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   Three cyanobacteria (Anabaena sp., Plectonema boryanum, Chroococcidiopsis CCMEE171) and an alga (Chlorella ellipsoidea) were grown under simulated martian ecopoesis conditions.  A xenon arc lamp with a solar filter provided simulated martian sunlight, and temperature cycled diurnally from -80°C to 26°C.  A Mars-like atmosphere of 100% CO₂ was provided at 25, 100, 300, 500 and 1000 mbar.  The cyanobacteria and alga were inoculated into JSC Mars-1 soil simulant and exposed to each atmospheric pressure for five weeks.  Survival and growth were determined via extractable chlorophyll a and total esterase (fluorescein diacetate hydrolysis) activity.  Maximum survival occurred at 100-300 mbar.  At 25, 500 and 1000 mbar, esterase activity was near zero, and extractable chlorophyll a was less than 10% of control samples.  Overall, the cyanobacteria survived better than the alga.  Low survival at 25 mbar was probably due to desiccation.  Low survival at 500 and 1000 mbar may have been due to CO₂ toxicity.  This research was supported by the NASA Institute for Advanced Concepts and by the Arkansas Space Grant Consortium.