Jean-Claude Duval

INFLUENCE OF THE POOL SIZE OF THE XANTHOPHYLL CYCLE ON THE EFFECTS OF LIGHT STRESS IN A DIATOM: COMPETITION BETWEEN PHOTOPROTECI'ION AND PHOTOINHIBITION

Abstract In a study of the relationship between nonphotochemical quenching of fluorescence and the xanthophyll cycle, we show that the diatom Phaeodactylum tricornutum exhibits several interesting characteristics. This xanthophyll cycle consists of only one reversible epoxidating/deepoxidating step (diadinoxanthiddiatoxanthin). Diadinoxan‐thin, which increases from 8 to 17 molecules/100 chlorophyll a (Chl a) during the ageing of the culture, was present as two separate pools, with a portion (of about 5 molecules/100 Chl a) which was never deepoxidated. Under a defined irradiance, the time necessary to abolish net photosynthesis increases with the pool size of diadinoxanthin available for deepoxidation. A close correlation is found between nonphotochemical quenching and the relative ratio of diatoxanthin until the photosytem II center is inactivated. The photoprotective effect of diadinoxanthin deepoxidation is limited to the phase during which quenching of the minimum fluorescence (F0) develops.

Changes in yield ofin-vivo fluorescence of chlorophyll a as a tool for selective herbicide monitoring

Triazines and derivatives of phenylurea, which are often found in outdoor water samples, induce specific changes in the yield of thein-vivo chlorophyll α-fluorescence of PSII. These changes are correlated quantitatively with the concentration of the herbicides and can therefore be used to set-up a low-price monitor system. In order to detect selectively the herbicide-sensitive part of the fluorescence emission a pulse amplitude modulated fluorimeter was used. The bioassay system was optimised with respect to test organism, growing and measuring conditions. The relationship between fluorescence yield and herbicide concentrations were experimentally determined for the triazines atrazine and simazine and the phenylurea herbicide DCMU and mathematically fitted (r=0.99). The I50-values were 0.9 µM for DCMU, 2.2 µM for simazine and 3.3 µM for atrazine. The detection limit of about 0.5 µM clearly shows that the sensitivity of this bioassay system is too low to reach the requirements of the drinking water regulation. However, due to its insensitivity against complex water matrices, there is good hope to combine this fluorometric bioassay with a potent herbicide preconcentration method like a solid-phase extraction procedure.

Effects of high light and desiccation on the operation of the xanthophyll cycle in two marine brown algae

Two brown algae, Pelvetia canaliculata and Laminaria saccharina, from the higher and lower mediolittoral belts respectively, have been tested for their capacity to overcome high-light stress in water and in air (in both fully hydrated and desiccated states). When exposed to supersaturating light irradiance in water, the two species developed non-photochemical quenching of fluorescence (NPQ) which was correlated with an increase in the de-epoxidation ratio (DR) of the xanthophyll cycle carotenoids (violaxanthin, antheraxanthin and zeaxanthin) and was followed by a slower decrease in oxygen evolution. NPQ reached values of up to 9 in P. canaliculata but only 4·5 in L. saccharina, at DRs of 0·65 and 0·5, respectively. In air, the xanthophyll cycle was also operative but the efficiency of de-epoxidation decreased linearly with the degree of hydration of the thallus. Photoprotection capacities in air also appeared higher in P. canaliculata than in L. saccharina, probably due to the higher molar content of the ...

Chlorophyll fluorescence transients from the diatom Phaeodactylum tricornutum: relative rates of cyclic phosphorylation and chlororespiration

In Phaeodactylum tricornutum cells kept 30 min in the dark, induction of fluorescence showed the well-known levels OIDPSMT. The decrease of MT was the most important when the intensity of excitation light was high. It was mainly due to the photochemical quenching. After addition of DCMU (2 to 20 μM), a quenching qE was still observed: this quenching, cancelled by NH4Cl (2 to 20 mM) is attributed to ΔpH. This qE was also inhibited by antimycin, an inhibitor of cyclic phosphorylation and may be of chlororespiration above plastoquinones. Anaerobiosis also decreased it. We can infer that chlororespiration also plays a part in the formation of the ΔpH in the presence of DCMU. After 30 mn of preillumination in red light, the levels P and M were lower and the quenching in presence of DCMU was no more observed: thus, neither the chlororespiration nor the cyclic phosphorylation were active, unless the activity of ATPase was much more important. So, in diatoms, one at least of the above cited phenomena can be modulated by light.

FLUORESCENCE INDUCTION KINETICS AS A TOOL TO DETECT A CHLORORESPIRATORY ACTIVITY IN THE PRASINOPHYCEAN ALGA, MANTONIELLA SQUAMATA

Abstract Pulse-modulated fluorescence measuring systems were used to study the fluorescence induction kinetics of the primitive prasinophycean alga, Mantoniella squamata . DCMU-treated cells show a very strong non-photochemical quenching which is insensitive to uncouplers and can not be attributed to state transitions or photoinhibition. In order to analyze the origin for this quenching, different inhibitors of the photosynthetic electron flow were applied. It was found that DCCD, an inhibitor of the ATP-synthase as well as of the cytochrome- c oxidase, enhances the fluorescence quenching, whereas antimycin A, which blocks the cyclic electron flow around PS I, stimulates it. The effect of both inhibitors can be abolished only under the condition of anaerobiosis, when all components of the electron transport chain were reduced. As a consequence of this, we postulate that this non-photochemical quenching is attributed to the redox state of the plastoquinone pool. In DCMU-treated cells it becomes oxidized under saturating light intensities, but will be reduced by the activity of an electron influx from NAD(P)H. This electron flow is considered to create a pH gradient in the dark, which is probably used for ATP synthesis under dark and extremely low incident light conditions. This chlororespiration requires the existence of an intrathylakoid bound cytochrome- c oxidase, which may be a conserved phylogenetic relict in primitive chloroplasts.

