Photosynthetic Carbon Sources of Stream Macrophytes

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Abstract
Rates of photosynthesis of four submerged stream macrophytes were examined under varying pH and composition of inorganic carbon species. Callitriche stagnatis and Sparganium simplex used only CO2 for photosynthesis. Potamogeton crispus and P. pectinatus used HCO3 in addition to CO2, but with much lower efficiency. The photosynthetic rates at air equilibrium and a total inorganic carbon concentration of 5.0 mM were 2–3 times lower than maximum rates at CO2 saturation for the HCO3 users and 10–14 times lower for the CO2 users. The CO2 compensation point of entire plants of Callitriche (2.5 μM) and Sparganium (6.0μM) was well below the equilibrium concentration (15 μM). and the low saturation points (250–500 μM) also pointed to efficient use of CO2. Callitriche and Sparganium compete successfully with HCO3− users in hardwater streams, which have a higher exchange and generation capacity of CO2 than stagnant and more soft waters. Rates of photosynthesis of Potamogeton crispus and P. pectinatus decreased at high pH. Depending on the two alternative hypotheses for HCO3use, this decline can be explained by CO3−− inhibition of HCO3− uptake or by increasing capacity to buffer H+efflux from the plant. Habitats subject to high pH, e. g. small ponds with dense vegetation, may have a strong selection for efficient mechanisms of HCO3 use.