Dark respiration in the light and in darkness of three marine macroalgal species grown under ambient and elevated CO₂ concentrations

Dark respiration (non-photorespiratory mitochondrial respiration), which occurs both in the light and in darkness, is vital for growth and survival of algae and plays a critical role in modulating the carbon balance of them. In the present study, we have investigated dark respiration in the light (R_L) and in darkness (R_D) in three marine macroalgal species, Hizikia fusiformis (phaeophyta), Gracilaria lemaneiformis (Rhodophyta) and Ulva lactuca (Chlorophyta), cultured at 20 ℃ using aeration with two CO₂ conditions:current ambient (CO₂ concentration about 380 μl/L) and elevated CO₂ (approximately 720 μl/L) air. R_L was estimated by using the Kok method, whereas R_D was determined as the rate of O₂ influx at zero light. The results showed that both R_L and R_D were unchanged for the elevated CO₂-grown algae relative to ambient CO₂ concentration for all the algal species tested. However, R_L was significantly lower than R_D across all the algal species and growth CO₂ treatments, demonstrating that daytime respiration was partly depressed by the light. The percentage of inhibition of respiration by light was similar between ambient and elevated CO₂-grown algae. The ratio of respiration to photosynthesis, which tended to decrease when estimated using R_L instead of R_D, was not altered for the elevated relative to ambient CO₂ concentration. The results suggest that R_L, rather than R_D, is a more accurate estimate of nonphotorespiratory carbon loss in marine macroalgae during the daytime. It would not be anticipated that elevated atmospheric CO₂ would exert a substantial influence on respiratory flux either in the light or in darkness in these particular marine macroalgal species.