Photoacclimation of Light Harvesting Systems in Eukaryotic Algae

Abstract

Photoacclimation is a suite of phenotypically expressed, developmentally independent, reversible physiological feedback responses to short-term (minutes to days) variations in spectral irradiance. These responses are observed in all eukaryotic algal taxa and involve alterations in the optical absorption cross section, the effective absorption cross section, and the rate of electron transfer from water to a terminal acceptor (e.g., carbon dioxide or nitrate). In this chapter, we review the primary physical processes in aquatic ecosystems that provided selection pressure for photoacclimation responses. These processes include the passage of clouds across the sky, vertical mixing, and diel variability in incident solar irradiance. The physiological responses to variations in the spectral irradiance are transduced via the redox state of intersystem electron transport components, especially plastoquinone. In a ‘nested’ series, responses include state transitions, alterations in the xanthophylls, and net synthesis/degradation of light harvesting complexes. The three processes have different time constants and dynamic ranges, but all result in alterations of the effective absorption cross section of photochemistry, such that light absorption and electron transport are balanced. The balance between light absorption and electron transport optimizes (not maximizes) photosynthesis under a very wide range of light conditions found in natural aquatic ecosystems.