Light-induced degradation of cytochrome b559 during photoinhibition of the photosystem II reaction center.

The behaviour of cytochrome (cyt) b559 during acceptor- and donor-side photoinhibition has been investigated in oxygen-evolving and non-evolving photosystem II (PSII) membranes. Strong illumination at 20 degrees C under aerobiosis induced a strong decrease in the absorbance of the cyt b559 alpha-band in the two preparations. This absorbance decline was observed only in non-oxygen-evolving PSII samples when illumination was performed under aerobiosis but at 4 degrees C, or under anaerobiosis at 20 degrees C. These results suggest that acceptor-side photoinhibition induces the degradation of cyt b559 by a mechanism related to an enzymatic reaction mediated by singlet oxygen. Donor-side photoinhibition may induce, however, a non-enzymatic photocleavage of the protein.     

Mathematical modelling of the light response curve of photoinhibition of Photosystem II

The light response curves of the acceptor and donor side mechanisms of photoinhibition of Photosystem II were calculated, using Arabidopsis as a model organism. Acceptor-side photoinhibition was modelled as double reduction of Q_A, noting that non-photochemical quenching has the same effect on the quantum yield of Q_A double reduction in closed PSII centres as it has on the quantum yield of electron transport in open centres. The light response curve of acceptor-side photoinhibition in Arabidopsis shows very low efficiency under low intensity light and a relatively constant quantum yield above light saturation of photosynthesis. To calculate the light response curve of donor-side photoinhibition, we built a model describing the concentration of the oxidized primary donor P₆₈₀⁺ during steady-state photosynthesis. The model is based on literature values of rate constants of electron transfer reactions of PSII, and it was fitted with fluorescence parameters measured in the steady state. The modelling analysis showed that the quantum yield of donor-side photoinhibition peaks under moderate light. The deviation of the acceptor and donor side mechanisms from the direct proportionality between photoinhibition and photon flux density suggests that these mechanisms cannot solely account for photoinhibition in vivo, but contribution of a reaction whose quantum yield is independent of light intensity is needed. Furthermore, a simple kinetic calculation suggests that the acceptor-side mechanism may not explain singlet oxygen production by photoinhibited leaves. The theoretical framework described here can be used to estimate the yields of different photoinhibition mechanisms under different wavelengths or light intensities.     

Damage to the oxygen-evolving complex by superoxide anion, hydrogen peroxide, and hydroxyl radical in photoinhibition of photosystem II

Under strong illumination of a photosystem II (PSII) membrane, endogenous superoxide anion, hydrogen peroxide, and hydroxyl radical were successively produced. These compounds then cooperatively resulted in a release of manganese from the oxygen-evolving complex (OEC) and an inhibition of oxygen evolution activity. The OEC inactivation was initiated by an acceptor-side generated superoxide anion, and hydrogen peroxide was most probably responsible for the transportation of reactive oxygen species (ROS) across the PSII membrane from the acceptor-side to the donor-side. Besides ROS being generated in the acceptor-side induced manganese loss; there may also be a ROS-independent manganese loss in the OEC of PSII. Both superoxide anion and hydroxyl radical located inside the PSII membrane were directly identified by a spin trapping-electron spin resonance (ESR) method in combination with a lipophilic spin trap, 5-(diethoxyphosphoryl)-5-phenethyl-1-pyrroline N-oxide (DEPPEPO). The endogenous hydrogen peroxide production was examined by oxidation of thiobenzamide.     

Photobleaching of photosynthetic pigments in spinach thylakoid membranes. Effect of temperature, oxygen and DCMU

The time dependence of photobleaching of photosynthetic pigments under high light illumination of isolated spinach thylakoid membranes at 22 and 4 degrees C was investigated. At 22 degrees C, the bleaching at 678, 472 and 436 nm was prominent but lowering the temperature up to 4 degrees C during illumination prevented the pigments from bleaching almost completely. The accelerating effect on pigment photobleaching by the presence of 3-(3,4 dichlorophenyl)-1,1-dimethyl-urea)-(DCMU), a well-known inhibitor of the electron transport and known to prevent photosystem I (PSI) and photosystem II (PSII) against photoinhibitory damage, was also suppressed at low temperature. At 22 degrees C in the presence and absence of DCMU, the decrease of the absorption at 678 and 472 nm was accompanied by a shift to the shorter wavelengths. To check the involvement of reactive oxygen species in the process, pigment photobleaching was followed in anaerobiosis. The effects of the three different environmental factors--light, temperature and DCMU--on the dynamics of photobleaching are discussed in terms of different susceptibility of the main pigment-protein complexes to photoinhibition.     

