Diurnal changes in the xanthophyll cycle pigments of freshwater algae correlate with the environmental hydrogen peroxide concentration rather than non-photochemical quenching

Background and Aims In photosynthetic organisms exposure to high light induces the production of reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂), which in part is prevented by non-photochemical quenching (NPQ). As one of the most stable and longest-lived ROS, H₂O₂ is involved in key signalling pathways in development and stress responses, although in excess it can induce damage. A ubiquitous response to high light is the induction of the xanthophyll cycle, but its role in algae is unclear as it is not always associated with NPQ induction. The aim of this study was to reveal how diurnal changes in the level of H₂O₂ are regulated in a freshwater algal community.Methods A natural freshwater community of algae in a temporary rainwater pool was studied, comprising photosynthetic Euglena species, benthic Navicula diatoms, Chlamydomonas and Chlorella species. Diurnal measurements were made of photosynthetic performance, concentrations of photosynthetic pigments and H₂O₂. The frequently studied model organisms Chlamydomonas and Chlorella species were isolated to study photosynthesis-related H₂O₂ responses to high light.Key Results NPQ was shown to prevent H₂O₂ release in Chlamydomonas and Chlorella species under high light; in addition, dissolved organic carbon excited by UV-B radiation was probably responsible for a part of the H₂O₂ produced in the water column. Concentrations of H₂O₂ peaked at 2 µm at midday and algae rapidly scavenged H₂O₂ rather than releasing it. A vertical H₂O₂ gradient was observed that was lowest next to diatom-rich benthic algal mats. The diurnal changes in photosynthetic pigments included the violaxanthin and diadinoxanthin cycles; the former was induced prior to the latter, but neither was strictly correlated with NPQ.Conclusions The diurnal cycling of H₂O₂ was apparently modulated by the organisms in this freshwater algal community. Although the community showed flexibility in its levels of NPQ, the diurnal changes in xanthophylls correlated with H₂O₂ concentrations. Alternative NPQ mechanisms in algae involving proteins of the light-harvesting complex type and antioxidant protection of the thylakoid membrane by de-epoxidized carotenoids are discussed.     

The Kok Effect in Chlamydomonas reinhardi

A Haxo-Blinks rate-measuring oxygen electrode together with a modulated light source gave an average current signal (change in net O₂ exchange) and a modulated current signal (photosynthetic O₂ evolution). Using this apparatus, net O₂ exchange and photosynthetic O₂ evolution at low intensities have been studied in the green alga, Chlamydomonas reinhardi. At both 645 nm and 695 nm, the curves of net O₂ exchange as a function of light intensity were steeper at lowest intensities than about compensation, indicative of the Kok effect. The effect was greater at 695 nm than at 645 nm. The corresponding curves of photosynthetic O₂ evolution, on the other hand, showed no Kok effect; here, the slope was lowest at lowest intensity. The absence of the Kok effect in O₂ evolution, together with its sensitivity to monofluoroacetic acid, show that it is due to an interaction of photosynthesis and respiration. The effect was exaggerated by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. In the presence of concentrations of this inhibitor sufficient to inhibit O₂ evolution completely, a light-induced change in net O₂ exchange remained. This was interpreted as a system I dependent depression of respiratory O₂ uptake. The Kok effect remained undiminished in concentrations of carbonyl cyanide m-chlorophenylhydrazone and 2,4-dinitrophenol which partially uncoupled either oxidative phosphorylation alone or both oxidative and photosynthetic phosphorylations. The above results can be explained within a model of the Kok effect in which O₂ uptake is depressed by diversion of reductant away from respiratory electron transport and into photosystem I. The same photodepression of O₂ uptake also appears to account for a transient in net O₂ exchange seen in several algae upon turning off the light.     

