Acclimation to variable CO(2)was studied in floating leaves of the freshwater monocotOttelia cordatagrown in either low or high CO2. The most striking anatomical variations responding to high CO(2)included the enlarged upper epidermal cells and the decreased area of epidermal chloroplasts. Stomata that distributed on the upper surface, and the stomatic chamber area, showed no significant response to high CO2. pH-drift experiments indicated that floating leaves ofO. cordatawere able to use bicarbonate regardless of CO(2)concentrations. Photosynthetic enzyme activities and patterns of organic acids fluctuation confirmed that floating leaves ofO. cordatacan operate CAM only at low CO2, and perform C-4-like metabolism at both high and low CO2. Overall, the present results imply that the floating leaves ofO. cordatadoes not just rely on the atmospheric CO(2)for its inorganic carbon, but is also dependent on CO(2)and bicarbonate in the water. By showing these effects of CO(2)variation, we highlight the need for further experimental studies on the regulatory mechanisms inO. cordatafloating leaves, that prevent futile cycling among the three CO(2)concentrating mechanisms (bicarbonate use, C-4, and CAM metabolism) and the strategy for exploiting atmospheric CO2, as well as studies on the detailed biochemical pathway for C(4)and CAM metabolism in this species.