Oxygen Evolution Measurements Research Articles

A gene required for the regulation of photosynthetic light harvesting in the cyanobacterium Synechocystis 6803.

A gene required for the short-term regulation of photosynthetic light harvesting (the state transition) has been identified in the cyanobacterium Synechocystis sp. PCC6803. The open reading frame is designated sll1926 in the complete Synechocystis gene sequence. The deduced amino acid sequence has no homologues in current sequence databases and no recognizable sequence motifs. It encodes a putative integral membrane protein of 16 kDa, which we have designated RpaC (regulator of phycobilisome association C). Fluorescence measurements of an insertional inactivation mutant of rpaC (Deltasll1926) show that it is specifically unable to perform state transitions. Deltasll1926 has approximately wild-type levels of PS1, PS2 and phycobilisomes. Measurements of oxygen evolution and uptake show Deltasll1926 to have no deficiency in electron transport rates. In vitro [gamma-32P]-ATP labelling experiments suggest that RpaC is not the 15 kDa membrane phosphoprotein previously implicated in state transitions. Deltasll1926 grows more slowly than the wild type only at very low light intensities.

A gasometric method to determine erythrocyte catalase activity.

We describe a new gasometric method to determine erythrocyte catalase activity by the measurement of the volume of oxygen produced as a result of hydrogen peroxide decomposition in a system where enzyme and substrate are separated in a special reaction test tube connected to a manometer and the reagents are mixed with a motor-driven stirrer. The position of the reagents in the test tube permits the continuous measurement of oxygen evolution from the time of mixing, without the need to stop the reaction by the addition of acid after each incubation time. The enzyme activity is reported as K Hb, i.e., mg hydrogen peroxide decomposed per second per gram of hemoglobin (s-1 g Hb-1). The value obtained for catalase activity in 28 samples of hemolyzed human blood was 94.4 +/- 6.17 mg H2O2 s-1 g Hb-1. The results obtained were precise and consistent, indicating that this rapid, simple and inexpensive method could be useful for research and routine work.

Oxygen‐dependent electron flow influences photosystem II function and psbA gene expression in the cyanobacterium Synechococcus sp. PCC 7942

During acclimated growth in Synechococcus sp. PCC 7942 a substantial proportion of the electrons extracted from water by photosystem II ultimately flow back to oxygen. This flow increases rapidly under high light, which allows Synechococcus to maintain photosystem II centers largely open, even under excessive excitation. The electron flow to oxygen with increasing light accounts for the progressive discrepancy between the light response curve of measured oxygen evolution, and the light response curve of photosystem II activity estimated from fluorescence measures. In cells under anoxia this flexible electron sink is lost and photosystem II centers suffer partial closure at the growth light intensity, with closure becoming more severe under excess light. As predicted from earlier work this PSII closure results in rapid loss of psbAI message, encoding the D1:1 protein of PSII, and induction of psbAII/AIII encoding the alternate D1:2 protein. The changes in the mRNA pool are not, however, reflected at the protein level, and D1:1 remains in the thylakoid membranes. There is no accumulation of D1:2, despite some continued synthesis of other proteins. PSII closure, therefore, results in repression of psbAI and induction psbAII/AIII expression, but D1:1/D1:2 exchange is blocked by anoxia, downstream from transcription. D1:1 protein and PSII activity are quite stable under anoxia and moderate illumination. Nevertheless, upon recovery under oxygenic conditions, the existing D1:1 is lost from the membranes, resulting in a transient drop in PSII activity. This suggests that under normal conditions the cells use oxygen to facilitate preemptive turnover of D1 proteins.

Construction and characterization of a functional mutant of Synechocystis 6803 harbouring a eukaryotic PSII-H subunit.

A Synechocystis 6803 mutant carrying a chimaeric photosystem II (PSII), in which the Zea mays PsbH subunit (7.7 kDa calculated molecular mass) replaces the cyanobacterial copy (7.0 kDa), was constructed. With the exception of the N-terminal 12 amino acid extension, which has a phosphorylatable threonine, the eukaryotic polypeptide is 78% homologous to its bacterial counterpart. Biochemical characterization of this mutant shows that it expresses the engineered gene correctly and is competent for photoautotrophic growth. Fluorescence analysis and oxygen evolution measurements in the presence of exogenous acceptors indicate that the observed phenotype results from a chimaeric PSII rather than from the absence of function associated with PsbH, suggesting that the heterologous protein is assembled into a functional PSII. Inhibition of oxygen evolution by herbicides belonging to different classes shows that the sensitivity of the mutant PSII is changed only towards phenolic compounds. This result indicates slight conformational modification of the QB/herbicide binding pocket of the D1 polypeptide caused by the bulky PsbH protein in the mutant, and also suggests close structural interaction of the D1 and PsbH subunits in the topological arrangement of PSII.