Effects of pH and Oxygen on Photosynthetic Reactions of Intact Chloroplasts

Oxygen inhibition of photosynthesis was studied with intact spinach (Spinacia oleracea L.) chloroplasts which exhibited very high rates of photosynthetic CO₂ reduction and were insensitive to additions of photosynthetic intermediates when CO₂ was available at saturating concentrations. Photosynthetic rates were measured polarographically as O₂ evolution, and the extent of the reduction of substrate was estimated from the amount of O₂ evolved. With CO₂ as substrate, inhibition of photosynthesis by O₂ was dependent on pH. At pH values above 8, rates of O₂ evolution were strongly inhibited by O₂ and only a fraction of the added bicarbonate was reduced before O₂ evolution ceased. The extent of O₂ evolution declined with increasing O₂ concentration and decreasing initial bicarbonate concentration. At pH 7.2, the initial photosynthetic rate was inhibited about 30% at high O₂ levels, but the extent of O₂ evolution was unaffected and most of the added bicarbonate was reduced. Photosynthetic O₂ evolution with 3-phosphoglycerate as substrate was similarly dependent on pH and O₂ concentration. In contrast, there was little effect of O₂ and pH on oxaloacetate-dependent oxygen evolution. Acid-base shift experiments with osmotically shocked chloroplasts showed that ATP formation was not affected by O₂. The results are discussed in terms of a balance between photosynthetic O₂ evolution and O₂ consumption by the ribulose diphosphate oxygenase reaction.     

Adaptation to quantum flux by the emerson photosynthetic unit

The size of the Emerson photosynthetic unit was measured in Chlorella pyrenoidosa strain no. 252 grown at light intensities between 50 and 1000 foot candles. The Emerson photosynthetic unit changed from a minimum size of 1970 molecules chlorophyll a + b/O₂ per flash in cells grown at 1000 foot candles to a maximum size of 3150 molecules chlorophyll a + b/O₂ per flash for cells grown at 50 foot candles. The size changes were interpreted as a partial adaptation where the trapping center antenna responded to changes in incident light intensity. Light-induced changes in chlorophyll content and size of the Emerson photosynthetic unit were directly related.     

An improved method for the simultaneous determination of photosynthetic O2 evolution and CO2 consumption in Rhizophora mucronata leaves

The photosynthetic gas-exchange has been assessed traditionally either as O₂ evolution or CO₂ consumption. In this study, we used a liquid-phase O₂ electrode combined with CO₂ optodes to examine simultaneously photosynthesis in intact leaves of mangrove Rhizophora mucronata. We verified suitable conditions for leaf photosynthetic rates by assessing pH levels and NaHCO₃ concentrations and compared these to the gas-exchange method at various PAR levels. The photosynthetic rate in response to pH exhibited a similar pattern both for O₂ evolution and CO₂ consumption, and higher rates were associated with intermediate pH compared with low and high pH values. The net photosynthetic quotient (PQ) of R. mucronata leaves ranged from 1.04–1.28. The PQ values, which were never lesser than 1, suggested that photorespiration did not occur in R. mucronata leaves under aqueous conditions. The similar maximum photosynthetic rates suggested that all measurements had a high capacity to adjust the photosynthetic apparatus under a light saturation condition. The simultaneous measurements of O₂ evolution and CO₂ consumption using the Clark oxygen electrode polarographic sensor with the CO₂ optode sensor provided a simple, stable, and precise measurement of PQ under aqueous and saturated light conditions.     

Assay of photosynthetic oxygen evolution from single protoplasts

A semiquantitative assay for light-dependent O₂ evolution by a single mesophyll protoplast is described. The assay indicator is the density of aerotactic bacteria (Pseudomonas aeruginosa, ATCC 10145; `Engelmann experiment') attracted to the protoplast. Quantification is by dark field microphotometry. The sensitivity is about 50 femtomoles O₂ per protoplast per minute. The results demonstrate the biphasic nature of O₂ evolution of a single protoplast during photosynthetic induction. Computerized data acquisition yields traces which, until a steady state of photosynthetic O₂ evolution is reached, are identical to ordinary O₂ electrode traces.     

