Inorganic carbon transport and fixation in cells of Anabaena variabilis adapted to mixotrophic conditions

Abstract

The cyanobacterium Anabaena variabilis ATCC 29413 grown at low CO2 concentration under mixotrophic conditions with fructose showed a repression in the ability to fix inorganic carbon. This repression was not due to a diminution in the ability to transport external inorganic carbon but could be explained by a decrease of two enzymatic activities involved in the assimilation of inorganic carbon: carbonic anhydrase and Rubisco. Carbonic anhydrase activity was close to 50% lower in amixtrophis than in autotrophic cells. Moreover growth under mixotrophic conditions reduced Rubisco activity at all dissolved inorganic carbon concentrations assayed (5-60 mM). Maximum Rubisco activity (Vmax) decreased from 4.7 μmol CO2 mg protein-1 h-1 in autotrophic cells to 2.3 μmol CO2 mg protein-1 h-1 in mixotrophic cells. No significant differencess in Km (Ci) between autotrophic and mixotrophic cells were however observed. The possible mechanisms involved in the inhibition of Rubisco are discussed.