Relationship of rapid light curves of variable fluorescence to photoacclimation and non-photochemical quenching in a benthic diatom

The response of rapid light–response curves (RLCs) of variable fluorescence to changes in short- and long-term photoacclimation status was studied in an estuarine benthic diatom. The diatom Nitzschia palea was grown under low- (LL, 20 μmol m⁻² s⁻¹) and high-light (HL, 400 μmol m⁻² s⁻¹) conditions, with the purpose of characterising the effects of long-term photoacclimation on (i) steady-state light–response curves (LC) of relative electron transport rate, rETR, (ii) the response of RLCs to changes in ambient irradiance (E, the irradiance to which the sample is acclimated to immediately before the RLCs), (iii) the relationship of RLCs to LC parameters and non-photochemical quenching (NPQ). Photoacclimation to LL and HL conditions induced distinct light–response patterns of rETR and NPQ. Higher growth light resulted in rETR vs. E curves with lower initial slopes (α, 0.591 μmol⁻¹ m² s vs. 0.661 μmol⁻¹ m² s, for HL and LL, respectively) and markedly higher maximum rates (rETR_m, 95.9 vs. 29.3), reached under higher E levels (higher light-saturation coefficient, E_k: 162.4 μmol m⁻² s⁻¹ vs. 44.3 μmol m⁻² s⁻¹). Acclimation to HL induced bi-phasic NPQ vs. E curves, with minimum values reached under low E levels (15–25 μmol m⁻² s⁻¹) and not on dark-acclimated samples. The response of RLCs to changes in ambient irradiance varied with the long-term photoacclimation status of the samples. The initial slope, α_RLC, decreased monotonically with E in LL cultures, from 0.68 to 0.25 μmol⁻¹ m² s, while varied bi-phasically in HL-acclimated samples. Typically, α_RLC of HL cultures increased under low E, reaching a maximum of 0.61 μmol⁻¹ m² s under 25–55 μmol m⁻² s⁻¹, and decreased gradually under higher E levels to 0.25 μmol⁻¹ m² s. RLC maximum rETR, rETR_m,RLC, and saturation coefficient E_k,RLC, increased with E following a saturation-like pattern, with the HL cultures presenting markedly higher values for all the E range (maximum rETR_m,RLC values were 108.6 and 33.4 for HL and LL cultures, respectively). An inverse relationship was consistently found between α_RLC and NPQ, both on LL and HL cultures, causing strong correlations (P < 0.001 in all cases) between NPQ and the high light-induced decrease of α_RLC, Δα_RLC. RLCs were confirmed to also provide information on the long-term photoacclimation status, as significant correlations (P < 0.001 both for HL and LL cultures) were verified between E_k and an index based on RLC parameters, Ê_k, both for LL and HL cultures. These results reinforce the usefulness of RLCs as a tool for inferring on the short- and long-term photoacclimation status of samples with different long-term light histories, through the estimation of LC parameters and the monitoring of NPQ levels.     

Chlorophyll fluorescence in the leaves of Tradescantia species of different ecological groups: Induction events at different intensities of actinic light

Chlorophyll fluorescence analysis is one of the most convenient and widespread techniques used to monitor photosynthesis performance in plants. In this work, after a brief overview of the mechanisms of regulation of photosynthetic electron transport and protection of photosynthetic apparatus against photodamage, we describe results of our study of the effects of actinic light intensity on photosynthetic performance in Tradescantia species of different ecological groups. Using the chlorophyll fluorescence as a probe of photosynthetic activity, we have found that the shade-tolerant species Tradescantia fluminensis shows a higher sensitivity to short-term illumination (≤20 min) with low and moderate light (≤200 μE m⁻² s⁻¹) as compared with the light-resistant species Tradescantia sillamontana. In T. fluminensis, non-photochemical quenching of chlorophyll fluorescence (NPQ) and photosystem II operational efficiency (parameter Φ_PSII) saturate as soon as actinic light reaches ≈200 μE m⁻² s⁻¹. Otherwise, T. sillamontana revealed a higher capacity for NPQ at strong light (≥800 μE m⁻² s⁻¹). The post-illumination adaptation of shade-tolerant plants occurs slower than in the light-resistant species. The data obtained are discussed in terms of reactivity of photosynthetic apparatus to short-term variations of the environment light.     