A Photorespiratory Mutant of Chlamydomonas reinhardtii

A mutant strain of Chlamydomonas reinhardtii, designated 18-7F, has been isolated and characterized. 18-7F requires a high CO₂ concentration for photoautrophic growth in spite of the apparent induction of a functional CO₂ concentrating mechanism in air-adapted cells. In 2% O₂ the photosynthetic characteristics of 18-7F and wild type are similar. In 21% O₂, photosynthetic O₂ evolution is severely inhibited in the mutant by preillumination in limiting CO₂, although the apparent photosynthetic affinity for inorganic carbon is similar in preilluminated cells and in cells incubated in the dark prior to O₂ evolution measurements. Net CO₂ uptake is also inhibited when the cells are exposed to air (21% O₂, 0.035% CO₂, balance N₂) for longer than a few minutes. [¹⁴C]Phosphoglycolate accumulates within 5 minutes of photosynthetic ¹⁴CO₂ fixation in cells of 18-7F. Phosphoglycolate does not accumulate in wild type. Phosphoglycolate phosphatase activity in extracts from air-adapted cells of 18-7F is 10 to 20% of that in wild-type Chlamydomonas. The activity of phosphoglycolate phosphatase in heterozygous diploids is intermediate between that of homozygous mutant and wild-type diploids. It was concluded that the high-CO₂ requiring phenotype in 18-7F results from a phosphoglycolate phosphatase deficiency. Genetic analyses indicated that this deficiency results from a single-gene, nuclear mutation. We have named the locus pgp-1.     

Photosynthetic oxygen production and the size of the photosynthetic unit in a cell-free preparation from Cyanidium caldarium

A cell-free preparation has been isolated from a mutant of Cyanidium caldarium, grown under conditions such that there is 15 times less chlorophyll per photosynthetic unit than in normal green algae. The preparation is sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea and shows the well-characterized oscillation of O₂ yield, from saturating flashes, following a period of dark adaptation. Greening experiments with dark-grown, wild-type Cyanidium show that the synthesis of photosynthetic units precedes that of bulk chlorophyll and that the O₂-producing system is assembled before the total system coupled to CO₂. No large-scale cooperation of chlorophyll molecules is required for O₂ production.     

Regulation of Soybean Net Photosynthetic CO(2) Fixation by the Interaction of CO(2), O(2), and Ribulose 1,5-Diphosphate Carboxylase

Kinetic properties of soybean net photosynthetic CO₂ fixation and of the carboxylase and oxygenase activities of purified soybean (Glycine max [L.] Merr.) ribulose 1, 5-diphosphate carboxylase (EC 4.1.1.39) were examined as functions of temperature, CO₂ concentration, and O₂ concentration. With leaves, O₂ inhibition of net photosynthetic CO₂ fixation increased when the ambient leaf temperature was increased. The increased inhibition of CO₂ fixation at higher temperatures was caused by a reduced affinity of the leaf for CO₂ and an increased affinity of the leaf for O₂. With purified ribulose 1,5-diphosphate carboxylase, O₂ inhibition of CO₂ incorporation and the ratio of oxygenase activity to carboxylase activity increased with increased temperature. The increased O₂ sensitivity of the enzyme at higher temperature was caused by a reduced affinity of the enzyme for CO₂ and a slightly increased affinity of the enzyme for O₂. The similarity of the effect of temperature on the affinity of intact leaves and of ribulose 1,5-diphosphate carboxylase for CO₂ and O₂ provides further evidence that the carboxylase regulates the O₂ response of photosynthetic CO₂ fixation in soybean leaves. Based on results reported here and in the literature, a scheme outlining the stoichiometry between CO₂ and O₂ fixation in vivo is proposed.     

Inhibition of photosynthesis by oxygen in isolated spinach chloroplasts

The inhibition of photosynthetic CO₂ fixation by O₂, commonly referred to as the Warburg effect, was examined in isolated intact spinach (Spinacia oleracea) chloroplasts. The major characteristics of this effect in isolated chloroplasts are rapid reversibility when O₂ is replaced by N₂, an increased inhibition by O₂ at low concentrations of CO₂ and a decreased effect of O₂ with increased concentrations of CO₂.     

Effects of CO(2) and O(2) on the Photosynthetic O(2) Evolution of Spirodela polyrrhiza Turions

Net photosynthetic rates of Spirodela polyrrhiza turions, at low O₂ levels, were 6.2 and 38.8 micromoles O₂ per gram fresh weight per hour at 1 millimolar HCO₃⁻ and CO₂ saturation, respectively, and much lower in a regular low-pH growth solution. Air equilibration O₂ concentrations decreased rates considerably, except at CO₂ saturation. The surfacing rate of turions in various inorganic carbon surroundings correlated positively with their photosynthetic rates, but were the same at high and low O₂ levels. The relevance of these findings in relation to environmental conditions conductive to germination of autotrophically growing turions is discussed.     

