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Testing the Oxygen Paradox with Antioxidant-Deficient Cyanobacteria     Michelle Eubanks, John Boling, Marie Crowell, Tiffany McSpadden, Carl Rector, Christy L. Schuchardt, CaSandra J. Spurlock, Jaime Warrington and David J. Thomas (P.I.). Lyon College, Science Division, Batesville, AR 72501.

   The light reactions of oxygenic photosynthesis produce reactive oxygen species (ROS) that can damage cellular components and lead to cell death.  Thus, the co-evolution of an antioxidant system was necessary for the survival of photosynthetic organisms.  This project investigated the sequence of antioxidant system evolution in cyanobacteria.  The first organisms that possessed antioxidants would have been able to live closer to the water's surface, and would have been pre-adapted for dealing with photosynthetically produced ROS.  We tested this hypotheses by growing wild type and mutant cyanobacteria (Synechococcus PCC7942 katG^-,  sodB^-, and tplA^-) under primordial atmospheres (2.5% CO­₂ in N₂).  The sodB^- strain had a lower growth rate than the wild type under both aerobic and anaerobic conditions, indicating a preexisting need for FeSOD before the evolution of oxygenic photosynthesis.  The katG^- strain grew somewhat faster under aerobic conditions versus anaerobic conditions; the reason for this is unclear.  There were no significant differences in growth rates of the tplA^- strain.  Overall, our results support the hypothesis that superoxide dismutase evolved before oxygenic photosynthesis, and was a necessary preadaptation.  Catalase would necessarily have evolved even earlier in order to detoxify the hydrogen peroxide produced by SOD. 

(This research was supported by grants from the Arkansas Space Grant Consortium.)