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EFFECTS OF VERY HIGH CO₂ ATMOSPHERES ON THE CYANOBACTERIAL PHOTOSYNTHETIC APPARATUS.   D. J. Thomas, S. L. Sullivan and S. M. Zimmerman.  Science Division, Lyon College, Batesville, Arkansas, USA.

   According to current theories of early biosphere evolution, cyanobacteria and similar organisms produced most of the oxygen found in Earth's atmosphere.  Early photosynthetic organisms would have adapted to an atmosphere that was rich in CO₂ and poor in O₂.  In many ways (except for the pressure), Earth's early atmosphere may have resembled Mars' current atmosphere.  However, many modern cyanobacterial species do not grow under these conditions.  We are currently investigating the tolerance of several cyanobacterial species to very high (>20%) concentrations of atmospheric CO₂.  Cultures of Synechococcus, Synechocystis, Plectonema boryanum and Anabaena were grown in liquid culture and bubbled with CO₂-enriched air.  Culture growth was monitored by measuring optical density.  Photosystem II activity was monitored by measurements of variable chlorophyll fluorescence (F_V/F_M).  Synechococcus, Plectonema, and Anabaena tolerated CO₂ concentrations up to 100% when the CO₂ content was gradually increased from ambient by 10-15% per day. However, Synechocystis did not tolerate high CO₂.  Additional experiments to differentiate between CO₂, O₂ and pH effects showed that strains that were sensitive to high CO₂ were also sensitive to low initial pH (pH 5-6), indicating that the formation of carbonic acid is probably responsible for the inhibited growth in high CO₂ environments.  Research in progress to determine the effects of high CO₂ environments on photosystem II will be presented.  In addition to providing insight as to the adaptations necessary on the early Earth, this research has applications for Mars exploration (e.g. a martian exploratory base or greenhouse).  Also, this research provides insight into the possibilities, however remote, of forward-contamination of Mars by robotic and human exploration, and the survival of such contaminants.  This research was supported by grants from the Arkansas Space Grant Consortium.