[8]

Directed Selection for CO₂-Tolerant Cyanobacteria and Algae.  K. Marie Crowell, Camille Chan and David J. Thomas.  Lyon College, Science Division, Batesville, Arkansas 72501.

   The oxygen in Earth's atmosphere was produced by early photosynthetic microorganisms (i.e. cyanobacteria).  Fossil evidence indicates that these early photosynthesizers may have evolved as early as 3.5 billion years ago.  Unlike the present-day atmosphere, Earth's early atmosphere probably was comprised mainly of carbon dioxide and nitrogen, with small amounts of methane, ammonia and other trace gases.  Notably, no oxygen was present.  Given their early history, one might assume that cyanobacteria should grow and even thrive in atmospheres with very high partial pressures of CO₂.  However, previous research in our lab has shown that while several strains of cyanobacteria tolerate concentrations of CO₂ in excess of 60% (v/v), other strains die at concentrations of CO₂ greater than 20%.  The mechanism of toxicity involves several factors, including the CO₂ itself, low pO₂ and low pH.  Additional research in our lab shows that photosystems I and II are detrimentally affected in cyanobacteria that don't tolerate elevated CO₂.  Currently, we are growing cyanobacteria and algae in atmospheres of >30% CO₂ in N₂ in order to artificially select for strains that not only survive, but thrive in high CO₂.  As of July 2007, we have grown Plectonema boryanum in 60% CO₂/N₂ at faster rates than the original Plectonema culture grown in air.  Similar results have been obtained at with strains of Chlorella ellipsoidea and Chroococcidiopsis, but at only 40% CO₂/N₂.  Additional results of experiments in progress will be presented.  Cyanobacteria and algae that grow in high CO₂ have the potential for use in bioregenerative life support systems and planetary engineering on Mars, and would also provide insight into how they evolved on Earth.  This research is funded by the Arkansas Space Grant Consortium.