Photoreduction of Molecular Oxygen in Preparations of Photosystem II under Photoinhibitory Conditions

Preparations of photosystem II (PSII) from pea (Pisum sativum L.) leaves were used to study the evolution and reduction of molecular oxygen under photoinhibitory conditions. Under these conditions, the photoinduced oxygen uptake did not exceed 10% of the total oxygen-evolving activity in PSII preparations. Both the Hill and the Mehler reactions were found to occur simultaneously under long-term illumination of PSII preparations with high-intensity light in the presence of potassium ferricyanide. During this light treatment in the presence of potassium ferricyanide, the rate of oxygen uptake increased gradually reaching 30% of the oxygen-evolving activity. The photogeneration of superoxide anion radical at increasing light intensities followed a typical light-response curve with a light saturation at 800 W/m². The results provide evidence that the Mehler reaction is the major source for superoxide and hydrogen peroxide in PSII preparations under photoinhibitory conditions and that the Mehler reaction in PSII proceeds more effectively at high light intensities. The relatively low and sustained rate of oxygen photoreduction in PSII preparations under photoinhibitory conditions substantiates the hypothesis on the involvement of Mehler reaction in cell signaling and regulation.     

Strong-light photoinhibition treatment accelerates the changes of protein secondary structures in triton-treated photosystem I and photosystem II complexes

Changes in the protein secondary structure and electron transport activity of the Triton X-100-treated photosystem I (PSI) and photosystem II (PSII) complexes after strong illumination treatment were studied using Fourier transform-infrared (FT-IR) spectroscopy and an oxygen electrode. Short periods of photoinhibitory treatment led to obvious decreases in the rates of PSI-mediated electron transport activity and PSII-mediated oxygen evolution in the native or Triton-treated PSI and PSII complexes. In the native PSI and PSII complexes, the protein secondary structures had little changes after the photoinhibitory treatment. However, in both Triton-treated PSI and PSII complexes, short photoinhibition times caused significant loss of -helical content and increase of -sheet structure, similar to the conformational changes in samples of Triton-treated PSI and PSII complexes after long periods of dark incubation. Our results demonstrate that strong-light treatment to the Triton-treated PSI and PSII complexes accelerates destruction of the transmembrane structure of proteins in the two photosynthetic membranes.     

The characterization of photoinhibition and recovery during cold acclimation in Arabidopsis thaliana using chlorophyll fluorescence imaging

The responses to photoinhibition of photosynthesis at low temperature and subsequent recovery were examined in Arabidopsis thaliana (ecotype Columbia) developed at 4°C cold-acclimating conditions, 23°C non-acclimating conditions and for non-acclimated plants shifted to 4°C (cold-shifted). These responses were determined in planta using Chl fluorescence imaging. We show that cold acclimation results in an increased tolerance to photoinhibition in comparison with non-acclimated plants and that growth and development at low temperature is essential for this to occur. Cold-shifted plants were not as tolerant as the cold-acclimated plants. In addition, we demonstrate this tolerance is as a result of growth under high PSII excitation pressure, that can be modulated by growth temperature or growth irradiance. Cold-acclimated and cold-shifted plants fully recover from photoinhibition in the dark, whereas non-acclimated plants show reduced levels of recovery and demonstrate a requirement for light. The role of the PSII repair cycle, PSII quenching centres, and the use of Chl fluorescence imaging to monitor photoinhibitory responses in planta are discussed.     

Probing the events of photoinhibition by altering electron-transport activity and light-harvesting capacity in chloroplast thylakoids

