Glycolate Metabolism and Excretion by Chlamydomonas reinhardtii

Abstract

The flux of glycolate through the C2 pathway in Chlamydomonas reinhardti was estimated after inhibition of the pathway with aminooxyacetate (AOA) or aminoacetonitrile (AAN) by measurement of the accumulation of glycolate and glycine. Cells grown photoautotropically in air excreted little glycolate except in the presence of 2 mM AOA when they excreted 5 micromoles glycolate per hour per milligram chlorophyll. Cells grown on high CO2 (1-5%) when transferred to air produced three times as much glycolate, with half of the glycolate metabolized and half excreted. The lower amount of glycolate produced by the air-grown cells reflects the presence of a CO2 concentrating mechanism which raises the internal CO2 level and decreases the ribulose-1,5-bisP oxygenase reaction for glycolate production. Despite the presence of the CO2 concentrating mechanism, there was still a significant amount of glycolate produced and metabolized by air-grown Chlamydomonas. The capacity of these cells to metabolize between 5 and 10 micromoles of glycolate per hour per milligram chlorophyll was confirmed by measuring the biphasic uptake of added labeled glycolate. The initial rapid (< 10 seconds) phase represented uptake of glycolate; the slow phase represented the metabolism of glycolate. The rates of glycolate metabolism were in agreement with those determined using the C2-cycle inhibitors during CO2 fixation.