The role of growth rate, redox-state of the plastoquinone pool and the trans-thylakoid Delta pH in photoacclimation of Chlorella vulgaris to growth irradiance and temperature

The long-term photoacclimation of Chlorella vulgaris Beijer (UTEX 265) to g rowth irradiance and growth temperature under ambient CO2 conditions was ex amined. While cultures grew at a faster rate at 27 than at 5 degreesC, grow th rates appeared to be independent of irradiance. Decreases in light-harve sting polypeptide accumulation, increases in xanthophyll pool size and chan ges in the epoxidation state of the xanthophyll cycle pigments were correla ted linearly with increases in the relative reduction state of QA, the prim ary quinone receptor of photosystem II, when estimated as 1-qp under steady -state growth conditions. However, we show that there is also a specific te mperature-dependent component, in addition to the redox-state of the QA, in volved in regulating the content and composition of light-harvesting comple x II of C. vulgaris. In contrast, modulation of the epoxidation state of th e xanthophyll pool in response to increased 1-qp in cells grown at 5 OC was indistinguishable from that of cells grown at 27 degreesC, indicating that light and temperature interact in a similar way to regulate xanthophyll cy cle activity in C. vulgaris. Because C. vulgaris exhibited a low-light phen otype in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), bu t a high-light phenotype upon addition of 2,5-dibromo-6-isopropyl-3-methyl- 1,4-benzoquinone, we conclude that the plastoquinone pool acts as a sensor regulating the accumulation of light-harvesting polypeptides in C. vulgaris . However, concomitant measurements of non-photochemical fluorescence quenc hing (q(N)) and the epoxidation state of the xanthophyll pool appear to ind icate that, in addition to the redox-state of the plastoquinone pool, the t rans-thylakoid Delta pH may also contribute to sensing changes in irradianc e and temperature that would lead to over-excitation of the photosynthetic apparatus. We suggest that sink capacity as reflected in photosynthate util ization and cell growth ultimately regulate photoacclimation in C. vulgaris .