Abstract. The effect of photoinhibition on the activity of photosystem II (PSII) in spinach chloroplasts was investigated. Direct light-induced absorbance change measurements at 320 nm (Δ A ₃₂₀) provided a measure of the PSII charge separation reaction and revealed that photoinhibition prevented the stable photoreduction of the primary quinone acceptor Q_A. Sensitivity to photoinhibition was substantially enhanced by treatment of thylakoids with NH₂OH which extracts manganese from the H₂O-splitting enzyme and prevents electron donation to the reaction centre. Incubation with 3-(3,4,-dichlorophenyl)-1,1-dimethylurea (DCMU) during light exposure did not affect the extent of photoinhibitory damage. The chlorophyll (Chl) b -less chlorina (2 mutant of barley displayed a significantly smaller light-harvesting antenna size of PSII (about 20% of that in wild type chloroplasts) and, simultaneously, a lower sensitivity to photoinhibition. These observations suggest that photoinhibition depends on the amount of light absorbed by PSII and that the process of photoinhibition is accelerated when electron donation to the reaction centre is prevented. It is postulated that the probability of photoinhibition is greater when excitation energy is trapped by P680⁺, the oxidized form of the PSII reaction centre. The results are discussed in terms of the D1/D2 heterodimer which contains the functional PSII components P680, pheophytin, Q_A and Q_B.     

Inhibition of Water Splitting Increases the Susceptibility of Photosystem II to Photoinhibition

Photosystem II (PSII)-enriched membrane particles were isolated from peas (Pisum sativum L.) and treated in several different ways to inhibit the water oxidation reactions, but not reaction center function itself, as judged by the light-induced rate of reduction of 2,6-dichlorophenol indophenol with and without the artificial electron donor, diphenyl carbazide. It was shown that such treatments increased the susceptibility of the PSII-enriched membranes to photoinhibition. This trend was further observed if 2,6-dichlorophenol indophenol was present during the illumination with photoinhibitory light. On the other hand, protection against the enhanced photoinhibition was found when the water-splitting activity was reconstituted or when the artificial electron donor diphenyl carbazide was present during the preillumination. The results indicate that irreversible photodamage occurred within the PSII reaction center as a consequence of illumination with strong light and that the rate of this damage was enhanced under conditions that are expected to give rise to a photoaccumulation of oxidizing species such as P680⁺ on the donor side of PSII. This mechanism of photoinhibitory damage occurred under both aerobic and anaerobic conditions.     

The involvement of photosystem II-generated H2O2 in photoinhibition.

The involvement of H2O2 generated by photosystem II (PSII) in the process of photoinhibition of thylakoids with a functional oxygen-evolving complex (OEC) was investigated. The rate of photoinhibition was decreased to the rate of loss of activity in the dark when bovine Fe-catalase was present during the photoinhibitory illumination. Photoinhibition was accelerated for both Cl(-)-depleted and Cl(-)-sufficient thylakoids when KCN was present to inhibit the thylakoid-bound Fe-catalase. We propose that these preparations become photoinhibited by reactions with H2O2 produced via oxidation of water by the Cl(-)-depleted OEC and by reduction of O2 at the QB site when PSII is illuminated without an electron acceptor.     

MPH1 is a thylakoid membrane protein involved in protecting photosystem II from photodamage in land plants

Photosystem II (PSII) is highly susceptible to photoinhibition caused by environmental stimuli such as high light; therefore plants have evolved multifaceted mechanisms to efficiently protect PSII from photodamage. We previously published data suggesting that Maintenance of PSII under High light 1 (MPH1, encoded by AT5G07020), a PSII-associated proline-rich protein found in land plants, participates in the maintenance of normal PSII activity under photoinhibitory stress. Here we provide additional evidence for the role of MPH1 in protecting PSII against photooxidative damage. Two Arabidopsis thaliana mutants lacking a functional MPH1 gene suffer from severe photoinhibition relative to the wild-type plants under high irradiance light. The mph1 mutants exhibit significantly decreased PSII quantum yield and electron transport rate after exposure to photoinhibitory light. The mutants also display drastically elevated photodamage to PSII reaction center proteins after high-light treatment. These data add further evidence that MPH1 is involved in PSII photoprotection in Arabidopsis. MPH1 homologs are found across phylogenetically diverse land plants but are not detected in algae or prokaryotes. Taken together, these results suggest that MPH1 protein began to play a role in protecting PSII against excess light following the transition from aquatic to terrestrial conditions.     

