The Uptake of Urea by Chlorella

S ummary The uptake of 14 C by urea‐grown cells of Chlorella (strain 211/8p) supplied with [ 14 C]urea was studied. Uptake was rapid in light with an apparent half‐saturation value ( K m ) for urea of 16·5 μM. Uptake was inhibited competitively by thiourea. Uptake of 14 C was inhibited by 6 × 10 −6 DCMU in light and was very much slower in darkness; nevertheless, under these conditions, rate of loss of 14 C from the medium was much the same as in illuminated non‐inhibited cultures. [ 14 C]urea was metabolized rapidly by the cells and, in illuminated cells after a 5 min exposure to [ 14 C]urea, less than 50 % of the soluble 14 C within the cells was in [ 14 C]urea. It is concluded that much of the measured 14 C uptake may have been due to conversion of [ 14 C]urea to 14 CO 2 followed by photosynthetic 14 CO 2 fixation. Nevertheless, when [ 14 C]urea in the cells was extracted and separated by thin‐layer chromatgraphy, there was clear evidence of accumulation at a concentration above that in the external medium and of light stimulation of uptake. It is suggested that, in view of the rapid metabolism of [ 14 C]urea by Chlorella , it may be preferable to study the uptake mechanism of this organism by using the urea analogue, thiourea.