Cadmium (Cd), classified as a human carcinogen, is dispersed into aquatic ecosystems mainly through industrial processes or via the application of phosphate fertilizers. High solubility of Cd in water facilitates its wide distribution in aquatic systems. Cd can be readily taken up by aquatic plants and cause phytotoxicity. For aquatic plants, carbon-shortage is another common problem/stress. Nevertheless, the aquatic plants have evolved carbon dioxide-concentrating mechanisms (CCMs) to counter the problem of inorganic carbon limitation. Very few reports are available on the combined effects of Cd and low inorganic carbon stress in aquatic plants.

Ottelia alismoides is the only known species to perform three CO₂-concentrating mechanisms (CCMs): facultative Crassulacean acid metabolism (CAM), constitutive bicarbonate (HCO₃^–)use and C₄ photosynthesis.  

The Aquatic Plant Biology Research Group led by Prof. LI Wei from CAS Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden studied the combined effects of Cd and low inorganic carbon stress on CCMs in O. alismoides, and discovered that O. alismoides exhibited an elevated Cd accumulation along with the increasing Cd concentration.

Cd treatment induced appreciable phytotoxicities in O. alismoides, including the damaged leave anatomy and chloroplast ultrastructure, as well as the reduced pigment biosynthesis and chlorophyll fluorescence.

The pH-drift technique showed that both Cd-treated O. alismoides plants could not uptake HCO₃^-. The diurnal change of acidity was absent, as well as the significant decrease in photosynthetic enzyme activity, indicating the disturbance within C₄ and CAM cycle. The alterations in the functionality of CCMs in O. alismoides induced by Cd might be related with the inhibition of the enzymes involved in inorganic carbon fixation, and the destruction of chloroplasts, as well as the re-allocation of energy and nutrients involved in CCMs and Cd detoxification. 

Research was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences and the National Natural Science Foundation of China. The results have been published in Ecotoxicology and Environmental Safety entitled “Responses of CO₂-concentrating mechanisms and photosynthetic characteristics in aquatic plantOttelia alismoides following cadmium stress under low CO₂”.