The metabolic cost of bicarbonate use in the submerged plant Elodea nuttallii

The aquatic plant Elodea nuttallii (Planch.) St. John has been shown to express plasticity in the source of inorganic carbon it uses for photosynthesis. An investigation was undertaken to determine what effect the switch from CO₂ to HCO₃^? use had on the growth of E. nuttallii. Plants were grown under reduced CO₂ availability that favoured the switch, together with control plants (CO₂ at equilibrium with air) that continued to use CO₂ only. The extent to which both sets of plants could utilise HCO₃^? was determined (as the ratio of oxygen evolution at pH 9 and 6.5), and several measures of growth were made. Although reduced CO₂ availability produced an increase in HCO₃^? utilisation, no differences were found in the measured growth of the plants. Therefore, it was possible to estimate, from the difference between the estimated rate of photosynthesis of the plants utilising HCO₃^? and those using CO₂ only, the approximate cost of constructing, maintaining and running the bicarbonate utilisation mechanism in this species as 69 μmol photons m^?2 s^?1. This value can be used to estimate an irradiance of circa 80 μmol m^?2 s^?1 below which HCO₃^? use would not be expected in this species, an irradiance commonly experienced by submerged macrophytes in the field.