Independent fluctuations of malate and citrate in the CAM species Clusia hilariana Schltdl. under low light and high light in relation to photoprotection

Clusia hilariana Schltdl. is described in literature as an obligate Crassulacean acid metabolism (CAM) species. In the present study we assessed the effect of irradiance with low light (LL, 200 μmol m⁻² s⁻¹) and high light (HL, 650–740 μmol m⁻² s⁻¹), on the interdependency of citrate and malate diurnal fluctuations. In plants grown at HL CAM-type oscillations of concentration of citrate and malate were obvious. However, at LL daily courses of both acids do not seem to indicate efficient utilization of these compounds as CO₂ and NADPH sources. One week after transferring plants from LL to HL decarboxylation of malate was accelerated. Thus, in the CAM plant C. hilariana two independent rhythms of accumulation and decarboxylation of malate and citrate take place, which appear to be related to photosynthesis and respiration, respectively. Non photochemical quenching (NPQ) of photosystem II, especially well expressed during the evening hours was enhanced. Exposure to HL for 7 d activated oxidative stress protection mechanisms such as the interconversion of violaxanthin (V), antheraxanthin (A) and zeaxanthin (Z) (epoxydation/de-epoxydation) measured as epoxydation state (EPS). This was accompanied by a slight increase in the total amount of these pigments. However, all these changes were not observed in plants exposed to HL for only 2 d. Besides violaxanthin cycle components also lutein, which shows a small, but not significant increase, may be involved in dissipating excess light energy in C. hilariana.     

Ultrafast fluorescence study on the location and mechanism of non-photochemical quenching in diatoms

The diatom algae, responsible for at least a quarter of the global photosynthetic carbon assimilation in the oceans, are capable of switching on rapid and efficient photoprotection, which helps them cope with the large fluctuations of light intensity in the moving waters. The enhanced dissipation of excess excitation energy becomes visible as non-photochemical quenching (NPQ) of chlorophyll a fluorescence. Intact cells of the diatoms Cyclotella meneghiniana and Phaeodactylum tricornutum, which show different NPQ induction kinetics under high light illumination, were investigated by picosecond time-resolved fluorescence under dark and NPQ-inducing high light conditions. The fluorescence kinetics revealed that there are two independent sites responsible for NPQ. The first quenching site is located in an FCP antenna system that is functionally detached from both photosystems, while the second quenching site is located in the PSII-attached antenna. Notwithstanding their different npq induction and reversal kinetics, both diatoms showed identical NPQ via both mechanisms in the steady-state. Their fluorescence decays in the dark-adapted states were different, however. A detailed quenching model is proposed for NPQ in diatoms.     

Luminostat operation: a tool to maximize microalgae photosynthetic efficiency in photobioreactors during the daily light cycle?

The luminostat regime has been proposed as a way to maximize light absorption and thus to increase the microalgae photosynthetic efficiency within photobioreactors. In this study, simulated outdoor light conditions were applied to a lab-scale photobioreactor in order to evaluate the luminostat control under varying light conditions. The photon flux density leaving the reactor (PFD_out) was varied from 4 to 20 μmol photons m⁻² s⁻¹and the productivity and photosynthetic efficiency of Chlorella sorokiniana were assessed.Maximal volumetric productivity (1.22 g kg⁻¹ d⁻¹) and biomass yield on PAR photons (400-700 nm) absorbed (1.27 g mol⁻¹) were found when PFD_out was maintained between 4 and 6 μmol photons m⁻² s⁻¹. The resultant photosynthetic efficiency was comparable to that already reported in a chemostat-controlled reactor. A strict luminostat regime could not be maintained under varying light conditions. Further modifications to the luminostat control are required before application under outdoor conditions.     

The time course of non-photochemical quenching in phycobilisomes of Synechocystis sp. PCC6803 as revealed by picosecond time-resolved fluorimetry

