Ottelia alismoides is a constitutive C₄ plant and bicarbonate user, and has facultative crassulacean acid metabolism (CAM) at low CO₂.  Acclimation to a factorial combination of light and CO₂ showed that the ratio of phosphoenolpyruvate carboxylase (PEPC) to ribulose-bisphosphate carboxylase/oxygenase (Rubisco) (>5) is in the range of that of C₄ plants. This and short-term response experiments showed that the activity of PEPC and pyruvate phosphate dikinase (PPDK) was high even at the end of the night, consistent with night-time acid accumulation and daytime carbon fixation. The diel acidity change was maximal at high light and low CO₂ at 17–25 µequiv g⁻¹ FW. Decarboxylation proceeded at ~2–3 µequiv g⁻¹ FW h⁻¹, starting at the beginning of the photoperiod, but did not occur at high CO₂; the rate was greater at high, compared with low light. There was an inverse relationship between starch formation and acidity loss. Acidity changes account for up to 21% of starch production and stimulate early morning photosynthesis, but night-time accumulation of acid traps <6% of respiratory carbon release. Ottelia alismoides is the only known species to operate CAM and C₄ in the same tissue, and one of only two known aquatic species to operate CAM and bicarbonate use.