Abstract
Oxygen gas exchange was monitored in the unicellular green alga Chlorella vulgaris 211 - 11 h by means of a mass spectrometer equipped with a special membrane gas-inlet-system and a photosynthetic reaction vessel. CO2-dependent 18O2-uptake as well as 16O2-evolution were analyzed in both High- and Low-CO2 cells. In High-CO2 cells, the 18O2-ruptake in the light (UL) decreased by 65% upon addition of 3 mᴍ NaHCO3 , while 16O2-evolution (E) was increased approx. 1.8 times by the same treatment. 18O2-uptake in the dark (UD) was not affected by the addition of external inorganic carbon (Ci). The addition of 3.3 mᴍ NaHCO3 also affected UL and E in Low CO2-cells, however, to a minor extent. UL under CO2-saturating conditions was light intensity-independent up to 2 klux and 1.2 klux in High- and Low-CO2 cells, respectively.
Above these light intensities UL increased approx. 4-fold in High- and approx. 6-fold in Low-CO2 cells. Under CO2-limiting conditions, however, UL increased in High-CO2 cells even under very low light intensities, showing that photorespiratory oxygen uptake occurred even in the near vicinity of the light compensation point. Under C02-saturating and strong light conditions UL represented almost half of E in Low-CO2 cells and about 30 % of E in High-CO2 cells. In Low-CO2 cells addition of ethoxyzolamide (EZA), an inhibitor of carbonic anhydrase, enhanced UL and suppressed E and NET under CO2-limiting conditions, whereas the compound had only a minor effect on High-CO2 cells.
DCMU (3 μᴍ) strongly inhibited E and UL under CO2-saturating conditions, with the remaining UL being smaller than UD . KCN (1 mᴍ) and SHAM (1.5 mᴍ) added to DCMU-treated Low-CO2 cells suppressed UL by approx. 50 % . The resulting value corresponded to half of UD . KCN also inhibited E under CO2-saturating conditions, with UL being strongly enhanced showing a maximal uptake at 0.4 mᴍ KCN . Under these conditions NET was nearly zero. The effect seems to be due to an inhibition of RubisCO and an enhancement of Mehler reactions. At 0.7 mᴍ KCN , DCMU entirely inhibited UL , but oxygen uptake appeared increased after turning the light off. This uptake corresponded to approx. 60 % of UD . Whereas KCN and SHAM inhibited approx. 70 % of UD , only 16% of UL was suppressed. These results suggest that the contribution of mitochondrial respiration to UL was negligeable, since UL seemed to be suppressed in the light under CO2-saturated conditions. Iodoacetamide, which is an inhibitor of the Calvin cycle and thereby diverts carbon into the respiratory pathway, inhibited E and NET under CO2-saturating conditions, but did not affect UL . This result also shows that UL is not due to mitochondrial respiration. A hydroxylamine derivative [20, 21] which changes the ratio of the RuBP carboxylation to oxygenation activity in tobacco leaves did not affect this ratio in Chlorella.
Assessment of the optimum growth medium and the effect of different light intensities on growth and photosynthetic pigments of Chlorella vulgaris and Scenedesmus arvernensis
