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.