Influence of the Diadinoxanthin Pool Size on Photoprotection in the Marine Planktonic Diatom Phaeodactylum tricornutum  

Abstract

The pool size of the xanthophyll cycle pigment diadinoxanthin (DD) in the diatom Phaeodactylum tricornutum depends on illumination conditions during culture. Intermittent light caused a doubling of the DD pool without significant change in other pigment contents and photosynthetic parameters, including the photosystem II (PSII) antenna size. On exposure to high-light intensity, extensive de-epoxidation of DD to diatoxanthin (DT) rapidly caused a very strong quenching of the maximum chlorophyll fluorescence yield (F m , PSII reaction centers closed), which was fully reversed in the dark. The non-photochemical quenching of the minimum fluorescence yield (F o , PSII centers open) decreased the quantum efficiency of PSII proportionally. For both F m and F o , the non-photochemical quenching expressed as F/FЈ Ϫ 1 (with FЈ the quenched level) was proportional to the DT concentration. However, the quenching of F o relative to that of F m was much stronger than random quenching in a homogeneous antenna could explain, showing that the rate of photochemical excitation trapping was limited by energy transfer to the reaction center rather than by charge separation. The cells can increase not only the amount of DT they can produce, but also its efficiency in competing with the PSII reaction center for excitation. The combined effect allowed intermittent light grown cells to down-regulate PSII by 90% and virtually eliminated photoinhibition by saturating light. The unusually rapid and effective photoprotection by the xanthophyll cycle in diatoms may help to explain their dominance in turbulent waters.Photosynthetic organisms have developed strategies to optimize light harvesting at low intensities while minimizing photoinhibitory damage due to excess energy at high-light intensity. From hours onwards, they regulate the quantity and composition of the light-harvesting complexes (LHCs) and of a number of other components of their photosynthetic machinery (for review, see Falkowski and Laroche, 1991; Anderson et al., 1995). On shorter time scales, they react to an unbalance between light intensity and photosynthetic capacity (e.g. due to a change in light intensity, temperature, or nutrient supply) by rapid structural modifications within the LHC of PSII (Horton et al., 1996; Bassi and Caffarri, 2000). These modifications lead to a decrease in the fluorescence yield designated as the non-photochemical quenching (NPQ) of chlorophyll (Chl) a fluorescence. NPQ is supposed to dissipate excess excitation energy through a harmless non-radiative pathway. The partitioning of absorbed energy between transfer to the reaction center and photoprotective non-radiative dissipation is controlled by the trans-thylakoid pH gradient (for recent review, see Mü ller et al., 2001) and by the reversible conversion of epoxidized to de-epoxidized forms of xanthophylls (the so-called xanthophyll cycle; Gilmore, 1997). The molecular mechanisms of photoprotection have mostly been studied in higher plants (Demmig-Adams and Adams, 1996). Several mutants ...

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

“…These experiments indicated that PSII electron transport might over-estimate the primary production under some conditions, because oxygen evolution rates were found to be lower than PSII electron flow (Gilbert et al 2000). Several explanations for this disparity have been suggested: (a) Cyclic electron flow around PSII (Prasil et al 1996;Lavaud et al 2002) (b) Water-water cycle (Asada 1999) where oxygen uptake on the acceptor side of PSI leads to superoxide which is then dismutated to H 2 O 2 and then detoxified to water and (c) Cyclic electron flow around PSI (Bendall and Manesse 1996) These processes can be summarised as alternative electron cycling (AEC) which are not energetic losses (such as non-photochemical quenching: NPQ), because at least the water-water cycle and the cyclic flow around PSI generate a proton gradient which can be used for additional ATP synthesis. Therefore, it is suggested that alternative electron cycling is a normal stress response and might be of less importance under balanced growth conditions.…”

“…3a). For that reason, it is calculated as (F m -F¢ m )/F¢ m (or F o -F¢ o /F¢ o ; Lavaud et al 2002). In higher plants, green algae and dinoflagellates, where the NPQ mechanism has been investigated, it consists of three components ( Fig.…”

Nonphotochemical Quenching of Chlorophyll Fluorescence inChlamydomonas reinhardtii

“…On the other hand, a wide variability of the capacity for nonphotochemical quenching is commonly seen in photosynthetic organisms. When calculated according to Bilger and Björkman This value increases to 0.5 in cyanobacteria (22) and up to 12 in diatoms (23). In addition, it appears that not only the amplitude but also the nature of the NPQ response can vary between organisms, even within the two model organisms mentioned above.…”

An atypical member of the light-harvesting complex stress-related protein family modulates diatom responses to light

“…For example, they show an outstanding capacity to cope with light stress. Their ability to dissipate excess energy in high light can surpass that of plants, as witnessed by the impressive levels of nonphotochemical quenching (NPQ) of chlorophyll fluorescence observed in the model diatom Phaeodactylum tricornutum in some conditions (7). The term NPQ describes the enhancement of thermal dissipation of absorbed energy that occurs in the pigment-containing proteins of photosystem (PS) II, whenever light absorption exceeds the maximum rate of CO 2 assimilation.…”