The Calvin Cycle and Its Regulation

Metagenomics and metatranscriptomics insights into microbial enhancement of H2S removal and CO2 assimilation

This study focuses on the coupled process of bio-enhanced absorption and biodesulfurization for the toxic gas H2S and the greenhouse gas CO2. The results show that on the basis of stabilized absorption of H2S and CO2 by alkaline solution (Stage I), the addition of air-lift bioreactor process solution in the absorption column enhanced their absorption (Stage II). Specifically, at constant inlet concentrations of H₂S and CO₂ of 3% (30,000 ppmv) and 30% (300,000 ppmv), respectively, the outlet gases were primarily H₂S, CO₂, and N₂. And the outlet H2S and CO2 concentrations decreased from 10,038 ± 1166 ppmv and 49,897 ± 2545 ppmv in Stage I to 940 ± 163 ppmv and 21,000 ± 2165 ppmv in Stage II. S0-producing performance (348 ± 20-503 ± 23 mg S/L) and biomass concentration (467 ± 13-677 ± 55 mg/L) in the subsequent bioreactor also increased in response to the enhanced absorption of H2S and CO2. Biologically enhanced H2S and CO2 absorption differs from physicochemical factors in that it depends on several physiological parameters such as microbial community composition and gene expression levels. In this study, the sulfur autotrophic denitrifying bacteria Thioalkalivibrio and Arenimonas had high abundance and activity (abundance: 69.5% and 21.1%, expression: 82.4% and 13.9%), and they were the main contributors to the bio-enhanced absorption of H2S and CO2 in this system. In addition, the main factor for enhanced H2S absorption could be the high expression of sulfide:quinone oxidoreductase (SQR, encoding gene sqr) (45 ± 9 to 821 ± 102 transcripts per million). Enhanced CO2 absorption could have been achieved by the oxidation of more H2S generating more energy to increase the carboxylation activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, encoding genes rbcLS). Enhanced H2S absorption enhances CO2 absorption and facilitates microbial growth, which in turn benefits the metabolism of H2S, creating a complementary biologically enhanced absorption. This study provides a novel strategy, demonstrating the potential of autotrophic sulfide-oxidizing microorganisms in the simultaneous removal of H₂S and assimilation of CO₂, and offers a deeper understanding of the underlying mechanisms.

Metabolite interactions in the bacterial Calvin cycle and implications for flux regulation

Metabolite-level regulation of enzyme activity is important for microbes to cope with environmental shifts. Knowledge of such regulations can also guide strain engineering for biotechnology. Here we apply limited proteolysis-small molecule mapping (LiP-SMap) to identify and compare metabolite-protein interactions in the proteomes of two cyanobacteria and two lithoautotrophic bacteria that fix CO2 using the Calvin cycle. Clustering analysis of the hundreds of detected interactions shows that some metabolites interact in a species-specific manner. We estimate that approximately 35% of interacting metabolites affect enzyme activity in vitro, and the effect is often minor. Using LiP-SMap data as a guide, we find that the Calvin cycle intermediate glyceraldehyde-3-phosphate enhances activity of fructose-1,6/sedoheptulose-1,7-bisphosphatase (F/SBPase) from Synechocystis sp. PCC 6803 and Cupriavidus necator in reducing conditions, suggesting a convergent feed-forward activation of the cycle. In oxidizing conditions, glyceraldehyde-3-phosphate inhibits Synechocystis F/SBPase by promoting enzyme aggregation. In contrast, the glycolytic intermediate glucose-6-phosphate activates F/SBPase from Cupriavidus necator but not F/SBPase from Synechocystis. Thus, metabolite-level regulation of the Calvin cycle is more prevalent than previously appreciated.

