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Specific rates of photosynthesis at saturating irradiances (PO,J by laboratory populations

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The growth of Volvox globator L. and Volvox aureus Ehr. was measured at five temperatures and nine phosphorus concentrations. Growth rates were hyperbolically related to phosphorus concentrations for all temperatures using a Monod growth model. Optimal growth rates of 1.17 and 1.00 doublings d−1 were obtained at 20°C for V. globator and V. aureus, respectively. Neither species grew at 5°C. The half‐saturation constants for growth, Ks, were lower for V. aureus. Phosphorus uptake by both species was also dependent upon external phosphorus concentrations and temperature. At all temperatures, maximum phosphorus uptake (μmol P colony−1 min−1) was similar for both species; however, the half‐saturation constants for uptake showed significant differences between the species. Comparisons of the kinetic constants for growth and phosphorus uptake suggest that V. aureus will outcompete V. globator under phosphorus limited, conditions.

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