As high-intensity solar radiation can lead to extensive damage of the photosynthetic apparatus, cyanobacteria have developed various protection mechanisms to reduce the effective excitation energy transfer (EET) from the antenna complexes to the reaction center. One of them is non-photochemical quenching (NPQ) of the phycobilisome (PB) fluorescence. In Synechocystis sp. PCC6803 this role is carried by the orange carotenoid protein (OCP), which reacts to high-intensity light by a series of conformational changes, enabling the binding of OCP to the PBs reducing the flow of energy into the photosystems. In this paper the mechanisms of energy migration in two mutant PB complexes of Synechocystis sp. were investigated and compared. The mutant CK is lacking phycocyanin in the PBs while the mutant ΔPSI/PSII does not contain both photosystems. Fluorescence decay spectra with picosecond time resolution were registered using a single photon counting technique. The studies were performed in a wide range of temperatures — from 4 to 300 K. The time course of NPQ and fluorescence recovery in darkness was studied at room temperature using both steady-state and time-resolved fluorescence measurements. The OCP induced NPQ has been shown to be due to EET from PB cores to the red form of OCP under photon flux densities up to 1000 μmol photons m^− 2 s^− 1. The gradual changes of the energy transfer rate from allophycocyanin to OCP were observed during the irradiation of the sample with blue light and consequent adaptation to darkness. This fact was interpreted as the revelation of intermolecular interaction between OCP and PB binding site. At low temperatures a significantly enhanced EET from allophycocyanin to terminal emitters has been shown, due to the decreased back transfer from terminal emitter to APC. The activation of OCP not only leads to fluorescence quenching, but also affects the rate constants of energy transfer as shown by model based analysis of the decay associated spectra. The results indicate that the ability of OCP to quench the fluorescence is strongly temperature dependent. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.     

Identification of a slowly inducible zeaxanthin-dependent component of non-photochemical quenching of chlorophyll fluorescence generated under steady-state conditions in Arabidopsis

The induction and relaxation of non-photochemical quenching (NPQ) under steady-state conditions, i.e. during up to 90 min of illumination at saturating light intensities, was studied in Arabidopsis thaliana. Besides the well-characterized fast qE and the very slow qI component of NPQ, the analysis of the NPQ dynamics identified a zeaxanthin (Zx) dependent component which we term qZ. The formation (rise time 10-15 min) and relaxation (lifetime 10-15 min) of qZ correlated with the synthesis and epoxidation of Zx, respectively. Comparative analysis of different NPQ mutants from Arabidopsis showed that qZ was clearly not related to qE, qT or qI and thus represents a separate, Zx-dependent NPQ component.     

Functions of the water soluble chlorophyll-binding protein in plants

Functional aspects of water soluble chlorophyll-binding protein (WSCP) in plants were investigated during the courses of leaf senescence, chlorophyll biogenesis, stress response and photoprotection. The cDNA sequence encoding WSCP from cauliflower was cloned into a binary vector to facilitate Agrobacterium tumefaciens mediated transformation of Nicotiana tabacum. The resultant transgenic tobacco plants overexpressed the CauWSCP gene under the control of a 35S-promoter. Analyses of protein and pigment contents indicate that WSCP overexpression does not enhance chlorophyll catabolism in vivo, thus rendering a role of WSCP in Chl degradation unlikely. Accumulation of higher levels of protochlorophyllide in WSCP overexpressor plants corroborates a proposed temporary storage and carrier function of WSCP for chlorophyll and late precursors. Although WSCP overexpressor plants did not show significant differences in non-photochemical quenching of chlorophyll fluorescence, they are characterized by significantly lower zeaxanthin accumulation and peroxidase activity at different light intensities, even at high light intensities of 700-900 μmol photons m⁻² s⁻¹. These results suggest a photoprotective function of the functional chlorophyll binding-WSCP tetramer by shielding of chlorophylls from molecular oxygen.     

Diurnal movements of chloroplasts in Halophila stipulacea and their effect on PAM fluorometric measurements of photosynthetic rates

Since diurnal chloroplast movements in Halophila stipulacea were described by Drew in 1979, this phenomenon has not been studied further for seagrasses. In addition to an apparent photoprotective role, such movements may affect the measurements of photosynthetic rates based on pulse amplitude modulated (PAM) fluorometry. This is because calculations of electron transport rates (ETR) are directly affected by the light absorption of the leaves (or the so-called absorption factor, AF), the latter of which changes with the movements of the chloroplasts. In this work, we therefore determined chloroplast clumping and dispersal, and measured AFs, chlorophyll contents and PAM fluorescence diurnally for H. stipulacea grown under two irradiance regimes. Diurnal chloroplast clumping occurred in high-light grown (HL) plants (∼450 μmol photons m⁻² s⁻¹ during midday), which was accompanied by a decrease in AF values (from 0.56 in the early morning to 0.34 at midday) but not in the chlorophyll content. Also, non-photochemical quenching (measured as NPQ) increased during the day in these plants. No such chloroplast movements and, thus, no diurnal changes in AF values (0.60 ± 0.04 throughout the day), and no changes in NPQ, were found in low-light grown (LL) plants (∼150 μmol photons m⁻² s⁻¹ during midday). As a consequence of the chloroplast clumping in HL plants, and its effect on AF values, maximal ETRs did not differ significantly between HL and LL plants. This finding thus shows the importance of taking into account changing AF values along the day when calculating ETRs of H. stipulacea, and other seagrasses potentially featuring diurnally changing AFs, under high-irradiance conditions.     