Proteomics Reveals Damaging Effect of Alpha-Cypermethrin Exposure in a Non-Target Freshwater Microalga Chlorella sp. NC-MKM

Alpha-cypermethrin, a pyrethroid pesticide, is frequently used on crops to prevent insect attacks. However, occasionally, due to drift, leaching, or with rainwater, it enters the aquatic environment and poses a serious threat to the growth of non-target aquatic organisms. In the current study, we were interested in investigating the damaging effect of alpha-cypermethrin on a local freshwater non-target green alga Chlorella sp. NC-MKM in terms of its protein levels. This was achieved by exposing Chlorella sp. NC-MKM to an EC₅₀ concentration of alpha-cypermethrin for 1 day, followed by the two-dimensional (2-D) gel electrophoresis and MALDI-TOF MS. Fifty-three proteins, which had showed significant differential accumulation (> 1.5 fold, P < 0.05) after exposure to alpha-cypermethrin, were considered as differentially accumulated proteins (DAPs). These DAPs were further divided into several functional categories, and the expressions of each in control and treatment samples were compared. Comparison revealed that alpha-cypermethrin exposure affects the accumulation of proteins related with photosynthesis, stress response, carbohydrate metabolism, signal transduction and transporters, translation, transcription, cell division, lipid metabolism, amino acid and nucleotide biosynthesis, secondary metabolites production, and post-translational modification, and thus rendered the tested algal isolate sensitive toward this pesticide. The overall findings of this research thus offer a fundamental understanding of the possible mechanism of action of the insecticide alpha-cypermethrin on the microalga Chlorella sp. NC-MKM and also suggest potential biomarkers for the investigation of pesticide exposed microalgae.

Evolution of an intermediate C4 photosynthesis in the non-foliar tissues of the Poaceae

Carbon concentrating mechanisms (CCMs) in plants are abaptive features that have evolved to sustain plant growth in unfavorable environments, especially at low atmospheric carbon levels and high temperatures. Uptake of CO₂ and its storage in the aerenchyma tissues of Lycopsids and diurnal acidity fluctuation in aquatic plants during the Palaeozoic era (ca. 300 Ma.) would represent the earliest evolution of a CCM. The CCM parts of the dark reactions of photosynthesis have evolved many times, while the light reactions are conserved across plant lineages. A C₄ type CCM, leaf C₄ photosynthesis is evolved in the PACMAD clade of the Poaceae family. The evolution of C₄ photosynthesis from C₃ photosynthesis was an abaptation. Photosynthesis in reproductive tissues of sorghum and maize (PACMAD clade) has been shown to be of a weaker C₄ type (high CO₂ compensation point, low carbon isotope discrimination, and lack of Rubisco compartmentalization, when compared to the normal C₄ types) than that in the leaves (normal C₄ type). However, this does not fit well with the character polarity concept from an evolutionary perspective. In a recent model proposed for CCM evolution, the development of a rudimentary CCM prior to the evolution of a more efficient CCM (features contrasting to a weaker C₄ type, leading to greater biomass production rate) has been suggested. An intermediate crassulacean acid metabolism (CAM) type of CCM (rudimentary) was reported in the genera, Brassia, Coryanthes, Eriopsis, Peristeria, of the orchids (well-known group of plants that display the CAM pathway). Similarly, we propose here the evolution of a rudimentary CCM (C₄-like type pathway) in the non-foliar tissues of the Poaceae, prior to the evolution of the C₄ pathway as identified in the leaves of the C₄ species of the PACMAD clade.

Effects of suppression of chloroplast phosphoglycerate kinase on photosynthesis in rice

As chloroplast phosphoglycerate kinase (cpPGK) is one of the enzymes which has the highest capacity among the Calvin-Benson cycle enzymes, it has not been regarded as a determinant for photosynthetic capacity. However, it was reported that the rate of CO₂ assimilation decreased under high irradiance and normal [CO₂] levels in the Arabidopsis cpPGK-knockdown mutant, implying that cpPGK has a control over photosynthetic capacity at a normal [CO₂] level. In the present study, the contribution of cpPGK to photosynthetic capacity was evaluated in transgenic rice plants with decreased amounts of cpPGK protein under high irradiance and various [CO₂] levels. The gene encoding cpPGK was suppressed using RNA interference techniques. Two lines of transgenic plants, Pi3 and Pi5, in which the amount of cpPGK protein decreased to 21% and 76% of that in wild-type plants, respectively, were obtained. However, there was no substantial difference in the rates of CO₂ assimilation between wild-type and transgenic plants. The rates of CO₂ assimilation decreased only slightly at elevated [CO₂] levels in the transgenic line Pi3 and did not differ between wild-type plants and the transgenic line Pi5, irrespective of [CO₂] level. These results clearly indicate that cpPGK does not have a strong control over photosynthetic capacity at various [CO₂] levels in rice.

