INORGANIC CARBON TRANSPORT AND FIXATION IN CELLS OF ANABAENA-VARIABILIS ADAPTED TO MIXOTROPHIC CONDITIONS

The cyanobacterium Anabaena variabilis ATCC 29413 grown at low CO2 con centration under mixotrophic conditions with fructose showed a repress ion in the ability to fix inorganic carbon. This repression was not du e to a diminution in the ability to transport external inorganic carbo n but could be explained by a decrease of two enzymatic activities inv olved in the assimilation of inorganic carbon: carbonic anhydrase and Rubisco. Carbonic anhydrase activity was close to 50% lower in mixotro phic than in autotrophic cells. Moreover growth under mixotrophic cond itions reduced Rubisco activity at all dissolved inorganic carbon conc entrations assayed (5-60 mM). Maximum Rubisco activity (V-max decrease d from 4.7 mu mol CO2 mg protein(-1) h(-1) in autotrophic cells to 2.3 mu mol CO2 mg protein(-1) h(-1) in mixotrophic cells. No significant differences in K-m (C-i) between autotrophic and mixotrophic cells wer e however observed. The possible mechanisms involved in the inhibition of Rubisco are discussed.