Uptake of phosphate by two cyanophytes: cation effects and energetics

Abstract

Phosphate uptake in the cyanophytes Synechococcus leopoliensis and Oscillatoria limnetica did not require exogenous metallic cations. In O. limnetica phosphate uptake was described by a single Michaelis–Menten relationship between 0.2 and 10.0 μM phosphate. Calcium increased the maximum velocity and decreased the half-saturation concentration for this system; phosphate uptake was also enhanced by Sr2+ and by Mg2+ but not by Na+, K+, or Zn2+ at up to 1 mM. Calcium appeared to act upon the phosphate transporter and its action is suggested to be ecologically significant. Uptake of phosphate in the light by both cyanophytes was unaffected by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) but was abolished by carbonylcyanide m-chlorophenylhydrazone (CCCP) or dicyclohexylcarbodiimide (DCCD) at concentrations that abolished apparent photosynthesis; however, phosphate uptake was not inhibited by concentrations of CCCP or DCCD that were only slightly inhibitory to apparent photosynthesis. At 5 μM, CCCP abolished light hyperpolarization of the membrane potential but was noninhibitory to phosphate uptake. Phosphate uptake in O. limnetica was accompanied by hyperpolarization of the membrane potential. Gramicidin hyperpolarized the membrane but did not influence phosphate uptake. It is suggested that phosphate uptake in both cyanophytes is independent of the proton motive force.