Overproduction of Chloroplast Glyceraldehyde-3-Phosphate Dehydrogenase Improves Photosynthesis Slightly under Elevated [CO2] Conditions in Rice

Chloroplast glyceraldehyde-3-phosphate dehydrogenase (GAPDH) limits the regeneration of ribulose 1,5-bisphosphate (RuBP) in the Calvin-Benson cycle. However, it does not always limit the rate of CO2 assimilation. In the present study, the effects of overproduction of GAPDH on the rate of CO2 assimilation under elevated [CO2] conditions, where the capacity for RuBP regeneration limits photosynthesis, were examined in transgenic rice (Oryza sativa). GAPDH activity was increased to 3.2- and 4.5-fold of the wild-type levels by co-overexpression of the GAPDH genes, GAPA and GAPB, respectively. In the transgenic rice plants, the rate of CO2 assimilation under elevated [CO2] conditions increased by approximately 10%, whereas that under normal and low [CO2] conditions was not affected. These results indicate that overproduction of GAPDH is effective in improving photosynthesis under elevated [CO2] conditions, although its magnitude is relatively small. By contrast, biomass production of the transgenic rice plants was not greater than that of wild-type plants under elevated [CO2] conditions, although starch content tended to increase marginally.

Influence of nutrient formulations on growth, lipid yield, carbon partitioning and biodiesel quality potential of Botryococcus sp. and Chlorella sp

The study was conducted to analyse the influence of three nutrient formulations, namely BG-11 medium, BBM and TAP medium, on growth potential and lipid yield of two microalgal genera (Botryococcus sp. and Chlorella sp.) and to study the roles of N, P and other major nutrients. The study focussed on the general patterns of starch and lipid synthesis and storage and to further assess how photosynthetic carbon partitioning into starch and lipid is altered by conditions in growth media such as N and C presence as seen in BG11 medium which are known to induce neutral lipid production and the lack of it in BBM and TAP medium. BG-11 medium performed better as compared to BBM and TAP medium in terms of biomass productivity and lipid yield. The lipid yield was highest in Botryococcus sp. (63.03% dry wt.) and Chlorella sp. (50.27% dry wt.) at 30th day of incubation. Mean biomass productivity was highest for Botryococcus in BBM medium (6.14 mg/L/day) and for Chlorella in BG-11 medium (4.97 mg/L/day). Mean lipid productivity (50.78% and 39.36%) was highest in BG11 medium for both Botryococcus and Chlorella species, respectively. A sharp decline in sugar content was observed in the late stationary phase of growth from 30th day to 45th day. Fatty acid methyl ester (FAME) profile of the extracted lipids showed predominantly oleic acid, followed by palmitic acid and stearic acid in both the strains when grown in BG-11 medium. The other biodiesel quality parameters were in accordance with the international standards. A complex relationship was found between chemical composition and biodiesel properties. Proximity analysis indicated that the fuel properties of biodiesels are determined by a number of parameters and by the combination of different chemical compositions. The results provide an insight into organic carbon partitioning into lipid compounds and how the organism's lipid metabolism changes due to N-deplete culturing in TAP medium and inorganic carbon source availability as seen in BG-11 and BBM medium.

Photosynthetic and Stress Responsive Proteins Are Altered More Effectively in Nicotiana benthamiana Infected with Plum pox virus Aggressive PPV-CR versus Mild PPV-C Cherry-Adapted Isolates

Plum pox virus (PPV, family Potyviridae) is one of the most important viral pathogens of Prunus spp. causing considerable damage to stone-fruit industry worldwide. Among the PPV strains identified so far, only PPV-C, PPV-CR, and PPV-CV are able to infect cherries under natural conditions. Herein, we evaluated the pathogenic potential of two viral isolates in herbaceous host Nicotiana benthamiana. Significantly higher accumulation of PPV capsid protein in tobacco leaves infected with PPV-CR (RU-30sc isolate) was detected in contrast to PPV-C (BY-101 isolate). This result correlated well with the symptoms observed in the infected plants. To further explore the host response upon viral infection at the molecular level, a comprehensive proteomic profiling was performed. Using reverse-phase ultra-high-performance liquid chromatography followed by label-free mass spectrometry quantification, we identified 38 unique plant proteins as significantly altered due to the infection. Notably, the abundances of photosynthesis-related proteins, mainly from the Calvin-Benson cycle, were found more aggressively affected in plants infected with PPV-CR isolate than those of PPV-C. This observation was accompanied by a significant reduction in the amount of photosynthetic pigments extracted from the leaves of PPV-CR infected plants. Shifts in the abundance of proteins that are involved in stimulation of photosynthetic capacity, modification of amino acid, and carbohydrate metabolism may affect plant growth and initiate energy formation via gluconeogenesis in PPV infected N. benthamiana. Furthermore, we suggest that the higher accumulation of H2O2 in PPV-CR infected leaves plays a crucial role in plant defense and development by activating the glutathione synthesis.