Exposure times in rapid light curves affect photosynthetic parameters in algae

Short-and long-duration light curves were applied to four macroalgae (Ulva sp., Codium fragile, Ecklonia radiata and Lessonia variegata), and two microalgal species (Chlorella emersonii and Chaetoceros muellerii). With increasing light curve duration, the maximal relative electron transport rate increased by a factor of three in E. radiata, and by factors of 1.25 and 1.23 in C. emersonii and L. variegata, respectively, but did not change in C. fragile and Ch. muellerii. The light saturation point E_k increased by 26 μmol photons m⁻² s⁻¹ in C. emersonii and 20 μmol photons m⁻² s⁻¹ in Ch. muellerii and E. radiata with elevated light curve exposure times. Oscillatory patterns of the continuous fluorescence readings reflect accumulation of Q_A. Continuous fluorescence values increased, or decreased, by approximately 10% within light curve increments. However, oscillations of 25% were not uncommon, which shows that cells are changing their photo-physiological response state during steady light conditions. Increasing dark acclimation times prior to light curve application lowered maximal relative electron transport rates in the C. emersonii (from 28 ± 1.7 to 25 ± 1.2 for 15 and 95 min dark acclimation in short-duration light curves respectively). This effect was counterbalanced by longer light curve application. It can therefore be concluded that manipulation of light exposure and dark incubation prior to the experiment affects the photosynthetic response, presumably due to different activation states of photosynthetic and photoprotective mechanisms. The highly species-specific photo-response patterns imply that a common rapid light curve protocol will generate artefacts in some species.     

Action potential in a plant cell lowers the light requirement for non-photochemical energy-dependent quenching of chlorophyll fluorescence

This study deals with effects of membrane excitation on photosynthesis and cell protection against excessive light, manifested in non-photochemical quenching (NPQ). In Chara corallina cells, NPQ and pericellular pH displayed coordinated spatial patterns along the length of the cell. The NPQ values were lower in H⁺-extruding cell regions (external pH ∼ 6.5) than in high pH regions (pH ∼ 9.5). Generation of an action potential by applying a pulse of electric current caused NPQ to increase within 30-60 s. This effect, manifested as a long-lived drop of maximum chlorophyll fluorescence (F_m′), occurred at lower photosynthetic flux densities (PFD) in the alkaline as compared to acidic cell regions. The light response curve of NPQ shifted, after generation of an action potential, towards lower PFD. The release of NPQ by nigericin and the rapid reversal of action potential-triggered NPQ in darkness indicate its relation to thylakoid ΔpH. Generation of an action potential shortly after darkening converted the chloroplasts into a latent state with the F_m identical to that of unexcited cells. This state transformed to the quenched state after turning on weak light that was insufficient for NPQ prior to membrane excitation of the cells. The ionophore, A23187, shifted NPQ plots similarly to the action potential effect, consistent with a likely role of a rise in the cytosolic Ca²⁺ level in the action potential-induced quenching. The results suggest that a rapid electric signal, across the plasma membrane, might exert long-lived effects on photosynthesis and chlorophyll fluorescence through ion flux-mediated pathways.     

Exploring the mechanism(s) of energy dissipation in the light harvesting complex of the photosynthetic algae Cyclotella meneghiniana

Photosynthetic organisms have developed vital strategies which allow them to switch from a light-harvesting to an energy dissipative state at the level of the antenna system in order to survive the detrimental effects of excess light illumination. These mechanisms are particularly relevant in diatoms, which grow in highly fluctuating light environments and thus require fast and strong response to changing light conditions. We performed transient absorption spectroscopy on FCPa, the main light-harvesting antenna from the diatom Cyclotella meneghiniana, in the unquenched and quenched state. Our results show that in quenched FCPa two quenching channels are active and are characterized by differing rate constants and distinct spectroscopic signatures. One channel is associated with a faster quenching rate (16 ns^− 1) and virtually no difference in spectral shape compared to the bulk unquenched chlorophylls, while a second channel is associated with a slower quenching rate (2.7 ns^− 1) and exhibits an increased population of red-emitting states. We discuss the origin of the two processes in the context of the models proposed for the regulation of photosynthetic light-harvesting. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.     

