Effect of naphthalene and aqueous crude oil extracts on the green flagellate Chlamydomonas angulosa. V. Heterotrophic responses

Abstract

Chlamydomonas angulosa shows a relatively weak capacity for heterotrophic growth on acetate in the dark. The cell doubling time on acetate in the dark is about 62 h as compared with about 17 h for growth in the absence of acetate under optimal light conditions. The cell's capacity for acetate uptake and its isocitrate lyase activity are also relatively low. No [14C]glucose is taken up by the cells, in agreement with the inability of C. angulosa to grow on glucose in the dark or of glucose to stimulate growth under light-limiting conditions. The Km and Vmax for acetate uptake for cells cultured on 12 h light: 12 h dark cycle, at the end of a 12-h dark period, are 0.05 mM and 1.3 fmol cell−1 min−1, respectively. The uptake capacity increases strongly upon dark incubation, while the presence of light during short uptake experiments enhances the uptake rate. There is only a moderate 2.5 times increase in isocitrate lyase activity upon incubation of light-grown cells in the dark with acetate.Cells incubated in 100% naphthalene-saturated media undergo a rapid and complete loss of acetate uptake capacity. Media, 50% saturated with naphthalene or 100% saturated with aqueous crude oil extracts, cause an initial stimulation of acetate uptake. Media, 50% saturated with aqueous crude oil extracts cause a much longer (at least 7 days) stimulation of acetate uptake capacity. Similarly, 50% saturated-naphthalene media cause an increase in isocitrate lyase activity on a per cell basis over control cells. While 50% naphthalene-saturated media permit almost no cell division in closed acetate-containing cultures in the dark, very dramatic increases in cell size occur over long time periods. Moderate concentrations of crude oil components dissolved in algal culture media, thus, permit significant rates of acetate uptake and assimilation but inhibit cell division. It is possible that the observed stimulation of acetate uptake by moderate concentrations of crude oil components is due to the following reasons: (i) hydrocarbon-induced permeabilization of the cell membrane and (ii) an increased availability of energy from photosynthetic light reactions owing to decreased CO2 photoassimilation.