The Uptake of Thiourea by Chlorella

SUMMARY Cells of Chlorella fusca grown with urea as nitrogen source take up and accumulate thiourea; ammonium‐grown cells take up thiourea more slowly. Uptake results from a metabolic process superimposed on passive diffusion. The uptake system has a high affinity for thiourea and urea. Fifty μM thiourea allows half the maximum rate of uptake. Urea inhibits competitively uptake of thiourea and, from the kinetics of inhibition, it can be concluded that the half‐saturation constant for urea is about 5 μM. Uptake of thiourea by urea‐grown organisms is not inhibited by low concentrations of nitrate or of ammonium. There is evidence that the cells metabolize thiourea to a limited extent.