Mass Spectrometry Analysis of Oxygen Gas Exchange

Z. Naturforsch. 43c, 709-716 (1988); received M a y 24, 1988 Chlorella, Oxygen Gas Exchange, Carbonic Anhydrase, Mass Spectrometry 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. C02-dependent I802-uptake as well as ,602-evolution were analyzed in both Highand Low-C0 2 cells. In High-C0 2 cells, the 1 O ruptake in the light (U L) decreased by 6 5 % upon addition of 3 m M N a H C 0 3 , while l602-evolution (E) was increased approx. 1.8 times by the same treatment. 1802-uptake in the dark (U D) was not affected by the addition of external inorganic carbon (Ci). The addition of 3.3 m M N a H C O , also affected U L and E in Low C02-cells, however, to a minor extent. U L under C02-saturating conditions was light intensity-independent up to 2 klux and 1.2 klux in Highand L o w C 0 2 cells, respectively. Above these light intensities U L increased approx. 4-fold in Highand approx. 6-fold in LowC 0 2 cells. Under C02-limiting conditions, however, U L increased in High-CO : 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 U L represented almost half of E in Low-C0 2 cells and about 3 0 % of E in High-C0 2 cells. In Low-C0 2 cells addition of ethoxyzolamide (EZA), an inhibitor of carbonic anhydrase, enhanced U L and suppressed E and N E T under C02-limiting conditions, whereas the compound had only a minor effect on High-C0 2 cells. D C M U (3 |XM) strongly inhibited E and U L under C02-saturating conditions, with the remaining U L being smaller than U D . K C N (1 m M ) and S H A M (1.5 m M ) added to DCMU-treated LowC 0 2 cells suppressed U L by approx. 5 0 % . The resulting value corresponded to half of U D . K C N also inhibited E under CÓ2-saturating conditions, with U L being strongly enhanced showing a maximal uptake at 0.4 m M K C N . Under these conditions N E T was nearly zero. The effect seems to be due to an inhibition of RubisCO and an enhancement of Mehler reactions. At 0.7 m M K C N , D C M U entirely inhibited U L , but oxygen uptake appeared increased after turning the light off. This uptake corresponded to approx. 6 0 % of U D . Whereas K C N and S H A M inhibited approx. 7 0 % of U D , only 16% of U L was suppressed. These results suggest that the contribution of mitochondrial respiration to U L was négligeable, since U L seemed to be suppressed in the light under C02-saturated conditions. Iodoacetamide, which is an inhibitor of the Calvin cycle and thereby diverts carbon into the respiratory pathway, inhibited E and N E T under C02-saturating conditions, but did not affect U L . This result also shows that U L is not due to mitochondrial respiration. A hydroxylamine derivative [20, 21] which changes the ratio of the R u B P carboxylation to oxygenation activity in tobacco leaves did not affect this ratio in Chlorella.