Towards understanding the regulation of photosynthesis under natural environments: Analyses for the behavior of photoacclimation processes in the mutant strains of a green alga Chlamydomonas reinhardtii under outdoor conditions

Most living organisms usually adapt to environmental conditions to survive in their natural habitats. To keep efficient photochemical reactions even in dramatically changing light, photosynthetic organisms optimize their photosynthetic apparatus including light-harvesting systems. Among the chlorophyll protein complexes that involved in photosynthetic electron transport, photosystem II (PSII), the oxygen-evolving chlorophyll protein complex, easily undergoes photodamage by a strong light, especially on the reaction center protein D1. The damaged Dl is rapidly replaced with newly synthesized one (Damage-repair cycle), and thus the net PSII activity is determined by the rate balance between damage and repair. To avoid highly exceeded damage on photosystems, several photoacclimation processes have been developed. NPQ (non-photochemical quenching), more precisely qE quenching (energy-dependent quenching), is one of the thermal dissipation mechanisms against excess light energy. On the other hand, frequent changes in light quality and intensity are also stressful for photosynthetic organisms. State transitions are considered as the acclimation processes against those kinds of conditions, in which light energy captured by light-harvesting proteins is re-distributed between two photosystems. The analyses using the mutant strains of a green alga Chlamydomonas reinhardtii and a model plant Arabidopsis thaliana have shown the molecular mechanisms and physiological roles of both NPQ and state transitions in laboratory conditions. However, actual behavior of these photoacclimation processes in natural environments is poorly understood. This review describes the behaviors of photosynthetic apparatus and molecular mechanisms of NPQ and state transitions in laboratory conditions. Moreover, photobioreactor, an outdoor conditions-replaying system, is also introduced as one of the ways to connect the "lab" and "natural" photosynthesis.