THE DIFFERENTIAL ABILITY OF AQUATIC PLANTS TO UTILIZE THE INORGANIC CARBON SUPPLY IN FRESH WATERS

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Abstract
SUMMARY: The photosynthetic activity of 15 species of microalgae and macrophytes was measured by continuous recording of oxygen concentration, and/or the change in pH, in bathing solutions of known alkalinity and carbon concentration. The microalgae had considerably greater apparent affinities for HCO3− and slightly greater apparent affinities for CO2, than the macrophytes (including several previously established HCO3‐users), while the macrophytes had larger apparent affinities for CO2 than for HCO3− and larger diffusive resistances to CO2. For species with different affinities for CO2 and HCO3−, assimilation rate in a solution of constant alkalinity showed a distinct reduction when the CO2 concentration decreased to a value typical of each species; a list of such ‘CO2 compensation points’ is presented. (The same CO2 compensation point was obtained for Elodea canadensis by extrapolation of rate‐substrate curves in solutions of which the pH was varied at each of three constant total carbon concentrations). At still lower total carbon values, any assimilation was held to be due to HCO3− uptake, and HCO3− compensation points for some species are given. Rather than ‘users’ and ‘non‐users’ of HCO3− it is concluded that a gradation exists, use depending on HCO3− as affected by alkalinity and pH of the bathing solution, and on the species' HCO3‐compensation point. Natural rates of photosynthesis of macrophytes and some microalgae are usually functions of exogenous [CO2] while those of other microalgae are functions of [CO2] and [HCO3−], with competitive effects at high pH in solutions of prevailingly low alkalinity. In characterizing the response of aquatic plants to inorganic carbon supply, a knowledge of CO2 and HCO3− compensation points is therefore essential.