Efficiency and role of loss processes in light-driven water oxidation by PSII

Its superior quantum efficiency renders PSII a model for biomimetic systems. However, also in biological water oxidation by PSII, the efficiency is restricted by recombination losses. By laser-flash illumination, the secondary radical pair, P680(+)Q(-) (A) (where P680 is the primary Chl donor in PSII and Q(A), primary quinone acceptor of PSII), was formed in close to 100% of the PSII. Investigation of the quantum efficiency (or yield) of the subsequent steps by time-resolved delayed (10 micros to 60 ms) and prompt (70 micros to 700 ms) Chl fluorescence measurements on PSII membrane particles suggests that (1) the effective rate for P680(+) Q(-) (A) recombination is approximately 5 ms(-1) with an activation energy of approximately 0.34 eV, circumstantially confirming dominating losses by reformation of the primary radical pair followed by ground-state recombination. (2) Because of compensatory influences on recombination and forward reactions, the efficiency is only weakly temperature dependent. (3) Recombination losses are several-fold enhanced at lower pH. (4) Calculation based on delayed-fluorescence data suggests that the losses depend on the state of the water-oxidizing manganese complex, being low in the S(0)-->S(1) and S(1)-->S(2) transition, clearly higher in S(2)-->S(3) and S(3)-->S(4)-->S(0). (5) For the used artificial electron acceptor, the efficiency is limited by acceptor-side processes/S-state decay at high/low photon-absorption rates resulting in optimal efficiency at surprisingly low rates of approximately 0.15-15 photons s(-1) (per PSII). The pH and S-state dependence can be rationalized by the basic model of alternate electron-proton removal proposed elsewhere. A physiological function of the recombination losses could be limitation of the lifetime of the reactive donor-side tyrosine radical (Y(.) (Z)) in the case of low-pH blockage of water oxidation.     

Mechanism of copper-enhanced photoinhibition in thylakoid membranes

The effect of copper on photoinhibition of photosystem II (PSII) in vitro was studied in bean ( Phaseolus vulgaris L. cv. Dufrix) and pumpkin ( Cucurbita pepo L.) thylakoids. The thylakoids were illuminated at 200–2 000 μmol photons m⁻² s⁻¹ in the presence of 70–1 830 added Cu²⁺ ions per PSII. Three lines of evidence show that the irreversible damage of PSII caused by illumination of thylakoids in the presence of Cu²⁺ was mainly due to donor-side photoinhibition resulting from inhibition of the PSII donor side by Cu²⁺. First, addition of an artificial electron donor partially restored PSII activity of thylakoids that had been illuminated in the presence of Cu²⁺. Second, already moderate light was enough to cause rapid inhibition of PSII, and the inhibition could be saturated by light. Third, the extrinsic polypeptides of the oxygen-evolving complex were found to become oxidized by the combined effect of Cu²⁺ and light. The presence of oxygen was not necessary for the copper-induced enhancement of photoinhibition of PSII. When the illumination was prolonged, copper caused a gradual collapse of the thylakoid structure by increasing degradation of thylakoid proteins.     

Dual role of triplet localization on the accessory chlorophyll in the photosystem II reaction center: photoprotection and photodamage of the D1 protein

Infrared absorption and electron spin resonance studies have shown that the excited triplet state of chlorophyll formed by radical pair recombination in the PSII reaction center is mainly localized on the accessory chlorophyll, which is most probably located in the D1 protein (Chl(1)). This triplet localization plays two contrasting roles, depending on the redox state of Q(A), in the process of acceptor-side photoinhibition of PSII. In the early stage of photoinhibition, in which singly reduced Q(A) is reversibly stabilized, the triplet state of Chl(1) ((3)Chl(1)*) is rapidly quenched (t(1/2) = 2-20 micro s) by the interaction with Q(A)(-), preventing formation of harmful singlet oxygen. In the next inhibitory stage, in which Q(A) is doubly reduced and then irreversibly released from the Q(A) pocket, the lifetime of (3)Chl(1)* becomes longer by more than two orders of magnitude (t(1/2) = 1-3 ms). As a result, singlet oxygen is produced around Chl(1) in the D1 protein, causing damage preferably to the D1 protein, which induces subsequent proteolytic degradation. Thus, (3)Chl(1)* functions as a switch to change from the protective to the degradative phase of the PSII reaction center by sensing either reversible or irreversible inhibited state at the Q(A) site.     

Degradation pattern of photosystem II reaction center protein D1 in intact leaves. The major photoinhibition-induced cleavage site in D1 polypeptide is located amino terminally of the DE loop.

