Bicarbonate use reduces the photorespiration in Ottelia alismoides adapting to the CO 2 -fluctuated aquatic systems.

Underwater CO ₂ concentration fluctuates extremely in natural water bodies. Under low CO ₂ , the unique CO ₂ 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 CO ₂ -obligate users under CO ₂ -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 CO ₂ -obligate user Blyxa japonica were chosen to answer these questions. The results showed that bicarbonate use saved ~13% carbon loss under low CO ₂ via decreasing photorespiration in O. alismoides. Through bicarbonate use, the photorespiration of O. alismoides was kept stable both under high and low underwater CO ₂ concentrations, while the photorespiration significantly increased in the CO ₂ -obligate user B. japonica under low CO ₂ . However, B. japonica showed a significantly higher photosynthesis rate than O. alsimoides when CO ₂ 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 CO ₂ (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 CO ₂ -obligate users in the field.
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