Uptake of sulphate, taurine, cysteine and methionine by symbiotic and free-living dinoflagellates

Sulphate uptake by Amphidinium carterae, Amphidinium klebsii and Gymnodinium microadriaticum grown on artificial seawater medium with sulphate, cysteine, methionine or taurine as sulphur source occurred via an active transport system which conformed to Michaelis-Menten type saturation kinetics. Values for K_m ranged from 0.18-2.13 mM and V_max ranged from 0.2-24.2 nmol · 10⁵ cells^-1 · h^-1. K_m for symbiotic G. microadriaticum was 0.48 mM and V_max was 0.2 nmol · 10⁵ cells^-1 · h^-1. Sulphate uptake was slightly inhibited by chromate and selenate, but not by tungstate, molybdate, sulphite or thiosulphate. Cysteine and methionine (0.1 mM), but not taurine, inhibited sulphate uptake by symbiotic G. microadriaticum, but not by the two species of Amphidinium. Uptake was inhibited 45-97% under both light and dark conditions by carbonylcyanide 3-chlorophenylhydrazone (CCCP); under dark conditions sulphate uptake was 40-60% of that observed under light conditions and was little affected by 3-(3,4-dichlorophenyl) 1,1-dimethylurea (DCMU). The uptake of taurine, cysteine and methionine by A. carterae, A. klebsii, cultured and symbiotic G. microadriaticum conformed to Michaelis-Menten type saturation kinetics. K_m values of taurine uptake ranged from 1.9-10 mM; for cysteine uptake from 0.6-3.2 mM and methionine from 0.001-0.021 mM. Cysteine induced a taurine uptake system with a K_m of 0.3-0.7 mM. Cysteine and methionine uptake by all organisms was largely unaffected by darkness or by DCMU in light or darkness. CCCP significantly inhibited uptake of these amino acids. Thus energy for cysteine and methionine uptake was supplied mainly by respiration. Taurine uptake by A. carterae was independent of light but was inhibited by CCCP, whereas uptake by A. klebsii and symbiotic G. microadriaticum was partially dependent on photosynthetic energy. Taurine uptake by cultured G. microadriaticum was more dependent on photosynthetic energy and was more sensitive to CCCP. Cysteine inhibited uptake of methionine and taurine by cultured and symbiotic G. microadriaticum to a greater extent than in the Amphidinium species. Methionine did not greatly affect taurine uptake, but did inhibit cysteine uptake. Taurine did not affect the uptake of cysteine or methionine.