Urea-grown cells of Phaeodactylum absorbed [14C]urea by an active mechanism. Most of the urea taken up was present in the cells as free urea and the ratio of internal to external concentration could exceed 3000. The activity of the transport mechanism was greatly increased by depriving the cells of nitrogen for up to 24 h. The uptake mechanism had a high affinity for urea, the half-saturation constant being about 1·0 μM. Active uptake of urea occurred in darkness, particularly in nitrogen-deprived cells, but uptake was markedly stimulated by light. Uptake was not inhibited by DCMU but was abolished almost completely by 10−4m CCCP. It is concluded that active uptake is driven by phosphorylation. Uptake of [14C]urea continued until a constant level within the cells was attained, the plateau value probably resulting from a balance between rate of active uptake and rate of passive loss by diffusion.

The transport mechanism was absent from cells grown with ammonium as nitrogen-source. The urea uptake mechanism developed in such cells when they were deprived of nitrogen; cycloheximide prevented the development of the mechanism. The mechanism was lost when urea-grown cells were incubated in ammonium medium for 24 h but ammonium (at concentrations up to 10 mM) did not inhibit short-term uptake of urea into urea-grown cells.