Joint action of natural and synthetic photosystem II inhibitors

The inhibitory action on photosystem II of four sorgoleone analogues, isolated from Sorghum bicolor (L) Moench, and two synthetic inhibitors, diuron and bentazone, was tested by measuring oxygen evolution of thylakoid membranes. The inhibition of oxygen evolution for mixtures of inhibitors was compared with the Additive Dose Model (ADM). ADM assumes that, at a defined response level, the effect of a mixture of inhibitors can be unambiguously expressed by the potency of either of the inhibitors applied separately. The slope of the logistic dose-response curves differed between the inhibitors; sorgoleone analogues had the steepest and bentazone the shallowest slope. The difference in slopes makes the interpretation of the isoboles less general and may reflect the differences in the interaction between the natural and the synthetic inhibitors with the binding site. The results suggest that there may be some limitation to ADM, namely that compounds with the same site of action might have different response curves if their mechanism of binding is different. The joint action of inhibitors follows ADM at I 50 . Therefore, the inhibitors can replace each other in any mixture ratio, based on the relative potencies of the pure inhibitors, without changing each other's effect on oxygen evolution. The joint action at I 20 and I 80 sometimes diverged from ADM.

Joint action of natural and synthetic photosystem II inhibitors

The inhibitory action on photosystem II of four sorgoleone analogues, isolated from Sorghum bicolor (L) Moench, and two synthetic inhibitors, diuron and bentazone, was tested by measuring oxygen evolution of thylakoid membranes. The inhibition of oxygen evolution for mixtures of inhibitors was compared with the Additive Dose Model (ADM). ADM assumes that, at a defined response level, the effect of a mixture of inhibitors can be unambiguously expressed by the potency of either of the inhibitors applied separately. The slope of the logistic dose-response curves differed between the inhibitors; sorgoleone analogues had the steepest and bentazone the shallowest slope. The difference in slopes makes the interpretation of the isoboles less general and may reflect the differences in the interaction between the natural and the synthetic inhibitors with the binding site. The results suggest that there may be some limitation to ADM, namely that compounds with the same site of action might have different response curves if their mechanism of binding is different. The joint action of inhibitors follows ADM at I50 . Therefore, the inhibitors can replace each other in any mixture ratio, based on the relative potencies of the pure inhibitors, without changing each other's effect on oxygen evolution. The joint action at I20 and I80 sometimes diverged from ADM. © 1999 Society of Chemical Industry

LISTENING TO PHYTOPLANKTON: MEASURING BIOMASS AND PHOTOSYNTHESIS BY PHOTOACOUSTICS

A simple technique based on photoacoustic measurements allowed us to determine the biomass, as well as the efficiency of photosynthesis, for different taxa of phytoplankton in situ. The method yields rapid, direct results compared to tedious standard measurements of 14C fixation and oxygen evolution or compared to indirect results from measurement of fluorescence. The procedure has a high potential for following the effects of environmental parameters such as irradiance, nutrient status, and pollution on phytoplankton communities and their photosynthetic activity.

Effects of cholesterol and benzyl alcohol on fluorescence transients of pea thylakoids

Using the chlorophyll fluorescence induction and oxygen evolution measurements we investigated the influence of alteration of the fluidity of pea thylakoid membranes on PSH activity and heterogeneity. The treatment of thylakoid membranes with cholesterol, leading to a decrease of membrane fluidity, resulted in a decrase of the rate of PSII-mediated electron transport by 30% and more pronounced inhibition of PSI photochemical activity. These changes are accompanied by a decrease of PSIIα centres and a decrease of the F v/ F no obtained from the chlorophyll fluorescence induction curves. The fluidization of the thylakoid membranes achieved by incubation of thylakoid membranes with local anaesthetic benzyl alcohol influenced chlorophyll α fluorescence induction parameters to a smaller extent. The rates of PSI and PSII mediated electron transport are inhibited by 20% benzyl alcohol treatment. The level of Q B nonreducing centres seems not to be altered by either treatment.