Diurnal courses of net photosynthesis and photosystem II quantum efficiency of submerged Lagarosiphon major under natural light conditions

An optode device for net-photosynthesis measurements, based on oxygen-depending quenching of fluorescence from O₂-specific sensors, and PAM fluorometry have been used to study diurnal courses of net-photosynthesis and the F_v/F_m ratio of the submerged plant Lagarosiphon major. Plants were pre-cultivated and studied in large mesocosm flow-through outdoor tanks under 50% and 80% shade cloth, respectively. Growth under the different shade cloths resulted in similar light compensation points (∼20 μmol photons m⁻² s⁻¹), but strongly different light saturation levels, with about 150 μmol m⁻² s⁻¹ for plants grown under 80% shade cloth and about 350 μmol m⁻² s⁻¹ for plants grown under 50% shade cloth. Plants under both growth conditions showed a transient reduction of the maximum F_v/F_m value in the afternoon (down to 70% of the morning control values under 80% shade cloth and down to 85% under 50% shade cloth), which was not accompanied by a reduction of the net photosynthetic rate. This indicated that the fluorescence parameter F_v/F_m must not be a reliable indicator of the rate of photosynthesis under all conditions. The new photo-optical device became evidenced as a valuable tool not only for laboratory experiments, but also for field studies of gas exchange of submerged plants.     

Mechanism of strong quenching of photosystem II chlorophyll fluorescence under drought stress in a lichen, Physciella melanchla, studied by subpicosecond fluorescence spectroscopy

The mechanism of the severe quenching of chlorophyll (Chl) fluorescence under drought stress was studied in a lichen Physciella melanchla, which contains a photobiont green alga, Trebouxia sp., using a streak camera and a reflection-mode fluorescence up-conversion system. We detected a large 0.31 ps rise of fluorescence at 715 and 740 nm in the dry lichen suggesting the rapid energy influx to the 715-740 nm bands from the shorter-wavelength Chls with a small contribution from the internal conversion from Soret bands. The fluorescence, then, decayed with time constants of 23 and 112 ps, suggesting the rapid dissipation into heat through the quencher. The result confirms the accelerated 40 ps decay of fluorescence reported in another lichen (Veerman et al., 2007 [36]) and gives a direct evidence for the rapid energy transfer from bulk Chls to the longer-wavelength quencher. We simulated the entire PS II fluorescence kinetics by a global analysis and estimated the 20.2 ns^− 1 or 55.0 ns^− 1 energy transfer rate to the quencher that is connected either to the LHC II or to the PS II core antenna. The strong quenching with the 3-12 times higher rate compared to the reported NPQ rate, suggests the operation of a new type of quenching, such as the extreme case of Chl-aggregation in LHCII or a new type of quenching in PS II core antenna in dry lichens.     

Unusual features of the high light acclimation of Chromera velia

In the present study, the high light (HL) acclimation of Chromera velia (Chromerida) was studied. HL-grown cells exhibited an increased cell volume and dry weight compared to cells grown at medium light (ML). The chlorophyll (Chl) a-specific absorption spectra ( \(a_{\text{phy}}^{*}\) ) of the HL cells showed an increased absorption efficiency over a wavelength range from 400 to 750 nm, possibly due to differences in the packaging of Chl a molecules. In HL cells, the size of the violaxanthin (V) cycle pigment pool was strongly increased. Despite a higher concentration of de-epoxidized V cycle pigments, non-photochemical quenching (NPQ) of the HL cells was slightly reduced compared to ML cells. The analysis of NPQ recovery during low light (LL) after a short illumination with excess light showed a fast NPQ relaxation and zeaxanthin epoxidation. Purification of the pigment–protein complexes demonstrated that the HL-synthesized V was associated with the chromera light-harvesting complex (CLH). However, the difference absorption spectrum of HL minus ML CLH, together with the 77 K fluorescence excitation spectra, suggested that the additional V was not protein bound but localized in a lipid phase associated with the CLH. The polypeptide analysis of the pigment–protein complexes showed that one out of three known LHCr proteins was associated in higher concentration with photosystem I in the HL cells, whereas in ML cells, it was enriched in the CLH fraction. In conclusion, the acclimation of C. velia to HL illumination shows features that are comparable to those of diatoms, while other characteristics more closely resemble those of higher plants and green algae.     

The site of regulation of light capture in Symbiodinium: Does the peridinin–chlorophyll a–protein detach to regulate light capture?

