Ottelia alismoides is the only known species to perform three CO₂-concentrating mechanisms (CCMs) to overcome the problem of carbon shortage: facultative Crassulacean acid metabolism (CAM), constitutive bicarbonate (HCO₃^–) use and C₄ photosynthesis.  Llittle is known about the mechanism responsible for HCO₃^– use in this species.  

The Laboratory of Aquatic Plant Biology led by Prof. LI Wei from CAS Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden discovered that 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS), the inhibitor of an anion exchange protein, had a small effect on CO₂ uptake and prevented HCO₃^– use in O. alismoides; the acetazolamide (AZ), as the inhibitor of external carbonic anhydrase, also prevented HCO₃^– use in this species. Analysis of mRNA transcripts identified a homologue of solute carrier 4 (SLC4) responsible for HCO₃^– transport, likely to be the target of DIDS, and a periplasmic α-CA1. A model of carbon acquisition was constructed to quantify the contribution of three different pathways involved in inorganic carbon uptake in O. alismoides, which showed that passive CO₂ diffusion dominated inorganic carbon uptake at high CO₂ concentrations. However, as CO₂ concentrations fell, two other pathways became predominant: conversion of HCO₃^– to CO₂ at the plasmalemma by α-CA1 and transport of HCO₃^– across the plasmalemma by SLC4. 

Research was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences, Chinese Academy of Sciences President's International Fellowship Initiative to S.C.M. and B.G., and the National Natural Science Foundation of China.  

The results have been published in Journal of Experimental Botany entitled “External α-carbonic anhydrase and solute carrier 4 are required for bicarbonate uptake in a freshwater angiosperm”. 

A model of inorganic carbon acquisition in Ottelia alismoides (Image by WBG)