Measurement of photosynthetic oxygen evolution with a new type of oxygen sensor

We measured the light response curve of photosynthetic oxygen evolution by illuminating a leaf disc in an air-tight windowed chamber. Oxygen production was measured by monitoring the quenching of luminescence of an organometallic ruthenium compound. A photodiode based chlorophyll a fluorometer was used to measure the luminescence intensity. Oxygen evolution measurements with a traditional oxygen electrode gave the same numerical values at different light intensities when the same leaf disk was tested. The quality of the measurement signal of the new method was found to be similar to that obtained with the oxygen electrode method. The new luminescence based system is more stable against electrical disturbances than an oxygen electrode, its response to oxygen pressure changes is very rapid, and the new method allows the same basic equipment to be used for chlorophyll fluorescence and oxygen measurements.

A novel approach for gasification of carbon using a mixed conductor

The gasification of carbon to CO without the production of nitrogen was studied using an oxide ionic and electronic mixed conductor. The oxide ionic and electronic mixed conductor CaTi0.75Fe0.25O3–α was used as a selective oxygen permeable diaphragm. The selective electrochemical permeation of oxygen across a ceramic sample was confirmed by measuring oxygen evolution rate at the low PO2 side of the oxygen gas concentration cell using sample ceramics as a diaphragm. The gasification of carbon black to CO using this diaphragm was examined. It was observed that CO and CO2 were formed without nitrogen when the opposite side of diaphragm was exposed to air at high temperature. The dependence of the CO/CO2 ratio on the partial pressure of oxygen in the high PO2 compartment and on the thickness of diaphragm was examined.

Quantum Requirement of Photosynthesis in the Primarily Chlorophyll d Containing Prokaryote Acaryochloris marina

The recently isolated and characterized unicellular photosynthetic prokaryote Acaryochloris marina (Miyashita et al., 1996) contains chlorophylls a, d , and traces of a chlorophyll c-like pigment as well as phycocyanin. a type of allophycocyanin, zeaxanthin and cx-carotene, chlorophyll d being the predominant chlorophyll component. Quantum requirement measurements of the photosynthetic oxygen evolution resulted in about 12 quanta for excitation of chlorophylls a and d and 18 for phycocyanin. The data also revealed that these pigments are involved in energy absorption for photosynthetic oxygen evolution. Energy is transferred efficiently and equally well between the chlorophylls. Light absorbed by phycocyanin which is organized in phycobiliprotein aggregates (M arquardt et al., 1997), results in a less efficient energy transfer to the reaction center chlorophylls

Thermoluminescence studies on the photoinhibition of pothos leaf discs at chilling, room and high temperature

Photoinhibition in pothos leaf discs was studied by measurement of thermoluminescence inhibition and oxygen evolution. At chilling and high temperature (4 and 40 °C respectively), photoinhibition is at least 40%–50% greater than that at room temperature (25°C), suggesting greater protection from inhibition at room temperature. Flash excitation studies exhibit thermoluminescence oscillations in 50% photoinhibited and normal leaf discs, with the same periodicity of four, with maxima at the second and sixth flashes, suggesting normal functioning of the water oxidation complex in these samples. Of the four distinct thermoluminescence bands observed in pothos leaf discs, peak V (C band) is hardly photoinhibited even when the other bands show more than 80% inhibition. The lack of inhibition of peak V was further confirmed in leaf materials photoinhibited at different temperatures, indicating that this band is indeed resistant to photoinhibition. Oxygen evolution measurements in chloroplasts isolated from leaf discs photoinhibited for 30 and 60 min at 4, 25 and 40°C using dichlorobenzoquinone or ferricyanide as electron acceptor exhibit relatively higher inhibition with ferricyanide as acceptor. On the basis of these results, it is suggested that leaves under stress conditions show enhanced photoinhibition. The acceptor side of photosynthetic electron transport seems to be more susceptible to photoinhibition and the charge pair responsible for generating peak V appears to be insensitive to photoinhibition.

