Photosynthesis—light relationships of a mixed culture of phytoplankton in fluctuating light

Abstract

Rates of oxygen production and inorganic carbon uptake by a mixed culture of phytoplankton were determined in a highly variable light regime. Scatter diagrams of the results revealed a smoothly varying relationship between irradiance and photosynthetic oxygen release except when the rate of oxygen release was depressed by CO2 depletion. There was no indication that the instantaneous rate of oxygen production depended on light intensities received in the recent (5–30 min) past. However, the net rate of inorganic carbon uptake exhibited large, transient fluctuations in response to rapid changes in light as CO2 became depleted (<0.001 mg·liter−1). Spectral analysis showed that the transient fluctuations in inorganic carbon exchange were related to the rate of change in irradiance. Enhanced transport of inorganic carbon into the cells at high pH may account for the highest rates of carbon uptake; diffusional loss of intracellular CO2, photorespiration, or both may account for the highest rates of carbon release. Models derived from incubations at constant light intensities cannot account for the observed fluctuations in inorganic carbon uptake, indicating the importance of light variability, especially in systems that become depleted of CO2.