Light Harvesting and Utilization by Phytoplankton

Abstract

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.