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SUMMARYThe 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 HCO;j" and slightly greater apparent affinities for CO, than the macrophytes (including several previously established HCO3" users), while the macrophytes had larger apparent affinities for CO., than for HCOa" and larger diffusive resistances to COj. For species with different affinities for CO^ and HCO^", assimilation rate in a solution of constant alkalinity showed a distinct reduction when the COj concentration decreased to a value typical of each species; a list of such ' COj compensation points' is presented. (The same COj 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 HCOg" uptake, and ' HCOg" compensation points ' for some species are given. Rather than ' users' and 'non-users' of HCOg" 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' HCOj^-compensation point. Natural rates of photosynthesis of macrophytes and some microalgae are usually functions of exogenous [COj] while those of other microalgae are functions of [COo] and [HCO;,"], 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.

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