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

Rates of oxygen production and inorganic carbon uptake by a mixed culture of phytoplankton were determined in a highly variable light regime. Scatter diagrams ofthe 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-I). 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 • it is assumed that the rate of photosynthesis at any time does not depend on light intensities received in the recent past. There is reason to believe that this assumption is not always justified (Harris 1973; Harris and Lott 1973; Harris and Piccinin 1977; Marra 1978a), and extrapolation of the static models is likely to incur errors in the prediction of integrals of production (Marra 1978b). The potential problems arising from the use of static models are of considerable importance in aquatic ecology, since organisms in the natural environment are only rarely, if ever, exposed to constant light for long periods (Harris and Piccinin 1977). In particular, the changes in irradiance due to fluctuations in cloud cover may be quite large, may contain many frequencies, and may be transmitted to considerable depths in the water (Dera and Gordon 1968). In situ incubations are intermediate between static experiments and the natural habitat, because in situ experiments integrate the fluctuations in incident light but ignore the fluctuations received by cells circulating in a vertical light gradient. Several studies have indicated that the I Present address: Marine Ecology Laboratory, instantaneous rate of photosynthesis in a Bedford Institute of Oceanography, Dartmouth, fluctuating light field depends, in part, on Nova Scotia B2Y 4A2. light intensities received in the recent