Gas exchange and carbon fixation properties of the mat-forming cyanophyte Lyngbya birgei G.M. Smith

The macroscopic cyanophyte Lyngbya birgei G.M. Smith showed low, O₂-insentive rates of CO₂ release into CO₂-free media, as well as low CO₂ compensation points and O₂-insensitive photosynthetic rates. These gas exchange traits indicate that this organism can suppress photorespiration. The ability to thus optimize photosynthetic rates is based on HCO₃^- use as well as limited C₄-like metabolism; both systems could elevate internal CO₂ levels and thereby reduce the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase. It was further found that while the low photorespiration state did not change in Lyngbya grown at various inorganic carbon levels, the enzymatic potential for C₄ metabolism varied with external HCO₃^- levels. It is suggested that Lyngbya has adapted its photosynthetic systems to be efficient, so as to ensure optimal production in a variety of inorganic carbon and O₂ environments, and that this ability is of competitive advantage under conditions where it becomes an aquatic weed.