Nonphotochemical Quenching of Chlorophyll Fluorescence inChlamydomonas reinhardtii

Abstract

Unlike plants, Chlamydomonas reinhardtii shows a restricted ability to develop nonphotochemical quenching upon illumination. Most of this limited quenching is due to state transitions instead of ∆pH-driven high-energy state quenching, qE. The latter could only be observed when the ability of the cells to perform photosynthesis was impaired, either by lowering temperature to ∼0°C or in mutants lacking RubisCO activity. Two main features were identified that account for the low level of qE in Chlamydomonas. On one hand, the electrochemical proton gradient generated upon illumination is apparently not sufficient to promote fluorescence quenching. On the other hand, the capacity to transduce the presence of a ∆pH into a quenching response is also intrinsically decreased in this alga, when compared to plants. The possible mechanism leading to these differences is discussed.The absorption of light in excess of the capacity for photosynthetic electron transport is damaging to photosynthetic organisms (1). That capacity is, however, variable, depending mainly on the efficiency of CO 2 assimilation. For example, decreases in temperature lower the rate with which electrons can be transported within the electron transport chain as well as the capacity of metabolic processes to assimilate the reductant that has been photoproduced. In higher plants, the availability of CO 2 is liable to be limiting under conditions of water stress, due to the closure of stomata. This will again inhibit photosynthetic assimilation. Under such conditions, cells are likely to experience oxidative stress, due to the uncontrolled formation of reactive oxygen species associated with the absorption of light by chlorophyll.Under conditions of excess light, a variety of mechanisms are initiated which protect chloroplasts from damage. In higher plants, a number of processes have been identified, primarily through application of measurements of chlorophyll fluorescence yield. These are all associated with photosystem (PS) 1 II and are collectively referred to as nonphotochemical quenching mechanisms (NPQ; 1). However, this term comprises at least three processes: (i) qI, mainly related to photoinhibition, a slowly reversible damage to PSII reaction centers (2, 3), although data suggest that a zeaxanthindependent quenching might contribute substantially to this process (4, 5), (ii) qT, state transitions, a change in the relative antenna sizes of PSII and PSI, due to the reversible phosphorylation and migration of antenna proteins (LHCII) (6), and (iii) qE, also termed "high-energy state quenching", a form of quenching associated with the development of a low pH in the thylakoid lumen (e.g., ref 7).High-energy state quenching is largely thought to be associated with an increase in thermal dissipation within the light-harvesting apparatus (1,8,9), associated with the generation of a ∆pH (7, 10) and with the formation of zeaxanthin via deepoxidation of violaxanthin (11). However, in vitro at least, this term also encompasses acidic pHinduced proc...

Light-induced Dissociation of an Antenna Hetero-oligomer Is Needed for Non-photochemical Quenching Induction

“…Algae and plants also differ as to their PsbS content. In fact, although the gene is present in several algal genomes, the corresponding polypeptide is not accumulated in any algal species analyzed thus far (40,41). Furthermore, in algae, only C 2 S 2 supercomplexes have been found, while larger supercomplexes, such as those of land plants, have never been observed (47).…”

Molecular Remodeling of Photosystem II during State Transitions inChlamydomonas reinhardtii

“…This implies that the phosphorylated (dissociated) LhcbM1 and -2 may, in fact, be involved in q E quenching in addition to their roles in state transitions. Considering that C. reinhardtii displays little to no expression of the essential protein for q E quenching, PsbS (Niyogi et al, 2005), and only has a limited capacity of q E quenching when grown under normal light (Finazzi et al, 2006), the underlying mechanism for q E quenching in this green alga is not necessarily the same as that elucidated in higher plants (see reviews in Horton and Ruban, 2005;Niyogi et al, 2005). Further investigation on the significance of phosphorylation of LhcbM1 and -2 will clarify this point.…”

Fluorescence as a Tool to Understand Changes in Photosynthetic Electron Flow Regulation

“…3b) can be extremely divergent between taxa, especially among microalgal groups (Casper-Lindley and Bjorkman 1998;Hill et al 2005;Juneau and Harrison 2005), and even between species within a taxonomic group Lavaud et al 2007). For example, qE shows high amplitude and fast onset in diatoms and brown macroalgae, while being of minor importance in most of the green microalgae (Finazzi et al 2006) and cyanobacteria (Kirilovsky 2007). Nevertheless, within the diatoms (see Chapter 7) as well as higher plants (Johnson et al 1993) there are clear differences in qE amplitude that have been highlighted.…”