Bicarbonate use reduces the photorespiration in Ottelia alismoides adapting to the CO₂-fluctuated aquatic systems

Underwater CO₂ concentration fluctuates extremely in natural water bodies. Under low CO₂, the unique CO₂ concentrating mechanism in aquatic plants, bicarbonate use, can suppress photorespiration. However, it remains unknown (1) to what extent bicarbonate use reduces photorespiration, (2) how exactly photorespiration varies between bicarbonate-users and CO₂-obligate users under CO₂-fluctuated environments, and (3) what are differences in Rubisco characteristics between these two types of aquatic plants. In the present study, the bicarbonate user Ottelia alismoides and its phylogenetically close CO₂-obligate user Blyxa japonica were chosen to answer these questions. The results showed that bicarbonate use saved ~13% carbon loss under low CO₂ via decreasing photorespiration in O. alismoides. Through bicarbonate use, the photorespiration of O. alismoides was kept stable both under high and low underwater CO₂ concentrations, while the photorespiration significantly increased in the CO₂-obligate user B. japonica under low CO₂. However, B. japonica showed a significantly higher photosynthesis rate than O. alsimoides when CO₂ was sufficient. These differences could be related to the kinetic characteristics of Rubisco showing a higher carboxylation turnover rate (Kcat) in B. japonica, and the similar affinity to CO₂ (Kc) and specificity factor (Sc/o) in these two species that might be determined by the variation of six amino acid residuals in Rubisco large subunit sequences, especially the site 281 (A vs. S) and 282 (H vs. F). All these differences in photorespiration and kinetic characteristics of Rubisco could explain the distribution patterns of bicarbonate users and CO₂-obligate users in the field.