HCO 3− use and evidence for a carbon concentrating process in the mat-forming cyanophyte Lyngbya birgei G.M. Smith

Abstract

The macroscopic cyanophyte Lyngbya birgei G.M. Smith can form extensive floating mats and become an aquatic weed. This organism, when grown in the laboratory under low or relatively high inorganic carbon concentrations (0.1 or 1.0 mM HCO 3 −, representing a range found in nature), showed photosynthetic K 1/2 values for inorganic carbon of 20 and 40 μM when using mainly CO 2 (pH 5.5) and HCO 3 − (pH 9.0), respectively. Comparisons of photosynthetic rates and maximal uncatalyzed HCO 3 − dehydration rates indicated HCO 3 − uptake at the higher pH. Only at an unnaturally high inorganic carbon growth concentration (10 mM) did the affinity for HCO 3 − decrease. Inorganic carbon was concentrated internally in Lyngbya up to eight times external levels by a light and HCO 3 − (but apparently not CO 2) dependent process. Internal inorganic carbon levels were about 2 mM at a range of ambient inorganic carbon concentrations occurring in mats. It is suggested that a resulting elevated internal CO 2 level is the basis for the ability of this cyanophyte to suppress photorespiration. The efficient use of HCO 3 − and the capability to concentrate inorganic carbon are seen as adaptive features which optimize photosynthetic production of Lyngbya in mats, where HCO 3 − is the principal external carbon source available for photosynthesis and O 2 levels are high during the day.