Sunlight, the ultimate energy source for life on our planet, enters the biosphere as a direct consequence of the evolution of photoautotrophy. Photoautotrophs must balance the light energy absorbed and trapped through extremely fast, temperature-insensitive photochemistry with energy consumed through much slower, temperature-dependent biochemistry and metabolism. The attainment of such a balance in cellular energy flow between chloroplasts, mitochondria and the cytosol is called photostasis. Photoautotrophs sense cellular energy imbalances through modulation of excitation pressure which is a measure of the relative redox state of QA, the first stable quinone electron acceptor of photosystem II reaction centers. High excitation pressure constitutes a potential stress condition that can be caused either by exposure to an irradiance that exceeds the capacity of C, N, and S assimilation to utilize the electrons generated from the absorbed energy or by low temperature or any stress that decreases the capacity of the metabolic pathways downstream of photochemistry to utilize photosynthetically generated reductants. The similarities and differences in the phenotypic responses between cyanobacteria, green algae, crop plants, and variegation mutants of Arabidopsis thaliana as a function of cold acclimation and photoacclimation are reconciled in terms of differential responses to excitation pressure and the predisposition of photoautotrophs to maintain photostasis. The various acclimation strategies associated with green algae and cyanobacteria versus winter cereals and A. thaliana are discussed in terms of retrograde regulation and the “grand design of photosynthesis” originally proposed by Arnon (1982).

No abstract

Chlorella vulgaris acclimated to high light (HL) conditions exhibited a pale-green phenotype characterized by reduced chlorophyll and light harvesting polypeptide abundance compared with the dark green phenotype of the control, low-light-grown (LL) cultures. We hypothesized that if chloroplast redox status was the sole regulator of phenotype, exposure to darkness should cause reversion of the HL to LL phenotype. Surprisingly, HL cells transferred to darkness or dim light failed to green. Thus, phenotypic reversion is light-dependent with an optimal photon flux density (PFD) of 110 μmol photons·m−2·s−1. HL cells shifted to this PFD exhibited increased chlorophyll and light harvesting polypeptide abundance, which were inhibited by 2,5-dibromo-3-methyl-6-isopropyl-benzoquinone but not by 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea. We conclude that photoacclimation of HL-grown cells to LL is governed by the redox state of the intersystem photosynthetic electron transport chain (PETC) at this PFD. At lower light levels, cells maintained the HL phenotype, despite an oxidized status of the PETC. Because 110 μmol photons·m−2·s−1 was the optimal PFD for protochlorophyllide oxidoreductase accumulation, we suggest that stabilization of light-harvesting polypeptides by chlorophyll binding may also govern photoacclimation in C. vulgaris. The possible role of the metabolic balance between respiration and photosynthesis is also discussed.

The green alga Chlorella vulgaris Beij. exhibits minimal capacity to adjust exponential growth rates in response to photon flux density (PFD) when monitored on a discontinuous basis. We hypothesized that modulation of maximum growth rates in C. vulgaris by PFD is a photoperiod-dependent phenomenon. The use of the photobioreactors to monitor continuous growth allowed us to detect repetitive daily oscillations in growth which were photoperiod-dependent. The rate of change in optical density (OD735) during the daily light period was two-fold greater in cells grown at 28 °C with a PFD of either 2000 or 150 μmol photons·m−2·s−1 when C. vulgaris was grown under a daily light–dark cycle. Concomitantly, oscillations of the chlorophyll fluorescence parameters paralleled the oscillations observed in growth rate. When cultures were shifted from a 12 h photoperiod with low light to continuous light (CL), the growth oscillations disappeared. In contrast, oscillations in the fluorescence parameters persisted even after the shift from a 12 h photoperiod to CL. We suggest that the nocturnal catabolism of starch reserves in conjunction with changes in cellular volume coupled with the diurnal changes in DNA content, as quantified by changes in Vybrant Green fluorescence yield, indicate that these growth oscillations reflect synchronized cellular division in C. vulgaris that is not evident when growth is assayed discontinuously.