Effect of acetate on growth and ammonium uptake in the microalga <i>Scenedesmus obliquus</i>

Abstract

The effect of acetate on growth and rate of ammonium uptake in Scenedesmus obliquus (UTEX 78) was investigated under light‐limiting conditions. Addition of acetate to autotrophic cells with a growth constant of 0.71 day −1 resulted in an increase in the growth rate (mixotrophy, k = 1.3 day −1 ), and in the presence of acetate, growth occurred in the dark (heterophy, k = 0.44 day −1 ). The rate of ammonium uptake in autotrophy (17.8 amol cell −1 min −1 ) was similar to that in heterotrophy (17.4 amol cell −1 min −1 ) but was 3.7 times lower than that in mixotrophy (65.9 amol cell −1 min −1 ). In general, mixotrophic cells showed optimum ammonium uptake at the acetate concentration at which they were grown. In autotrophy, uptake of ammonium leveled off at about 12.5 μ M while no saturation was observed in mixotrophic cells. An increase in the rate of uptake of ammonium was observed in autotrophic cells within 1 h after the addition of acetate. The activity of isocitrate lyase (EC 4.1.3.1), a key enzyme for the regulation of the glyoxylate cycle responsible for acetate catabolism, showed a 3.9‐fold increase in activity after 24 h in the dark in the presence of acetate. The level of isocitrate lyase activity in cells grown for 24 h in the dark in the presence of 0–20 m M acetate also increased as a function of acetate concentration.