Acclimation of photosynthesis to supersaturated CO2 in aquatic plant bicarbonate users

1. The photosynthesis of submerged aquatic plants can be limited by carbon dioxide (CO2) because of the low diffusivity of dissolved gas in water. About half of the species are able to use bicarbonate (HCO3-) as an alternative source of dissolved inorganic carbon (DIC). HCO3- is usually available in several-fold higher concentrations than CO2 at pH above seven, but its affinity is lower and the costs are higher than for CO2 use. The effects of changes in CO2 concentrations on the growth and physiology of bicarbonate users remain to be explored comparatively across a wide range of species. 2. We studied, in the laboratory, the plasticity in photosynthetic carbon kinetics and tissue composition of 10 submerged species, all able to use bicarbonate, acclimated to air saturated and 22 times supersaturated CO2 concentrations with the same concentration of DIC (0.85 mM). 3. Growth rate of all was stimulated (1.1-2.5 fold) and plants allocated more biomass in roots under elevated CO2 over the 35 days of the acclimation period. The photosynthetic affinity for HCO3- was higher in species acclimated to air saturated rather than supersaturated CO2. Net photosynthesis was more stimulated (2-6 times) by CO2 supersaturation concentrations after acclimation to supersaturated CO2 concentration due to their reduced HCO3- use capacity. 4. Plants grown under elevated CO2 showed reduced investment in tissue nitrogen, protein and free amino acids. Moreover, the high CO2 treatments yielded overall lower gamma-aminobutyric acid (GABA) pools, an indicator of stress relief or changes in nitrogen metabolism. 5. The analysis of 10 freshwater plants confirms that availability of HCO3-, and CO2 is a key regulator of plant physiology affecting growth. Inorganic carbon availability affects the energy budget of plants thereby influencing an array of ecological processes, and hence the distribution of submerged macrophytes in the wild.