Engineering photosynthesis: progress and perspectives

Photosynthesis is the basis of primary productivity on the planet. Crop breeding has sustained steady improvements in yield to keep pace with population growth increases. Yet these advances have not resulted from improving the photosynthetic process per se but rather of altering the way carbon is partitioned within the plant. Mounting evidence suggests that the rate at which crop yields can be boosted by traditional plant breeding approaches is wavering, and they may reach a “yield ceiling” in the foreseeable future. Further increases in yield will likely depend on the targeted manipulation of plant metabolism. Improving photosynthesis poses one such route, with simulations indicating it could have a significant transformative influence on enhancing crop productivity. Here, we summarize recent advances of alternative approaches for the manipulation and enhancement of photosynthesis and their possible application for crop improvement.

Regulation of the Calvin-Benson-Bassham cycle in the enigmatic diatoms: biochemical and evolutionary variations on an original theme

In Plantae, the Calvin-Benson-Bassham (CBB) cycle is highly regulated and most of its enzymes have been thoroughly studied. Since diatoms arose as a result of secondary endosymbiosis with one or more Plantae ancestors, their precise evolutionary history is enigmatic and complex resulting in biochemical variations on the original CBB cycle theme. The Rubisco Michaelis constant for CO2 is higher in diatoms than land plants and the nuclear-encoded Rubisco activase in Plantae is replaced by an analogous chloroplast-encoded CbbX (Calvin-Benson-Bassham protein X) in diatoms. In the CBB cycle reduction phase, phosphoglycerate kinase in diatoms is redox-regulated and similar to that in red algae; however, glyceraldehyde phosphate dehydrogenase (GAPDH) is not redox-regulated, unlike in Plantae. The phosphoribulokinase (PRK)-GAPDH-CP12 complex found in many photosynthetic organisms has not yet been found in diatoms, but a ferredoxin-NADP reductase (FNR)-GAPDH-CP12 complex has been found in one species. In the CBB cycle regeneration phase, sedoheptulose 1,7-bisphosphatase and PRK are not redox-regulated in diatoms, unlike in Plantae. Regulation at the transcriptional level seems to be important in diatoms. CBB cycle enzyme properties appear to be variable among diatoms, but this view relies on results from a few model species: a greater range of diatoms need to be studied to test this.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

The glucose 6-phosphate shunt around the Calvin-Benson cycle

It is just over 60 years since a cycle for the regeneration of the CO2-acceptor used in photosynthesis was proposed. In this opinion paper, we revisit the origins of the Calvin-Benson cycle that occurred at the time that the hexose monophosphate shunt, now called the pentose phosphate pathway, was being worked out. Eventually the pentose phosphate pathway was separated into two branches, an oxidative branch and a non-oxidative branch. It is generally thought that the Calvin-Benson cycle is the reverse of the non-oxidative branch of the pentose phosphate pathway but we describe crucial differences and also propose that some carbon routinely passes through the oxidative branch of the pentose phosphate pathway. This creates a futile cycle but may help to stabilize photosynthesis. If it occurs it could explain a number of enigmas including the lack of complete labelling of the Calvin-Benson cycle intermediates when carbon isotopes are fed to photosynthesizing leaves.