Dinoflagellates from the genus Symbiodinium form symbiotic associations with cnidarians including corals and anemones. The photosynthetic apparatuses of these dinoflagellates possess a unique photosynthetic antenna system incorporating the peridinin–chlorophyll a–protein (PCP). It has been proposed that the appearance of a PCP-specific 77 K fluorescence emission band around 672–675 nm indicates that high light treatment results in PCP dissociation from intrinsic membrane antenna complexes, blocking excitation transfer to the intrinsic membrane-bound antenna complexes, chlorophyll a–chlorophyll c₂–peridinin–protein-complex (acpPC) and associated photosystems (Reynolds et al., 2008 Proc Natl Acad Sci USA 105:13674–13678).We have tested this model using time-resolved fluorescence decay kinetics in conjunction with global fitting to compare the time-evolution of the PCP spectral bands before and after high light exposure. Our results show that no long-lived PCP fluorescence emission components appear either before or after high light treatment, indicating that the efficiency of excitation transfer from PCP to membrane antenna systems remains efficient and rapid even after exposure to high light. The apparent increased relative emission at around 675 nm was, instead, caused by strong preferential exciton quenching of the membrane antenna complexes associated with acpPC and reaction centers. This strong non-photochemical quenching (NPQ) is consistent with the activation of xanthophyll-associated quenching mechanisms and the generally-observed avoidance in nature of long-lived photoexcited states that can lead to oxidative damage. The acpPC component appears to be the most strongly quenched under high light exposure suggesting that it houses the photoprotective exciton quencher.     

Acclimation of the intertidal red alga Bangiopsis subsimplex (Stylonematophyceae) to salinity changes

The effect of salinity on growth, photosynthetic performance and osmotic acclimation was investigated in the eulittoral red algal species Bangiopsis subsimplex (Stylonematophyceae). The strain grew in a broad salinity range between 1 and 70 psu showing optimum growth between 10 and 50 psu. The saturation point I_k of the photosynthesis irradiance curves ranged between 153 and 83 μmol photons m^− 2 s^− 1 at all salinities and indicates an adaptation of B. subsimplex to moderate radiation conditions. Adjustments on the photosynthetic level (non-photochemical quenching) were sufficient to prevent damage to the photosynthetic apparatus as F_v/F_m values were constantly high (> 0.7) even when grown at the most hypo- and hypersaline conditions. As main low molecular weight carbohydrates, B. subsimplex contains the heteroside digeneaside and the polyol sorbitol. Digeneaside concentration was low and almost unchanged after hypersaline treatment (< 20 μmol g^− 1 DW), i.e. it did not play a role in osmotic acclimation. By contrast, sorbitol levels increased linearly from 150 to 380 μmol g^− 1 DW with increasing salinities between 5 and 60 psu, indicating its important function as an osmolyte and compatible solute under hypersaline conditions. The data presented are consistent with the natural habitat of B. subsimplex, i.e. the upper eulittoral zone.     

Response of the diatom Phaeodactylum tricornutum to photooxidative stress resulting from high light exposure

The response of microalgae to photooxidative stress resulting from high light exposure is a well-studied phenomenon. However, direct analyses of photosystem II (PSII) D1 protein (the main target of photoinhibition) in diatoms are scarce. In this study, the response of the diatom model species Phaeodactylum tricornutum to short-term exposure to high light was examined and the levels of D1 protein determined immunochemically. Low light (LL) acclimated cells (40 μmol photons m(-2) s(-1)) subjected to high light (HL, 1,250 μmol photons m(-2) s(-1)) showed rapid induction of non-photochemical quenching (NPQ) and ca. 20-fold increase in diatoxanthin (DT) concentration. This resulted from the conversion of diadinoxanthin (DD) to DT through the activation of the DD-cycle. D1 protein levels under LL decreased about 30% after 1 h of the addition of lincomycin (LINC), a chloroplast protein synthesis inhibitor, showing significant D1 degradation and repair under low irradiance. Exposure to HL lead to a 3.2-fold increase in D1 degradation rate, whereas average D1 repair rate was 1.3-x higher under HL than LL, leading to decreased levels of D1 protein under HL. There were significant effects of both HL and LINC on P. tricornutum maximum quantum yield of PSII (F(v)/F(m)), showing a reduction of active PSII reaction centres. Partial recovery of F(v)/F(m) in the dark demonstrates the photosynthetic resilience of this diatom to changes in the light regime. P. tricornutum showed high allocation of total protein to D1 and an active D1-repair cycle to limit photoinhibition.