Light Harvesting and Utilization by Phytoplankton

In this study we use a model based on target theory to analyze steady-state photosynthesis-irradiance relationships in continuous light. From the average turnover time (τ) of photosynthetic units (PSUO₂), numerical analyses of the model coefficients, and measurements of the light field and cell absorptivity, apparent absorption crosssections of photosystem II ($σ¯$PSII) were determined for three species of marine unicellular algae grown at different irradiance levels. These cross-sections generally, but not always, increased with decreased growth irradiance. Additionally, the ratios of photosystem I/photosystem II reaction centers were calculated from measurements of oxygen flash yields and chlorophyll/P₇₀₀ ratios. From the ratios of the reaction centers, cell absorptivity and the apparent absorption cross-section of photosystem II, the apparent absorption cross-sections of photosystem I ($σ¯$PSI) were also calculated. Finally, on the basis of our calculated absorption cross-sections, we estimated the minimum quantum requirements for O₂ evolution. Our results suggest that the absorption cross-sections of PS I and PS II vary independently and the minimum quantum requirements for O₂ vary by more than twofold, increasing from 9.1 to 20.6 quanta/O₂, as growth irradiance increases. The increase in quantum requirement corresponds to larger apparent cross-sections for photosystem I and higher carotenoid/chlorophyll ratios.