Photoinhibition in the Mediterranean green alga Acetabularia mediterranea measured in the field under solar irradiation

Photoinhibition of photosynthesis was investigated in the Mediterranean green macroalga Acetabularia mediterranea using pulse amplitude modulated (PAM) chlorophyll fluorescence and oxygen evolution measurements in situ under solar radiation. Fluorescence parameters measured during the course of the day showed that moderate photoinhibition occured even in the natural environment of this alga when the sun was at high angles, but photosynthetic capacity was almost fully restored until sunset. A drastic decline in effective quantum yield was monitored when plants collected at 5 m depth were exposed to high irradiance close to the surface. Removal of the short wavelength band from solar radiation using cut-off filters revealed that UV has an overproportional inhibitory effect on photosynthesis. Especially the UV-B part of the spectrum contributes to photoinhibition whereas UV-A seems to be less effective in A. mediterranea. Total recovery of PAM chlorophyll fluorescence was obtained after 2 h of shading in all samples, indicating reversible photoinhibition rather than non-reversible photodamage. Oxygen evolution showed slower kinetics of inhibition.

Structural changes in photosystem II after treatment with the zero-length bifunctional cross-linker 1-ethyl-3-(3-dimethylaminopropyl)carbodi-imide: an electron microscopic study.

Two-dimensional (2D) crystals of photosystem II (PS II) treated with various concentrations of the zero-length crosslinker 1-ethyl-3-(3-dimethylaminopropyl))carbodi-imide (EDC) were analysed by electron microscopy in conjunction with crystallographic image processing. The preparations were characterized by SDS/PAGE and oxygen-evolution measurements, and the effectiveness of cross-linking was monitored by measuring the level of protection afforded against high concentrations of NaCl and CaCl2, which normally remove extrinsic proteins from PS II. We found that low concentrations of EDC (0.25%) increase the order of 2D crystals of PS II. Treatments with EDC concentrations higher than 0.5% did not improve the order of 2D crystals but induced gross structural changes, which were correlated with a decrease in oxygen evolution activity. Structural changes due to cross-linking did not affect packing or symmetry of the 2D crystals, further supporting the conclusion that PS II has a monomeric nature in vivo.

Complementary chromatic adaptation alters photosynthetic strategies in the cyanobacterium Calothrix.

The cyanobacterium Calothrix sp. strain PCC 7601 drastically changes phycobiliprotein composition and colour in response to light quality, through complementary chromatic adaptation (CCA). Red light promotes phycocyanin-II and inhibits phycoerythrin synthesis, while green light has the opposite effect, through changes in transcription regulated by a putative green/red photoreceptor(s). The effects of CCA on photosynthesis were characterized by measuring oxygen evolution and chlorophyll fluorescence parameters. Cells fully acclimated to either red or green light achieve a similar photosynthetic quantum yield of oxygen evolution (light-use efficiency). Shifting acclimated cells from green to red or from red to green light caused similar 40% drops in photosynthetic quantum yield. Therefore, full CCA significantly increases light use efficiency, which is of great importance under light-limited growth. Cells growing under red light are in state I, with very low PS II to PS I energy transfer, since red light is absorbed both by phycocyanin in the phycobilisome/PS II supracomplex and by PS I chlorophyll. Cells growing under green light are in state II, with high transfer of excitation energy from the phycobilisome/PS II supracomplex to PS I. This transfer allows green light captured by phycoerythrin to ultimately drive both PS I and PS II photochemistry.

Photosynthetic and Stomatal Responses to Variable Light in a Cool-Season and a Warm-Season Prairie Forb

Species differences in photosynthetic and stomatal responses to steady-state and variable light were examined in two co-occurring tallgrass prairie forbs, the cool-season legume Baptisia bracteata var. glabrescens and the warm-season composite Helianthus annuus. Previous studies indicated that these species might have similar responses to short-term, minutes-long shade because of their similar growth forms. However, photosynthetic carbon gain, oxygen evolution, transpiration, and leaf xylem pressure potential measurements showed that Helianthus was far more responsive than Baptisia to changes in light availability. Helianthus had higher photosynthetic capacity, photosynthetic temperature optimum, stomatal conductance, and transpiration rates, rapid stomatal closure during shade, and delayed photosynthetic recovery when light levels increased, traits common to species exposed to high temperatures or periodic drought stress. Baptisia, active under cooler, wetter conditions than Helianthus, had lower photosynthetic capacity, photosynthetic temperature optimum, stomatal conductance, and transpiration, and no stomatal response to shade, responses typifying species that experience little water stress. We suggest that environmental and physiological factors may combine to reinforce greatly different photosynthetic and stomatal responses to short-term shade in species with similar growth form, especially in habitats with long, seasonally varying growing conditions.

