Light is crucial for photosynthesis, but the amount of light that exceeds an organism's assimilation efficacy can lead to photo-oxidative damage and even cell death. In Chlamydomonas (C). reinhardtii cyclic electron flow (CEF) is very important for the elicitation of non-photochemical quenching (NPQ) by controlling the acidification of thylakoid lumen. This process requires the cooperation of proton gradient regulation (PGR) proteins, PGRL1 and PGR5. Here, we compared the growth pattern and photosynthetic activity between wild type (137c, t222+) and mutants impaired in CEF (pgrl1 and pgr5) under photoautotrophic and photoheterotrophic conditions. We have observed the discriminative expression of NPQ in the mutants impaired in CEF of pgrl1 and pgr5. The results obtained from the mutants showed reduced cell growth and density, Chl a/b ratio, fluorescence, electron transport rate, and yield of photosystem (PS)II. These mutants have reduced capability to develop a strong NPQ indicating that the role of CEF is very crucial for photoprotection. Moreover, the CEF mutant exhibits increased photosensitivity compared with the wild type. Therefore, we suggest that besides NPQ, the fraction of non-regulated non-photochemical energy loss (NO) also plays a crucial role during high light acclimation despite a low growth rate. This low NPQ rate may be due to less influx of protons coming from the CEF in cases of pgrl1 and pgr5 mutants. These results are discussed in terms of the relative photoprotective benefit, related to the thermal dissipation of excess light in photoautotrophic and photoheterotrophic conditions.
Keywords: Chlamydomonas reinhardtii; Chlorophyll fluorescence; Cyclic electron transport; High light; Non-photochemical quenching; Photosystems.