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Growth and shorter term incorporation measurements with both wild type Chiamydomonas reinWii and a mutant (F-60, lacking Recent gas exchange measurements with Chlamydomonas reinhardtii and other green algae (3, 5, 13) have indicated very low levels of photorespiration, thereby changing the perception of the role of glycolate in these organisms. An inducible mechanism for concentrating CO2 internally has been characterized (2). A high intracellular CO2 concentration enables the algal cell to avoid glycolate formation and photorespiratory gas exchange. This mechanism provides a good explanation for many of the well known changes in measurements of whole cell glycolate release; prominent glycolate excretion is seen only in cells grown with elevated levels of CO2 when there is little internal concentrating of CO2. The likelihood of variable glycolate formation and the observations of glycolate excretion by the cell raise questions about the maximum capacity and the variability of glycolate utilization.The very existence of glycolate excretion suggests that turnover is at times limited. However, radioisotopic and enzymic analyses have indicated that the potential for carbon flow through the glycolate pathway of higher plants exists in including the present study, has noted the limitations of such utilization by green algae (23).Before the awareness of active CO2 concentration it was concluded that differences in metabolism were responsible for changes in the excretion of glycolate. A decrease in the activity of the enzyme responsible for its initial oxidation, glycolate dehydrogenase, that corresponded with the increased excretion following growth on high levels of CO2 corroborated this notion (17). However, this correlation has been shown to break down in Chlorella pyrenoidosa (7) and Euglena (6) as well as in the following results with Chlamydomonas. In addition, the control of the glycolate balance by permeability and the availability of counterions was suggested in early work by Tolbert and Zill (24) and has been recently carefully examined in Scenedesmus by Findenegg (9). The latter study concluded that permeability can be the primary controlling factor in the uptake and release of glycolate.In this report the potential for utilization of internal glycolate is estimated by several methods. At a low pH, the permeability barrier of glycolate into the cell is relieved. The effects of various growth conditions are determined. Incorporation studies with the F-60 mutant are particularly important in measuring true heterotrophic glycolate utilization. Because of its phosphoribulokinase lesion, this mutant can produce little glycolate. The effects of the metabolic inhibitor isonicotinyl hydrazide are also measured. MATERIALS AND METHODSCell Culture. The MM3 of Sueoka (21) was used with the following modifications: 15 mm potassium acetate was added to make MA; 15 mm potassium glycolate was added to make MG; 10 mm glycylglycine buffer (pH 3.8) was added to medium which contained only one-sixth of the phosphate...

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