Stoichiometries of Photosystem I and Photosystem II in Rice Mutants Differently Deficient in Chlorophyll b

Stoichiometries of photosystem I (PSI) and photosystem II (PSII) reaction centers in a cultivar of rice, Norin No. 8, and three chlorophyll b-deficient mutants derived from the cultivar were investigated. Quantitation of PSI by photooxidation of P-700 and chromatographic assay of vitamin K1 showed that, on the basis of chlorophyll, the mutants have higher concentrations of PSI than the wildtype rice. Greater increases were observed in the PSII contents measured by photoreduction of QA, binding of a radioactive herbicide and atomic absorption spectroscopy of Mn. Consequently, the PSII to PSI ratio increased from 1.1–1.3 in the wild-type rice to 1.8 in chlorina 2, which contains no Chl b, and to 2.0–3.3 in chlorina 11 and chlorina 14, which have chlorophyll a/b ratios of 9 and 13, respectively. Measurement of oxygen evolution with saturating single-turnover flashes revealed that, whereas at most 20% of PSII centers are inactive in oxygen evolution in the wildtype rice, the non-functional PSII centers amount to about 50% in the three mutant strains. The fluorescence induction kinetics was also analyzed to estimate proportions of the inactive PSII in the mutants. The data obtained suggest that plants have an ability to adjust the stoichiometry of the two photosystems and the functional organization of PSII in response to the genetically induced deficiency of chlorophyll b.

Properties of the chloride-depleted oxygen-evolving complex of photosystem II studied by electron paramagnetic resonance.

The effects of different Cl- depletion treatments in photosystem II (PS-II)-enriched membranes have been investigated by electron paramagnetic resonance (EPR) spectroscopy and by measurements of oxygen-evolving activity. The results indicated that the oxygen-evolving complex of PS-II exhibits two distinct Cl(-)-dependent properties. (1) After Cl(-)-free washes at pH 6.3, a reversibly altered distribution of structural states of PS-II was observed, manifested as the appearance of a g = 4 EPR signal from the S2 state in a significant fraction of centers (20-40%) at the expense of the S2 multiline signal. In addition, small but significant changes in the shape of the S2 multiline EPR signal were observed. Reconstitution of Cl- to Cl(-)-free washed PS-II rapidly reversed the observed effects of the Cl(-)-free washing. The anions, SO4(2-) and F-, which are often used during Cl- depletion treatments, had no effect on the S2 EPR properties of PS-II under these conditions in the absence or presence of Cl-. Flash experiments and measurements of oxygen evolution versus light intensity indicated that the two structural states observed after the removal of Cl- at pH 6.3 originated from oxygen-evolving centers exhibiting a lowered quantum yield of water oxidation. (2) Depletion of Cl- in PS-II by pH 10 treatment reversibly inhibited the oxygen-evolving activity to approximately 15%. The pH 10 treatment depleted the Cl- from a site which is considered to be equivalent to that studied in most earlier work on Cl(-)-depleted PS-II. The S2 state in pH 10/Cl(-)-depleted PS-II was reversibly modified to a state from which no S2 multiline EPR signal was generated and which exhibited an intense S2 g = 4 EPR signal corresponding to at least 40% of the centers but possibly to a much larger fraction of centers. The state responsible for the intense S2 g = 4 signal generated under these conditions is unlike that observed after removal of Cl- from PS-II at pH 6.3, in that this state was more stable in the dark, showing a half-decay time of approximately 1.5 h at 0 degrees C, and was unable to undergo further charge accumulation. Nevertheless, a fraction of centers, probably different from those exhibiting the S2 g = 4 signal, was able to advance to the formal S3 state, giving rise to a narrow EPR signal around g = 2. Addition of the anions SO4(2-) or F- to pH 10/Cl(-)-depleted PS-II affected the properties of PS-II, resulting in EPR properties of the S2 state similar to those reported earlier following Cl- depletion treatment of PS-II in the presence of these anions. Surprisingly, after addition of F-, the g = 4 EPR signal showed a damped flash-dependent oscillation. In addition, a narrow signal around g = 2, corresponding to the formal S3 state, also showed a damped flash-dependent oscillation pattern. The presence of oscillating EPR signals (albeit damped) in F(-)-treated pH 10/Cl(-)-depleted PS-II indicates functional enzyme turnover. This was confirmed by measurements of the oxygen-evolving activity versus light intensity which indicated that in approximately 45% of oxygen-evolving centers the enzyme turnover was slowed by a factor of 2. The distinct Cl- depletion effects in PS-II observed under the two different Cl- depletion treatments are considered to reflect the presence of two distinct Cl(-)-binding sites in PS-II.

