Evidence of Reentrance of Glycolate Carbon into the Photosynthetic Carbon Reduction Cycle in Photosynthesizing Euglena gracilis Z

Abstract

Aminooxyacetate induced excretion of glycolate from airgrown cells of Euglena gracilis in both air and 1% CO2 atmospheres. The rate of the excretion reached 70% of the photosynthetic rate in the air on a carbon basis, and was 10% in 1% CO2. The compulsory loss of photosynthetically fixed carbon as glycolate at the high rate in air in the presence of aminooxyacetate caused a decrease of the rate of synthesis of paramylon, the reserve polysaccharide. Analyses of the steady levels of photosynthetic intermediates showed that a decrease of the 3-phosphoglycerate level was the cause of the slow rate of paramylon synthesis under these conditions. The photosynthetic CO2 fixation enzyme, Rubisco,2 inevitably fixes O2 in the presence of 02. The resulting phosphoglycolate is hydrolyzed in the chloroplast to glycolate, the substrate of photorespiration. The rate of glycolate synthesis reaches 25 to 50% ofthe rate ofCO2 fixation in air atmosphere in the organisms, where Rubisco is the primary C02-fixing enzyme in photosynthesis (2,5,7,20).Glycolate is metabolized via the glycolate pathway in C3 higher plants (8 function of the glycolate pathway in these experiments. However, since any metabolite accumulated in excess in the presence of the inhibitors or in the mutants can affect growth and photosynthesis through influencing nonspecified steps (3), it is difficult for these studies to reach firm conclusions on the metabolic and physiologic functions ofthe metabolism. Elimination of the secondary effects of accumulated metabolites must be essential to obtain evidence for the hypotheses.Euglena and algal cells excrete glycolate, unlike C3 plants, when its metabolism is inhibited. We have found that AOA induces the excretion of glycolate with a small accumulation of glycolate and glyoxylate in the cells of E. gracilis (13,14). Therefore, the metabolic study of the effects of AOA on photosynthesis in Euglena may lead to an understanding of the metabolic relationship between the glycolate pathway and the PCR cycle. This paper describes our results.

PHOTORESPIRATION IN CONTINUOUS CULTURE OF DUNALIELLA TERTIOLECTA (CHLOROPHYTA): RELATIONSHIPS BETWEEN SERINE, GLYCINE, AND EXTRACELLULAR GLYCOLATE

“…Part of this released DGA seems to be reassimilated, as previously demonstrated in other species in laboratory cultures (e.g. Yokota et al 1990 for Euglena gracilis), and up to 16% of added dissolved glycolate has been found to be incorporated into stable products of D. tertiolecta cells in batch cultures (Leboulanger et al 1995). Dissolved free amino acids-SER and GLY-are higher in cells from LL cyclostat than in cells from HL, possibly because of the relative nitrogen depletion in HL, where growth rate and average cell density are higher than in LL.…”

Expression of foreign type I ribulose-1,5-bisphosphate carboxylase/ oxygenase (EC 4.1.1.39) stimulates photosynthesis in cyanobacterium Synechococcus PCC7942 cells

“…The supernatant was lyophilized once, then the remaining material was dissolved in 50 ml of distilled water and lyophilized again. After the addition of a small volume of 400 mM Hepes-KOH buffer (pH 7.6), RuBP and PGA were measured using the methods of Yokota et al (1990) and Badger et al (1984). (Figure 3).…”

“…The concentrations of RuBP and PGA in the steady state of photosynthesis were determined according to the method of Yokota et al (1990) with a slight modification. The cells suspended in 50 mM Hepes-NaOH buffer (pH 8.0) at a chlorophyll density of 10 lg Chl ml )1 were put into a 200 ml volume chemostat vessel equipped with a YSI oxygen electrode.…”