Bicarbonate-use by aquatic macrophytes allows a reduction in photorespiration at low CO2 concentrations

Abstract Submerged macrophytes face the effects of the CO2 deficiency underwater that could result in higher photorespiration, nearly half of the submerged macrophytes examined to date have developed HCO3- use. In this study, the effects of HCO3- use on photorespiration were investigated in four submerged macrophytes, Cabomba caroliniana, Ceratophyllum demersum, Egeria densa and Ottelia alismoides. We used pH-drift experiments to show that three species, C. demersum, E. densa and O. alismoides, had the ability to use HCO3- (final pH exceeded 10) while C. caroliniana did not (final pH less than 9.5). The external carbonic anhydrase inhibitor, acetazolamide (AZA), was chosen to inhibit HCO3- use for oxygen evolution experiments under high CO2 and low CO2 conditions. The results showed that adding AZA significantly reduced oxygen production rate under low CO2 in the three HCO3- users, C. demersum, E. densa and O. alismoides. In addition, the activity of photorespiration-related enzymes, glycolate oxidase and catalase, significantly increased under AZA treatment in HCO3- users. However, for C. caroliniana, the CO2-only user, photorespiration and its related enzymes activities did not change under AZA addition.