UPTAKE OF UREA BY CHLORELLA

The uptake of 14C by urea‐grown cells of Chlorella (strain 211/8p) supplied with [14C]urea was studied. Uptake was rapid in light with an apparent half‐saturation value (Km) for urea of 16·5 μM. Uptake was inhibited competitively by thiourea. Uptake of 14C was inhibited by 6 × 10−6 DCMU in light and was very much slower in darkness; nevertheless, under these conditions, rate of loss of 14C from the medium was much the same as in illuminated non‐inhibited cultures. [14C]urea was metabolized rapidly by the cells and, in illuminated cells after a 5 min exposure to [14C]urea, less than 50 % of the soluble 14C within the cells was in [14C]urea. It is concluded that much of the measured 14C uptake may have been due to conversion of [14C]urea to 14CO2 followed by photosynthetic 14CO2 fixation. Nevertheless, when [14C]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 [14C]urea by Chlorella, it may be preferable to study the uptake mechanism of this organism by using the urea analogue, thiourea. Copyright © 1979, Wiley Blackwell. All rights reserved