On the nature of the oxygen uptake in the light by Chondrus crispus. Effects of inhibitors, temperature and light intensity.
The nature of the different processes of O2 uptake involved in the light in the red macroalga Chondrus crispus Stackhouse (Rhodophyta, Gigartinales) was investigated. At limiting CO2, INH (2.5 mM) did not alter the O2 uptake rate. Glycolate was not excreted and did not accumulate within the cells. KCN reduced the rate of O2 uptake in the light by 76% at limiting CO2 and by 43% at saturating CO2, but caused > 95% inhibition of O2 evolution. DCMU (5 μM) totally blocked the photosynthetic electron transport chain, but allowed a residual O2 uptake of 3.0±0.6 μmol O2 .h(-1).g(-1) FW, irrespective of the CO2 concentration. In saturating CO2, a high light intensity pretreatment significantly stimulated the rate of O2 uptake compared to net O2 evolution, suggesting the persistence, in the light, of mitochondrial respiration. Irrespective of the CO2 concentration, the optimum temperature for O2 evolution was 17°C whereas dark O2 uptake increased linearly with temperature. In contrast, O2 uptake in the light showed an optimum at 17°C in limiting CO2, and 21-25° C in saturating CO2; its Q10 was 2.4 at limiting CO2, a value close to that of RuBP oxygenase, and 3.1 at saturating CO2, a value close to that of dark respiration. It is concluded that: 1) mitochondrial respiration and Mehler reaction are both involved at all CO2 concentrations, 2) RuBP oxygenase activity cannot account for more than 45%, and Mehler reaction for less than 20%, of the total O2 uptake observed in the light at limiting CO2.