Linking phosphorus availability with photo-oxidative stress in plants

Plants have evolved a plethora of mechanisms to circumvent the potential damaging effects of living under low phosphorus availability in the soil. These mechanisms include different levels of organization, from root-shoot signalling at the whole-plant level to specific biochemical responses at the subcellular level, such as reductions in photosynthesis and the consequent activation of photo- and antioxidant mechanisms in chloroplasts. Some recent studies clearly indicate that severe phosphorus deficiency can lead to alterations in the photosynthetic apparatus, including reductions in CO2 assimilation rates, a down-regulation of photosynthesis-related genes and photoinhibition at the photosystem II level, thus causing potential photo-oxidative stress. Photo-oxidative stress is characterized by an increased production of reactive oxygen species in chloroplasts, which at low concentrations can serve a signalling, protective role, but when present at high concentrations can cause damage to lipids, proteins and nucleic acids, thus leading to irreversible injuries. We discuss here the mechanisms that phosphate-starved plants have evolved to withstand photo-oxidative stress, including changes at the subcellular level (e.g. activation of photo- and antioxidant protection mechanisms in chloroplasts), cellular and tissular levels (e.g. activation of photorespiration and anthocyanin accumulation) and whole-plant level (alterations in source-sink relationships modulated by hormones). Of particular importance is the current evidence demonstrating that phosphate-starved plants activate simultaneous responses at multiple levels, from transcriptional changes to root-shoot signalling, to prevent oxidative damage. In this review, we summarize current knowledge about the occurrence of photo-oxidative stress in phosphate-starved plants and highlight the mechanisms these plants have evolved to prevent oxidative damage under phosphorus limitation at the subcellular, cellular and whole-plant levels.

e-Photosynthesis: a comprehensive dynamic mechanistic model of C3 photosynthesis: from light capture to sucrose synthesis

Photosynthesis is arguably the most researched of all plant processes. A dynamic model of leaf photosynthesis that includes each discrete process from light capture to carbohydrate synthesis, e-photosynthesis, is described. It was developed by linking and extending our previous models of photosystem II (PSII) energy transfer and photosynthetic C3 carbon metabolism to include electron transfer processes around photosystem I (PSI), ion transfer between the lumen and stroma, ATP synthesis and NADP reduction to provide a complete representation. Different regulatory processes linking the light and dark reactions are also included: Rubisco activation via Rubisco activase, pH and xanthophyll cycle-dependent non-photochemical quenching mechanisms, as well as the regulation of enzyme activities via the ferredoxin-theoredoxin system. Although many further feedback and feedforward controls undoubtedly exist, it is shown that e-photosynthesis effectively mimics the typical kinetics of leaf CO₂ uptake, O₂ evolution, chlorophyll fluorescence emission, lumen and stromal pH, and membrane potential following perturbations in light, [CO₂] and [O₂] observed in intact C3 leaves. The model provides a framework for guiding engineering of improved photosynthetic efficiency, for evaluating multiple non-invasive measures used in emerging phenomics facilities, and for quantitative assessment of strengths and weaknesses within the understanding of photosynthesis as an integrated process.

Over-expressing the C(3) photosynthesis cycle enzyme Sedoheptulose-1-7 Bisphosphatase improves photosynthetic carbon gain and yield under fully open air CO(2) fumigation (FACE)

BACKGROUND

Biochemical models predict that photosynthesis in C(3) plants is most frequently limited by the slower of two processes, the maximum capacity of the enzyme Rubisco to carboxylate RuBP (V(c,max)), or the regeneration of RuBP via electron transport (J). At current atmospheric [CO(2)] levels Rubisco is not saturated; consequently, elevating [CO(2)] increases the velocity of carboxylation and inhibits the competing oxygenation reaction which is also catalyzed by Rubisco. In the future, leaf photosynthesis (A) should be increasingly limited by RuBP regeneration, as [CO(2)] is predicted to exceed 550 ppm by 2050. The C(3) cycle enzyme sedoheptulose-1,7 bisphosphatase (SBPase, EC 3.1.3.17) has been shown to exert strong metabolic control over RuBP regeneration at light saturation.

RESULTS

We tested the hypothesis that tobacco transformed to overexpressing SBPase will exhibit greater stimulation of A than wild type (WT) tobacco when grown under field conditions at elevated [CO(2)] (585 ppm) under fully open air fumigation. Growth under elevated [CO(2)] stimulated instantaneous A and the diurnal photosynthetic integral (A') more in transformants than WT. There was evidence of photosynthetic acclimation to elevated [CO(2)] via downregulation of V(c,max) in both WT and transformants. Nevertheless, greater carbon assimilation and electron transport rates (J and J(max)) for transformants led to greater yield increases than WT at elevated [CO(2)] compared to ambient grown plants.

CONCLUSION

These results provide proof of concept that increasing content and activity of a single photosynthesis enzyme can enhance carbon assimilation and yield of C(3) crops grown at [CO(2)] expected by the middle of the 21st century.