Photoinhibition-induced degradation of the D1 protein of the photosystem II reaction center was studied in intact pumpkin (Cucurbita pepo L.) leaves. Photoinhibition was observed to cause the cleavage of the D1 protein at two distinct sites. The main cleavage generated an 18-kD N-terminal and a 20-kD C-terminal degradation fragment of the D1 protein. this cleavage site was mapped to be located clearly N terminally of the DE loop. The other, less-frequent cleavage occurred at the DE loop and produced the well-documented 23-kD, N-terminal D1 degradation product. Furthermore, the 23-kD, N-terminal D1 fragment appears to be phosphorylated and can be detected only under severe photoinhibition in vivo. Comparison of the D1 degradation pattern after in vivo photoinhibition to that after in vitro acceptor-side and donor-side photoinhibition, performed with isolated photosystem II core particles, gives indirect evidence in support of donor-side photoinhibition in intact leaves.     

Light quality during growth of Tradescantia albiflora regulates photosystem stoichiometry, photosynthetic function and susceptibility to photoinhibition

Tradescantia albiflora (Kunth) was grown under two different light quality regimes of comparable light quantity: in red + far-red light absorbed mainly by photosystem I (PSI light) and yellow light absorbed mainly by photosystem II (PSII light). The composition, function and ultrastructure of chloroplasts, and photoinhibition of photosynthesis in the two types of leaves were compared. In contrast to regulation by light quantity (Chow et al. 1991. Physiol. Plant. 81: 175–182), light quality exerted an effect on the composition of pigment complexes, function and structure of chloroplasts in Tradescantia: PSII light-grown leaves had higher Chl a/b ratios, higher PSI concentrations, lower PSII/PSI reaction centre ratios and less extensive thylakoid stacking than PSI light-grown leaves. Light quality triggered modulations of chloroplast components, leading to a variation of photosynthetic characteristics. A larger proportion of primary quinone acceptor (Q_A) in PSI light-grown leaves was chemically reduced at any given irradiance. It was also observed that the quantum yield of PSII photochemistry was lower in PSI light-grown leaves. PSI light-grown leaves were more sensitive to photoinihibition and recovery was slower compared to PSII light-grown leaves, showing that the PSII reaction centre in PSI light-grown leaves was more easily impaired by photoinhibition. The increase in susceptibility of leaves to photoinhibition following blockage of chloroplast-encoded protein synthesis was greater in PSII light-grown leaves, showing that these leaves normally have a greater capacity for PSII repair. Inhibition of zeaxanthin formation by dithiothreitol slightly increased sensitivity to photoinhibition in both PSI and PSII light-grown leaves.     

Irreversible disassembly of chiral macrodomains in thylakoids due to photoinhibition

We investigated the effect of photoinhibitory illumination on the chiral macroorganization of the chromophores in spinach thylakoid membranes. By measuring circular dichroism (CD), we found that prolonged (15 min) illumination of membranes with intense white light led to irreversible diminishment of the main CD bands originating from the chiral macroorganization of the chromophores. The irreversible decrease of the main CD bands showed a nearly linear correlation with the extent of photoinhibition which was determined by chlorophyll fluorescence induction. CD measurements also revealed that the excitonic CD bands, which are given rise by short-range interactions between the chromophores inside the complexes or particles, were largely insensitive to the photoinhibitory illumination of the membranes. These data show that, whereas photoinhibitory treatment has no perceptible effect on the molecular architecture of the bulk of the pigment–protein complexes, it leads to a disorganization of their macroarray, and an irreversible disassembly of the chirally organized macrodomains.     