A new fluorometric device to measure the in vivo chlorophyll a fluorescence yield in microalgae and its use as a herbicide monitor

A new fluorometric device was developed to measure the fluorescence yield of microalgae in order to detect photosystem II specific herbicides in outdoor water samples. The system was tested with respect to sensitivity and reliability with two species: Chlorella fusca (a green alga) and Phaeodactylum tricornutum (a diatom). The main advantages are the use of cell cultures without preliminary centrifugation, the evaluation of variable fluorescence from steady-state levels, and the rapidity of the measurement. Concentrations around the binding constant of inhibitors (around 10-8 M) can be detected for dichlorophenyldimethylurea, atrazine and simazine. The new system was compared with the commercially available pulse amplitude fluorometer PAM 101; it was shown that the sensitivity was about 10 times greater than with PAM 101, working at optimal chlorophyll concentrations. The advantages and limits of both systems when applied to free-water samples are discussed.

Comparison of state 1-state 2 transitions in the green alga Chlamydomonas reinhardtii and in the red alga Rhodella violacea: effect of kinase and phosphatase inhibitors

Abstract In the green alga Chlamydomonas reinhardtii , state transitions occur upon phosphorylation of the light-harvesting complex. The protein kinase inhibitor staurosporine, and the phosphoprotein phosphatase inhibitor NaF, suppress state 2 and state 1 transitions, respectively. By contrast, in the red alga Rhodella violacea none of these inhibitors has any effect, suggesting that, in red algae, the mechanisms of redistribution of excitation energy are independent of protein phosphorylation.

77 K fluorescence quenching induced by reduction of Photosystem I primary electron acceptors in a cyanobacterium

A Pseudanabaena strain isolated in our laboratory exhibited a large amount of a 710 nm chlorophyll form, associated with Photosystem I, the chromophore of which was found to be parallel to the membrane plane as shown by linear dichroism measurements. Whole cells, thylakoid vesicles and Photosystem I particles obtained after digitonin treatment presented a particular 77 K fluorescent component at 750 nm (F750):a high initial level of fluorescence (F0) was obtained when samples were frozen in the dark; illumination induced a decrease to a low level (FL). Kinetic analysis showed that this decrease was biphasic, with a first phase 3-times faster than the second one. F0FL ratio was about 3. Action spectra demonstrate the origin and properties of chlorophyll 710. By poising Photosystem I electron acceptors at different oxidoreduction states, and from experiments with ferricyanide, we conclude that the fluorescence at 750 nm originates from a chlorophyll form absorbing at 710 nm in a close relation to P-700 and that its kinetics can be used to evaluate Photosystem I primary electron acceptor pools.

Correlated influence of cation concentration and excitation intensity on PS II activity-II. Comparative study between green plant and brown-alga chloroplasts

AbstractA preparation of photochemically active chloroplasts of Fucus was added to a low-salt medium with high osmolarity (HEPES AMPD buffer, 1M sorbitol). The rate of DCIP reduction (DCIPr) and the variable fluorescence (Fv) of these phaeoplasts were measured and compared with the same activities in spinach chloroplasts. A study of the influence of mono- and divalent-cations showed that salt effects on PS II activity also exist in Fucus. (i)Mg++ action on Fv is similar, although noticeably weaker in Fucus than in spinach chloroplasts.(ii)Na+ has no effect on Fv of Fucus chloroplasts, but its influence on DCIPr is more pronounced than in spinach.(iii)Mg++ influence on DCIPr is largely dependent upon excitation energy. In subsaturating light (100\2-1000 W m\t-2), Mg++ stimulation increases up to 100 mM, almost doubling the level. In very low wight conditions (3Wm\t02), however, this stimulation saturates at about 10 mM; higher concentrations are no longer effective but do not quench DCIPr noticeably, unlike the case in spinach. Therefore, cations act through similar pathways on Fucus as on spinach isolated chloroplasts but the effects on PS II centers are preponderant in Fucus whereas the modifications in non-radiative decay or pigment array size are weaker.

Kinetics of a Chlorophyll a Long Wavelength Fluorescing form: Quenching of F 750 in Relation to PS I Oxido-Reduction State in a Cyanobacterium (Pseudanabaena SP.)

Cyanobacteria synthetize only onetype of chlorophyll molecule, chlorophyll a, besides accessory pigments, carotenoids and phycobiliproteins. The major part of Chl a (80 %) is associated with PS I. Electrophoresis of chl a-protein complexes from thylakold pigments revealed heavy complexes (in the 100 KD to 300 KD range) which are related to PS I and are enriched in long wavelength Chl a absorbing forms. Fluorescence emission at 77 K above 700 nm largely originated from these comolexes (Oauist et al.1981). However, significant differences seemed to exist for these forms among complexes isolated from various strains (Thomas, Mousseau 1981). Emission fluorescence spectrum at 77 K of whole cells of a Pseudanabaena strain exhibited a shoulder in the 750 nm range. This component was not observed in spectra obtained from LiDS-PAGE chlorophyll-protein complexes. Our work deals with occurrence and some properties of the F 750 component.