Effect of Photon Fluence Rate on Oxygen Evolution and Uptake by Chlamydomonas reinhardtii Suspensions Grown in Ambient and CO2-Enriched Air

Abstract

A closed system consisting of an assimilation chamber furnished with a membrane inlet from the liquid phase connected to a mass spectrometer was used to measure O(2) evolution and uptake by Chlamydomonas reinhardtii cells grown in ambient (0.034% CO(2)) or CO(2)-enriched (5% CO(2)) air. At pH = 6.9, 28 degrees C and concentrations of dissolved inorganic carbon (DIC) saturating for photosynthesis, O(2) uptake in the light (U(o)) equaled O(2) production (E(o)) at the light compensation point (15 micromoles photons per square meter per second). E(o) and U(o) increased with increasing photon fluence rate (PFR) but were not rate saturated at 600 micromoles photons per square meter per second, while net O(2) exchange reached a saturation level near 500 micromoles photons per square meter per second which was nearly the same for both, CO(2)-grown and air-grown cells. Comparison of the U(o)/E(o) ratios between air-grown and CO(2)-grown C. reinhardtii showed higher values for air-grown cells at light intensities higher than light compensation. For both, air-grown and CO(2)-grown algae the rates of mitochondrial O(2) uptake in the dark measured immediately before and 5 minutes after illumination were much lower than U(o) at PFR saturating for net photosynthesis. We conclude that noncyclic electron flow from water to NADP(+) and pseudocyclic electron flow via photosystem I to O(2) both significantly contribute to O(2) exchange in the light. In contrast, mitochondrial respiration and photosynthetic carbon oxidation cycle are regarded as minor O(2) consuming reactions in the light in both, air-grown and CO(2)-grown cells. It is suggested that the "extra" O(2) uptake by air-grown algae provides ATP required for the energy dependent CO(2)/HCO(3) (-) concentrating mechanism known to be present in these cells.