The GapA/B Gene Duplication Marks the Origin of Streptophyta (Charophytes and Land Plants)

Independent evidence from morphological, ultrastructural, biochemical, and molecular data have shown that land plants originated from charophycean green algae. However, the branching order within charophytes is still unresolved, and contradictory phylogenies about, for example,the position of the unicellular green alga Mesostigma viride are difficult to reconcile. A comparison of nuclear-encoded Calvin cycle glyceraldehyde-3-phosphate dehydrogenases (GAPDH) indicates that a crucial duplication of the GapA gene occurred early in land plant evolution. The duplicate called GapB acquired a characteristic carboxy-terminal extension (CTE) from the general regulator of the Calvin cycle CP12. This CTE is responsible for thioredoxin-dependent light/dark regulation. In this work, we established GapA, GapB, and CP12 sequences from bryophytes, all orders of charophyte as well as chlorophyte green algae, and the glaucophyte Cyanophora paradoxa. Comprehensive phylogenetic analyses of all available plastid GAPDH sequences suggest that glaucophytes and green plants are sister lineages and support a positioning of Mesostigma basal to all charophycean algae. The exclusive presence of GapB in terrestrial plants, charophytes, and Mesostigma dates the GapA/B gene duplication to the common ancestor of Streptophyta. The conspicuously high degree of GapB sequence conservation suggests an important metabolic role of the newly gained regulatory function. Because the GapB-mediated protein aggregation most likely ensures the complete blockage of the Calvin cycle at night, we propose that this mechanism is also crucial for efficient starch mobilization. This innovation may be one prerequisite for the development of storage tissues in land plants.

The peculiar distribution of class I and class II aldolases in diatoms and in red algae

Diatom plastids probably evolved by secondary endocytobiosis from a red alga that was up by a eukaryotic host cell. Apparently, this process increased the complexity of the intracellular distribution of metabolic enzymes. We identified genes encoding fructose-bisphosphate aldolases (FBA) in two centric (Odontella sinensis, Thalassiosira pseudonana) and one pennate (Phaeodactylum tricornutum) diatoms and found that four different aldolases are present in both groups: two plastid targeted class II enzymes (FBAC1 and FBAC2), one cytosolic class II (FBA3) and one cytosolic class I (FBA4) enzyme. The pennate Phaeodactylum possesses an additional plastidic class I enzyme (FBAC5). We verified the classification of the different aldolases in the diatoms by enzymatic characterization of isolated plastids and whole cell extracts. Interestingly, our results imply that in plastids of centric and pennate diatoms mainly either class I or class II aldolases are active. We also identified genes for both class I and class II aldolases in red algal EST databases, thus presenting a fascinating example of the reutilization and recompartmentalization of different aldolase isoenzymes during secondary endocytobiosis but as well demonstrating the limited use of metabolic enzymes as markers for the interpretation of phylogenetic histories in algae.

Effects of Chloramphenicol on Photosynthesis, Protein Profiles and Transketolase Activity under Extremely High CO2 Concentration in an Extremely-high-CO2-tolerant Green Microalga, Chlorococcum littorale

An extremely-high-CO2-tolerant alga, Chlorococcum littorale, showed high quantum efficiency of PSII (PhiII) in the light at 40% CO2, as well as at 5% CO2. However, PhiII decreased greatly when chloramphenicol (CAP) was added at 40% CO2, while no such decrease was observed at 5% CO2. Cycloheximide showed no effect on PhiII at either 5% or 40% CO2. The amount of a 76 kDa polypeptide (p76) on SDS-PAGE decreased markedly in the presence of CAP at 40% CO2 but not at 5% CO2. A partial amino acid sequence of p76 was 71-100% identical (10-14 identical residues out of 14 amino acids determined) to those of transketolases (TKLs) reported in higher plants and a cyanobacterium. In agreement with these observations, the TKL activity in C. littorale was decreased by CAP at 40% CO2, but not at 5% CO2. The transient decrease in TKL activity caused by CAP under 40% CO2 was well correlated with that in PhiII. These results indicate that the addition of CAP directly or indirectly influences the stability of TKL in C. littorale at 40% CO2, but not at 5% CO2, and that photosynthetic activity was reduced by a decrease in TKL activity.