Photorespiration and Internal CO2 Accumulation in Chara corallina as Inferred from the Influence of DIC and O2 on Photosynthesis 1

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An O2 electrode system with a specially designed chamber for `whorl' cell complexes of Chara corallina was used to study the combined effects of inorganic carbon and O2 concentrations on photosynthetic O2 evolution. At pH = 5.5 and 20% O2, cells grown in HCO3− medium (low CO2, pH ≥ 9.0) exhibited a higher affinity for external CO2 (K½(CO2) = 40 ± 6 micromolar) than the cells grown for at least 24 hours in high-CO2 medium (pH = 6.5), (K½(CO2) = 94 ± 16 micromolar). With O2 ≤ 2% in contrast, both types of cells showed a high apparent affinity (K½(CO2) = 50 − 52 micromolar). A Warburg effect was detectable only in the low affinity cells previously cultivated in high-CO2 medium (pH = 6.5). The high-pH, HCO3−-grown cells, when exposed to low pH (5.5) conditions, exhibited a response indicating an ability to fix CO2 which exceeded the CO2 externally supplied, and the reverse situation has been observed in high-CO2-grown cells. At pH 8.2, the apparent photosynthetic affinity for external HCO3− (K½[HCO3−]) was 0.6 ± 0.2 millimolar, at 20% O2. But under low O2 concentrations (≤2%), surprisingly, an inhibition of net O2 evolution was elicited, which was maximal at low HCO3− concentrations. These results indicate that: (a) photorespiration occurs in this alga and can be revealed by cultivation in high-CO2 medium, (b) Chara cells are able to accumulate CO2 internally by means of a process apparently independent of the plasmalemma HCO3− transport system, (c) molecular oxygen appears to be required for photosynthetic utilization of exogenous HCO3−: pseudocyclic electron flow, sustained by O2 photoreduction, may produce the additional ATP needed for the HCO3− transport.

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