Effects of O(2) and CO(2) Concentration on the Steady-State Fluorescence Yield of Single Guard Cell Pairs in Intact Leaf Discs of Tradescantia albiflora: Evidence for Rubisco-Mediated CO(2) Fixation and Photorespiration in Guard Cells

A procedure for following changes in the steady-state yield of chlorophyll a fluorescence (F_s) from single guard cell pairs in variegated leaves of Tradescantia albiflora is described. As an indicator of photosynthetic electron transport, F_s is a very sensitive indirect measure of the balance of adenosine 5′-triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), producing reactions with the sink reactions that utilize those light-generated products. We found that F_s under constant light is sensitive to manipulation of ambient CO₂ concentrations, as would be expected if either phosphoenolpyruvate carboxylase or ribulose-1, 5 bisphosphate carboxylase/oxygenase (Rubisco)-dependent CO₂ fixation is the sink for photosynthetic ATP and NADPH in guard cells. However, we also found that changing O₂ concentration had a strong effect on fluorescence yield, and that O₂ sensitivity was only evident when the concentration of CO₂ was low. This finding provides evidence that both O₂ and CO₂ can serve as sinks for ATP and NADPH produced by photosynthetic electron transport in guard cell chloroplasts. Identical responses were observed with mesophyll cell chloroplasts in intact leaves. This finding is difficult to reconcile with the view that guard cell chloroplasts have fundamentally different pathways of photosynthetic metabolism from other chloroplasts in C₃ plants. Indeed, Rubisco has been detected at low levels in guard cell chloroplasts, and our studies indicate that it is active in the pathways for photosynthetic carbon reduction and photorespiration in guard cells.     

On the Mechanism of Resistance to Paraquat in Hordeum glaucum and H. leporinum: Delayed Inhibition of Photosynthetic O(2) Evolution after Paraquat Application

The mechanism of resistance to paraquat was investigated in biotypes of Hordeum glaucum Steud. and H. leporinum Link. with high levels of resistance. Inhibition of photosynthetic O₂ evolution after herbicide application was used to monitor the presence of paraquat at the active site. Inhibition of photosynthetic O₂ evolution after paraquat application was delayed in both resistant biotypes compared with the susceptible biotypes; however, this differential was more pronounced in the case of H. glaucum than in H. leporinum. Similar results could be obtained with the related herbicide diquat. Examination of the concentration dependence of paraquat-induced inhibition of O₂ evolution showed that the resistant H. glaucum biotype was less affected by herbicide compared with the susceptible biotype 3 h after treatment at most rates. The resistant H. leporinum biotype, in contrast, was as inhibited as the susceptible biotype except at the higher rates. In all cases photosynthetic O₂ evolution was dramatically inhibited 24 h after treatment. Measurement of the amount of paraquat transported to the young tissue of these plants 24 h after treatment showed 57% and 53% reductions in the amount of herbicide transported in the case of the resistant H. glaucum and H. leporinum biotypes, respectively, compared with the susceptible biotypes. This was associated with 62% and 66% decreases in photosynthetic O₂ evolution of young leaves in the susceptible H. glaucum and H. leporinum biotypes, respectively, a 39% decrease in activity for the resistant H. leporinum biotype, but no change in the resistant H. glaucum biotype. Photosynthetic O₂ evolution of leaf slices from resistant H. glaucum was not as inhibited by paraquat compared with the susceptible biotype; however, those of resistant and susceptible biotypes of H. leporinum were equally inhibited by paraquat. Paraquat resistance in these two biotypes appears to be a consequence of reduced movement of the herbicide in the resistant plants; however, the mechanism involved is not the same in H. glaucum as in H. leporinum.     