Characterization of carotenoid and chlorophyll photooxidation in photosystem II

Photosystem II (PSII) contains two accessory chlorophylls (Chl(Z), ligated to D1-His118, and Chl(D), ligated to D2-His117), carotenoid (Car), and heme (cytochrome b(559)) cofactors that function as alternate electron donors under conditions in which the primary electron-donation pathway from the O(2)-evolving complex to P680(+) is inhibited. The photooxidation of the redox-active accessory chlorophylls and Car has been characterized by near-infrared (near-IR) absorbance, shifted-excitation Raman difference spectroscopy (SERDS), and electron paramagnetic resonance (EPR) spectroscopy over a range of cryogenic temperatures from 6 to 120 K in both Synechocystis PSII core complexes and spinach PSII membranes. The following key observations were made: (1) only one Chl(+) near-IR band is observed at 814 nm in Synechocystis PSII core complexes, which is assigned to Chl(Z)(+) based on previous spectroscopic studies of the D1-H118Q and D2-H117Q mutants [Stewart, D. H., Cua, A., Chisholm, D. A., Diner, B. A., Bocian, D. F., and Brudvig, G. W. (1998) Biochemistry 37, 10040-10046]; (2) two Chl(+) near-IR bands are observed at 817 and 850 nm in spinach PSII membranes which are formed with variable relative yields depending on the illumination temperature and are assigned to Chl(Z)(+), and Chl(D)(+), respectively; (3) the Chl and Car cation radicals have significantly different stabilities at reduced temperatures with Car(+) decaying much faster; (4) in Synechocystis PSII core complexes, Car(+) decays by recombination with Q(A)(-) and not by Chl(Z)/Chl(D) oxidation, with multiphasic kinetics that are attributed to an ensemble of protein conformers that are trapped as the protein is frozen; and (5) in spinach PSII membranes, Car(+) decays mainly by recombination with Q(A)(-), but also partly by formation of the 850 nm Chl cation radical. The greater stability of Chl(Z)(+) at low temperatures enabled us to confirm that resonance Raman bands previously assigned to Chl(Z)(+) are correctly assigned. In addition, the formation and decay of these cations provide insight into the alternate electron-donation pathways to P680(+).     

Photosystem II electron transfer cycle and chlororespiration in planktonic diatoms

The dominance of diatoms in turbulent waters suggests special adaptations to the wide fluctuations in light intensity that phytoplankton must cope with in such an environment. Our recent demonstration of the unusually effective photoprotection by the xanthophyll cycle in diatoms [Lavaud et al. (2002) Plant Physiol 129 (3) (in press)] also revealed that failure of this protection led to inactivation of oxygen evolution, but not to the expected photoinhibition. Photo-oxidative damage might be prevented by an electron transfer cycle around Photosystem II (PS II). The induction of such a cycle at high light intensity was verified by measurements of the flash number dependence of oxygen production in a series of single-turnover flashes. After a few minutes of saturating illumination, the oxygen flash yields are temporarily decreased. The deficit in oxygen production amounts to at most 3 electrons per PS II, but continues to reappear with a half time of 2 min in the dark until the total pool of reducing equivalents accumulated during the illumination has been consumed by (chloro)respiration. This is attributed to an electron transfer pathway from the plastoquinone pool or the acceptor side of PS II to the donor side of PS II that is insignificant at limiting light intensity but is accelerated to milliseconds at excess light intensity. Partial filling of the 3-equivalents capacity of the cyclic electron transfer path in PS II may prevent both acceptor-side photoinhibition in oxygen-evolving PS II and donor-side photoinhibition when the oxygen-evolving complex is temporarily inactivated. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photosynthetic light-response curves and photoinhibition of the deep-water laminaria abyssalis and the intertidal laminaria digitata (phaeophyceae)

Photosynthetic light‐response curves of the deep‐water Laminaria abyssalis Oliveira and of the intertidal L. digitata Lamoroux were determined and related to photoinhibition phenomena as monitored by oxygen evolution and photosystem II efficiency (F_V/F_M). L. abyssalis has half the pigment content, number of cells and plastids, and photosynthetic capacity per unit area compared with L. digitata. L. abyssalis showed a higher in vivo Chl a absorption coefficient and higher photosynthetic efficiency on a Chl a basis, although the two algae showed somewhat similar light‐response curves on a Chl a basis. Both species showed similar Chl a/Chl c and Chl a/fucoxanthin ratios, and similar dark respiration rates and light compensation points. In addition, they also showed similar convexities in their light‐response curves and no differences in their light saturation of F_V/F_M. Room temperature chlorophyll fluorescence induction measurements of fronds incubated in 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) suggest that both species may have a similar PSII absorption cross section. Thus, L. abyssalis appears to optimize its light absorption at very low light intensities, not by increasing the pigment content, but by absorbing light more efficiently. However, L. abyssalis was more sensitive to photoinhibition than L. digitata and showed no recovery of F_V/F_M and O₂ evolution after a photoinhibitory treatment, even with a subsequent exposure to 24 h of dim light. L. digitata, on the other hand, recovered its photosynthetic capacity within 6 h under dim light. These results suggest that photosynthetic light‐induction curves based on Chl a are not a good indicator of either the photosynthetic capacity or the sensitivity to photoinhibition when macroalgae of different species are being compared. Based on their light‐response and photoinhibition characteristics, we suggest that L. abyssalis, a deep‐water oceanic macroalgae, is an atypical shade alga whereas L. digitata has the properties of a sun alga.