Dynamic responses of photosystem II and phycobilisomes to changing light in the cyanobacterium Synechococcus sp. PCC 7942

We have examined the molecular and photosynthetic responses of a planktonic cyanobacterium to shifts in light intensity over periods up to one generation (7 h). Synechococcus sp. PCC 7942 possesses two functionally distinct forms of the D1 protein, D1∶1 and D1∶2. Photosystem II (PSII) centers containing D1∶1 are less efficient and more susceptible to photoinhibition than are centers containing D 1∶2. Under 50 μmol photons· m−2·s−1, PSII centers contain D1∶1, but upon shifts to higher light (200 to 1000 μmol photons·m−2·s−1), D1∶1 is rapidly replaced by D 1∶2, with the rate of interchange dependent on the magnitude of the light shift. This interchange is readily reversed when cells are returned to 50 μmol photons·m−2·s−1. If, however, incubation under 200 μmol photons·m−2·s−1 is extended, D1∶1 content recovers and by 3 h after the light shift D1∶1 once again predominates. Oxygen evolution and chlorophyll (Chl) fluorescence measurements spanning the light shift and D1 interchanges showed an initial inhibition of photosynthesis at 200 μmol photons·m−2·s−1, which correlates with a proportional loss of total D1 protein and a cessation of growth. This was followed by recovery in photosynthesis and growth as the maximum level of D 1∶2 is reached after 2 h at 200 μmol photons·m−2·s−1. Thereafter, photosynthesis steadily declines with the loss of D1∶2 and the return of the less-efficient D1∶1. During the D1∶1/D1∶2 interchanges, no significant change occurs in the level of phycocyanin (PC) and Chl a, nor of the phycobilisome rod linkers. Nevertheless, the initial PC/Chl a ratio strongly influences the magnitude of photo inhibition and recovery during the light shifts. In Synechococcus sp. PCC 7942, the PC/Chl a ratio responds only slowly to light intensity or quality, while the rapid but transient interchange between D1∶1 and D 1∶2 modulates PSII activity to limit damage upon exposure to excess light.

Photosynthetic responses of the seagrass Halophila ovalis (R. Br.) Hook. f. to high irradiance stress, using chlorophyll a fluorescence

With the increasing threat of destruction to diminishing seagrass resources, there is a need for an accurate stress monitoring procedure to be developed for species of this community. In this study chlorophyll a fluorescence (20°C) was used to monitor the stress responses of the seagrass Halophila ovalis (R. Br.) Hook. f. when exposed to increased irradiance. Leaf tissue was exposed to the following light treatments: 100, 500 and 1000 μmol m −2 s −1 for 10, 20, 40, 60 and 120 min. Exposure to the higher irradiances resulted in significant photoinhibitory responses for both fluorescence and oxygen evolution measurements. There was evidence of both photoinhibitory responses; photoprotection and photodamage. Photoprotective processes appear to be operating in all 500 μmol m −2 s −1 treatments, and in the 1000 μmol m −2 s −1 treatments for at least the first 60 min. Responses included an elevated initial fluorescence, accompanied by a reduction in the variable/maximum fluorescence ratio, maximum fluorescence and variable fluorescence. Photodamage appeared to be involved after 120 min exposure at 1000 μmol m −2 s −1, where all fluorescence parameters including initial fluorescence were quenched and the maximum oxygen evolution rate saturated at a lower irradiance. A direct linear relationship between variable/maximum fluorescence ratio and the light saturated photosynthetic rate was demonstrated for H. ovalis. The oxygen evolution data corroborate the evidence of the stress responses indicated with the fluorescence results. Generally, the longer the exposure period or higher the irradiance, the greater the damage to the tissue and the associated stress symptoms. These results demonstrate the potential of chlorophyll a fluorescence to assess the relative photoinhibitory response of H. ovalis to short-term exposure to increased irradiance.