiments with aerobic bacteria (E. coli) were carried out. A procedure to determine the survivability of E.coli has been evaluated using live/dead fluorescent stains. Preliminary results are encouraging, though interferences from fluorescent inorganic matter need to be eliminated. Additional bacterial strains have been obtained. Methods for assaying DNA damage are being further developed and tested. Methods for assaying low concentrations of H2O2 are also currently tested.
One of the premises on which this study is based, the presence of peroxy in igneous rocks, has been confirmed. This opens the road to a series of experiments aimed at testing whether and to what extent the ROS activated by crushing and H2O2 liberated during simulated weathering will affect and damage bacteria.
Point of Contact: Lynn J. Rothschild, 650/604-6525,lrothschild@mail.arc.nasa.gov