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The Effects of Exogenous Hydrogen Peroxide on Cyanobacterial Antioxidant Mutants under Ambient and Primordial Conditions.  Neil McCarthy, K. Marie Crowell and David J. Thomas.  Lyon College, Science Division, Batesville, Arkansas.72501.

     The role of the antioxidant enzymes in photosynthetic organisms is an important one. The enzymes protect the cell from the harmful effects of reactive oxygen species (ROS), which form as byproducts of photosynthetic and respiratory electron transport.  The timing of the enzymes' co-evolution with photosynthesis is not entirely clear.  The early primordial atmosphere was much different from the present one, and was devoid of molecular oxygen. However, ROS (H₂O₂ in particular) can be created by other pathways including mineralogical and UV-mediated reactions.  This would mean that antioxidant enzymes could have evolved before photosynthesis in response to these ROS.  In our experiments, wild type and antioxidant mutant (katG^-, sodB^-, tplA^-) strains of Synechococcus were treated with 1.5 mM H₂O₂ and grown for 72 hours in ambient and simulated primordial (2.5% CO₂ in N­₂) atmospheres.  Growth was monitored by absorbance at 720 nm.  Photosynthetic electron transport was monitored by measurements of chlorophyll fluorescence, and redox-dependent changes in absorbance of P700 at 830 nm.  Results of in-progress experiments will be presented.  This research is funded by the Arkansas Space Grant Consortium.