Fructose utilization by the cyanobacterium Anabaena variabilis studied using whole filaments and isolated heterocysts

Abstract

We have investigated the utilization of [14C]-fructose by whole filaments and isolated heterocysts of Anabaena variabilis ATCC 29413, a strain which is capable of fructose-dependent heterotrophic growth. The experimental conditions were chosen such that both transport and subsequent metabolism were studied. The apparent Km for fructose was 60 μmM, close to the results of previous studies. Rates of fructose utilization were the same in light and darkness. When photosynthetic CO2 fixation was possible, almost all the label appeared as cell-carbon. In darkness or in the presence of DCMU appreciable amounts of label were released as CO2. Isolated heterocysts with high rates of endogenous metabolism were not capable of utilizing added fructose at significant rates. The effects of oxygen concentration on the metabolism of added fructose in darkness showed that uptake was saturated at low pO2 values. Increasing the pO2 values lead to an increase in the ratio between the lable released as CO2 and that recovred as cell-carbon. These results suggest that fructose is taken up only by the vegetative cells but carbon derived from added fructose can be released as CO2 as a result of respiration in the heterocysts. Fructose utilization was inhibited by uncouplers. The greatest inhibition was found when both (delta) (psi) and (delta) pH were abolished. High concentrations of erythrose inhibited fructose utilization. None of the other potential analogs tested had any effect.