STUDIES OF NITRATE TRANSPORT BY MARINE PHYTOPLANKTON USING 36 Cl‐ClO 3 − AS A TRANSPORT ANALOGUE. I. PHYSIOLOGICAL FINDINGS 1

Transport of a nitrate analogue, 36 Cl‐ClO 3 − , was examined in two diatoms, Skeletonema costatum (Greve.) Cleve and Nitzschia closterium (Ehrenb) W. Sm. A dinoflagellate, Gonyaulax polyedra did not transport ClO 3 − . Transport of 36 Cl‐ClO 3 − , by diatoms appeared to be active and showed saturation kinetics. The data were fitted by Michaelis‐Menten equation at all but the lowest chlorate concentrations (where plots of S vs. v showed a slight concave bend). Affinity of cells for nitrate was considerably higher than for chlorate. The K i for nitrate inhibition of chlorate transport was calculated assuming competitive inhibition. Light had little or no effect on chlorate transport. Pulse‐chase experiments demonstrated that (1) ClO 3 − (hence NO 3 − ) was stored in two intracellular compartments of equal size, (2) internal ClO 3 − was exchangeable with external ClO 3 − (rates of efflux and influx were measured), and (3) efflux of intracellular ClO 3 − showed transient states following a chase of ClO 3 − or NO 3 − which stabilized after 10–20 min. Transport of chlorate was a function of growth phase.