Mode of Action Studies on Nitrodiphenyl Ether Herbicides : II. The Role of Photosynthetic Electron Transport in Scenedesmus obliquus

The nitrodiphenyl ether herbicide 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitroacetophenone oxime-o-(acetic acid, methyl ester) (DPEI) induces light- and O₂-dependent lipid peroxidation and chlorophyll (Chl) bleaching in the green alga Scenedesmus obliquus. Under conditions of O₂-limitation, these effects are diminished by prometyne and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), both inhibitors of photosynthetic electron transport. Mutants in which photosynthetic electron transport is blocked are also resistant to DPEI under conditions of O₂-limitation. Light- and O₂-dependent lipid peroxidation and Chl bleaching are also induced by 5-[2-chloro-4-(trifluoromethyl)phenoxy]-3-methoxyphthalide (DPEII), a diphenyl ether whose redox properties preclude reduction by photosystem I. However, these effects of DPEII are also inhibited by DCMU. Under conditions of high aeration, DCMU does not protect Scenedesmus cells from Chl bleaching induced by DPEI, but does protect against paraquat. DPEI, but not paraquat, induces tetrapyrrole formation in treated cells in the dark. This is also observed in a mutant lacking photosystem I but is suppressed under conditions likely to lead to O₂ limitation. Our results indicate that, in contrast to paraquat, the role of photosynthetic electron transport in diphenyl ether toxicity in Scenedesmus is not to reduce the herbicide to a radical species which initiates lipid peroxidation. Its role is probably to maintain a sufficiently high O₂ concentration, through water-splitting, in the algal suspension.     

Photosynthetic Unit Size during the Synchronous Life Cycle of Scenedesmus

Apparent size of the photosynthetic unit (chlorophyll/O₂ per flash) was estimated by O₂ yield of repetitive short flashes on cell samples taken at various times from a synchronized culture (14-hour light, 10-hour dark) of Scenedesmus obliquus. Unit size was essentially invariant (< 10% variation) with a mean value of 1750 chlorophyll/O₂ per flash. In contrast, the light-saturated photosynthetic rate per unit chlorophyll, or turnover rate of the photosynthetic unit, varied with the life cycle, rising 40% in the first three hours of the light period and decaying slowly thereafter. The results are taken as evidence that the metabolic machinery is subject to far greater control and adjustment than is the photochemical machinery.     

Photosynthetic light-response curves

The shapes of photosynthetic light-response curves for leaves of Eucalyptus maculata (Hook) and E. pauciflora (Sieber ex Sprengel) were examined. Three different methods were used to measure photosynthesis: CO₂ and H₂O-vapour exchange, O₂ evolution at a 5-kPa CO₂ partial pressure, and chlorophyll fluorescence. The three methods were compared and gave good agreement when measured under equivalent conditions. However, O₂ evolution was inhibited by high CO₂ partial pressures. A non-rectangular hyperbolic curve has been used widely to describe photosynthetic light-response curves. It has three variables which define the maximum quantum yield (photosynthetic rate divided by absorbed irradiance at very low irradiances), the maximum capacity and the curvature (Θ). We found that Θ was affected by the CO₂ partial pressure, declining to a minimum of about 0.6 as CO₂ partial pressure increased to 100 Pa. Further increases in the CO₂ partial pressure began to inhibit the rate of O₂ evolution at 2000 μmol quanta · m^?2·^?1 and Θ increased back to 0.95 by 5 kPa CO₂ partial pressure. At low irradiances, photosynthesis is limited by the rate of electron transport while at high irradiances, photosynthesis is frequently limited by the activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco). The dependence of Θ on CO₂ partial pressure arises because the transition between limitations changes as a function of the CO₂ partial pressure. The light-response curve is truncated by the transition to a Rubisco limitation and the lower the irradiance at the transition, the higher the value of Θ. There is a gradient in light absorption through the leaf which influences the photosynthetic capacity of different layers within the leaf. The gradient in photosynthetic capacity can be demonstrated by the fact that the shape of the light-response curve changes when the leaf is illuminated unilaterally onto either the adaxial or abaxial surface. We compared two Eucalyptus species which had either isolateral or dorsiventral leaf anatomy. Leaves were able to reverse completely the gradients in photosynthetic capacity following inversion of the leaves for a week, irrespective of their anatomy.     

