Effects of aerobic and anaerobic conditions on growth and metabolism of blue-green algae

Abstract

The blue-green algae Anabaena flos-aquae and Nostoc muscorum may reduce acetylene to ethylene most actively at pO$_{2}$ levels below 0.2 atm. High pO$_{2}$ levels inhibit acetylene reduction, nitrogen fixation, respiration and $^{14}$CO$_{2}$ fixation in A. flos-aquae. The effect is not solely via an inhibition of nitrogenase activity because inhibition of $^{14}$CO$_{2}$ fixation by a species of Phormidium which does not fix nitrogen, and by A. flos-aquae grown on combined nitrogen also occurs. The inhibition of acetylene reduction in Anabaena is reversible in short-term experiments, the rate of recovery being rather similar irrespective of the pO$_{2}$ level to which the alga was subjected initially. 3(3,4-dichlorophenyl)-1-1-dimethyl urea (DCMU) at a concentration of 3 $\times $ 10$^{-5}$ M, which completely inhibits oxygen evolution and acetylene reduction but not respiration by aerobically grown Anabaena cultures also inhibits acetylene reduction in Na$_{2}$S-grown cultures. Salicylaldoxime at a concentration of 10$^{-4}$ M which partially inhibits electron flow from photosystem II inhibits acetylene reduction to a greater extent under aerobic conditions than in the presence of Na$_{2}$S. The presence of Na$_{2}$S also results in the removal of free oxygen from the medium in the pH range at which Anabaena normally grows. The data suggest that under our conditions (1) the photolysis of water, or photosystem II is essential for the growth of A. flos-aquae in the presence of H$_{2}$S; (2) H$_{2}$S may provide electrons when reducing power from the photolysis of water is reduced but not inhibited completely; (3) H$_{2}$S prevents an accumulation of oxygen in the medium during photosynthesis. These physiological findings from the laboratory may help to explain why blue-green algae live not only in aerobic environments but also in habitats where reducing conditions may prevail.