THE UPTAKE OF UREA BY MARINE PHYTOPLANKTON1

SUMMARY Half‐saturation constants for urea uptake by 4 clones of neritic diatoms capable of utilizing urea were determined from short‐term uptake studies with15N‐labeled urea.K8values obtained were similar to those determined, earlier for ammonium, and since ammonium and urea concentrations are similar in the marine environment, it was concluded that these species are capable of utilizing ecologically significant concentrations of urea. Two of 3 species unable to grow on urea showed patterns of short‐term uptake not unlike those of species capable of utilizing urea, which implies that, their assimilatory rather than uptake processes are defective with, regard to urea utilization. The third species initially took15N (supplied as urea) into the cells but subsequently released it back into the medium.