Photosynthesis and photorespiration in a mutant of the cyanobacterium Synechocystis PCC 6803 lacking carboxysomes

A mutant of the cyanobacterium Synechocystis PCC 6803 was obtained by replacing the gene of the carboxylation enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) with that of the photosynthetic bacterium Rhodospirillum rubrum. This mutant consequently lacks carboxysomes — the protein complexes in which the original enzyme is packed. It is incapable of growing at atmospheric CO₂ levels and has an apparent photosynthetic affinity for inorganic carbon (C_i) which is 1000 times lower than that of the wild type, yet it accumulates more C_i than the wild type. The mutant appears to be defective in its ability to utilize the intracellular C_i pool for photosynthesis. Unlike the carboxysomal carboxylase activity of Rubisco, which is almost insensitive to inhibition by O₂ in vitro, the soluble enzyme is competitively inhibited by O₂. The photosynthetic rate and C_i compensation point of the wild type were hardly affected by low O₂ levels. Above 100 μM O₂, however, both parameters became inhibited. The CO₂ compensation point of the mutant was linearly dependent on O₂ concentration. The higher sensitivity of the mutant to O₂ inhibition than that expected from in-vitro kinetics parameters of Rubisco, indicates a low capacity to recycle photorespiratory metabolites to Calvin-cycle intermediates.     

Photooxidative Damage in Photosynthetic Activities of Chromatium vinosum

The capacity of photosynthetic CO₂ fixation in the anaerobic purple-sulfur bacterium, Chromatium vinosum is markedly impaired by strong illumination (9 × 10⁴ lux) in the presence of 100% O₂. In the absence of HCO₃⁻, decline in activity occurred gradually, with about 40% of the initial activity remaining after a 1-hour incubation. The addition of 50 millimolar HCO₃⁻ to the incubation medium resulted in a measurable delay (about 30 minutes) of the inactivation process. Ribulose-1,5-bisphosphate carboxylase activity and light-dependent O₂ uptake (electron flow) or crude extracts prepared after pretreatment of the bacterial cells with O₂ and light were not affected but the photophosphorylation capacity of either bacterial cells or chromatophores was drastically reduced. The inhibition of photophos-phorylation in the chromatophore preparations was significantly reduced by the addition of either an O₂⁻ scavenger, Tiron, or an ¹O₂ scavenger, α-tocopherol. These results suggest that the active O₂ species, O₂⁻ or ¹O₂, might take part in the observed inactivation.

The pretreatment of the bacteria with O₂ and light inhibited CO₂ assimilation through the Calvin-Benson cycle, while relatively stimulating the formation of aspartate and glutamate. It also inhibited the conversion of glycolate to glycine, resulting in a sustained extracellular excretion of glycolate. The inactivation of photosynthetic CO₂ fixation by intact cells was enhanced by low temperature, KCN, or methylviologen addition during the pretreatment with O₂ and light. The mechanism(s) of O₂-dependent photoinactivation of photosynthetic activities in Chromatium are discussed in relation to the possible role of photorespiration as a means of producing CO₂ in the photosynthetic system.

     

Effects of heat stress on photosynthetic electron transport in a marine cyanobacterium <Emphasis Type="Italic">Arthrospira</Emphasis> sp.

Arthrospira (Spirulina) is widely used as human health food and animal feed. In cultures grown outdoors in open ponds, Arthrospira cells are subjected to various environmental stresses, such as high temperature. A better understanding of the effects of high temperature on photosynthesis may help optimize the productivity of Arthrospira cultures. In this study, the effects of heat stress on photosynthetic rate, chlorophyll a fluorescence transients, and photosystem (PS) II, PSI activities in a marine cyanobacterium Arthrospira sp. were examined. Arthrospira cells grown at 25 °C were treated for 30 min at 25 (control), 30, 34, 37, or 40 °C in the dark. Heat stress (30–37 °C) enhanced net photosynthetic O₂ evolution rate. Heat stress caused over-reduction PSII acceptor side, damage of donor side of PSII, decrease in the energetic connectivity of PSII units, and decrease in the performance of PSII. When the temperature changed from 25 to 37 °C, PSII activity decreased, while PSI activity increased, the enhancement of photosynthetic O₂ evolution was synchronized with the increase in PSI activity. When temperature was further increased to 40 °C, it induced a decrease in photosynthetic O₂ evolution rate and a more severe decrease in PSII activity, but an increase in PSI activity. These results suggest that PSI activity was the decisive factor determining the change of photosynthetic O₂ evolution when Arthrospira was exposed to a temperature from 25 to 37 °C, but then, PSII activity became the decisive factor adjusting the change of photosynthetic O₂ evolution when the temperature was increased to 40 °C.