The induction of inorganic carbon transport and external carbonic anhydrase in Chlamydomonas reinhardtii is regulated by external CO2 concentration

Induction of the carbon concentrating mechanism (CCM) has been investigated during the acclimation of 5% CO₂‐grown Chlamydomonas reinhardtii 2137 mt + cells to well‐defined dissolved inorganic carbon (C_i) limited conditions. The CCM components investigated were active HCO₃^? transport, active CO₂ transport and extracellular carbonic anhydrase (CA_ext) activity. The CA_ext activity increased 10‐fold within 6 h of acclimation to 0·035% CO₂ and there was a further slight increase over the next 18 h. The CA_ext activity also increased substantially after an 8 h lag period during acclimation to air in darkness. Active CO₂ and HCO₃^? uptake by C. reinhardtii cells were induced within 2 h of acclimation to air, but active CO₂ transport was induced prior to active HCO₃^? transport. Similar results were obtained during acclimation to air in darkness. The critical C_i concentrations effecting the induction of active C_i transport and CA_ext activity were determined by allowing cells to acclimate to various inflow CO₂ concentrations in the range 0·035–0·84% at constant pH. The total C_i concentration eliciting the induction and repression of active C_i transport was higher during acclimation at pH 7·5 than at pH 5·5, but the external CO₂ concentration was the same at both pHs of acclimation. The concentration of external CO₂ required for the full induction and repression of C_i transport and CA_ext activity were 10 and 100 μM , respectively. The induction of CA_ext and active C_i transport are not correlated temporally, but are regulated by the same critical CO₂ concentration in the medium.     

The induction of the CO2 concentrating mechanism in a starch-less mutant of Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii the formation of a starch sheath surrounding the pyrenoid is observed when cells grown under high CO₂ (5% CO₂ in air) are transferred to low CO₂ (0.03%) conditions. Formation of the starch sheath occurs coincidentally with induction of the CO₂ concentrating mechanism and with de novo synthesis of 5 polypeptides with molecular masses of 21, 36, 37, 42–44 kDa. We studied the effect of CO₂ concentrations on photosynthesis, ultrastructure and protein synthesis in Chlamydomonas reinhardtii cw-15 (wild phenotype for photosynthesis) and in the starch-less mutant BAFJ -6, with the aim to clarify the role of the pyrenoid starch sheath in the operation of the CO₂ concentrating mechanism and whether these low CO₂-inducible polypeptides are involved in the formation of starch sheath. When wild type and starch-less mutant cells were transferred from high to low CO₂, the CO₂ requirement for half-maximal rates of photosynthesis decreased from 40 μM to 2 μM CO₂. ³⁵SO₄^2- labeling analyses showed that the starch-less mutant induced the 5 low CO₂-inducible polypeptides. These observations suggest that the starch-less mutant was able to induce a fully active CO₂ concentrating mechanism. Since the starch-less mutant did not form a pyrenoid starch sheath, we suggest that the starch sheath is not involved in the operation of the CO₂ concentrating mechanism and that none of these 5 low CO₂-inducible proteins is involved in the formation of the starch sheath in Chlamydomonas .     

Effects of doubled atmospheric CO2 concentration on the growth and photosynthesis of Chlamydomonas reinhardtii (Volvocales, Chlorophyceae)

The freshwater microalga, Chlamydomonas reinhardtii Dangeard, was cultured under 350 and 700 ppmv CO₂ to determine the impact of doubled atmospheric CO₂ concentration on its growth and photosynthesis. No significant difference was observed in the specific growth rate, photosynthetic efficiency, maximal net photo‐synthetic rate and light‐saturating point between the low and high CO₂ cultures. Both the low‐ and high‐CO₂‐grown cells showed reduced light‐dependent O₂ evolution rate and photochemical efficiency (F_v/F_m) owing to photoinhibition when exposed to high photon flux density. However, high‐CO₂‐grown cells were less photoinhibited, and showed better recovery in dim light or darkness during the initial period of the recovery process.     

A Comparison of CO2 Uptake by the Green Algae Tetraedron minimum and Chlamydomonas monoica

Abstract: The ability of the green alga Tetraedron minimum to acquire inorganic carbon from its environment was investigated and compared with that of Chlamydomonas monoica. T. minimum showed a higher affinity for bicarbonate ions than C. monoica, regardless of whether it was grown at high or low CO₂ concentrations. Furthermore, T. minimum was distinguished by the fact that it maintained a large intracellular pool of inorganic carbon. These features may explain why this alga is able to proliferate in alkaline conditions.     

A role for mitochondrial carbonic anhydrase in limiting CO2 leakage from low CO2-grown cells of Chlamydomonas reinhardtii

A model is presented which quantifies a possible role for the carbonic anhydrase in the mitochondrial matrix of Chlamydomonas reinhardtii which incorporates the observation that the expression of this enzyme is increased under growth conditions in which the expression of the carbon dioxide-concentrating mechanism is increased. It is assumed that the inorganic carbon enters the cytosol from the medium, and leaves the cytosol to the plastids, as HCO₃⁻ and that there is negligible carbonic anhydrase activity in the cytosol. The role of the mitochondrial carbonic anhydrase is suggested to be the conversion to HCO₃^– of the CO₂ produced in the mitochondria in the light from tricarboxylic acid cycle activity and from decarboxylation of glycine in any photorespiratory carbon oxidation cycle activity which is not suppressed by the carbon concentrating mechanism. If there is a HCO₃⁻ channel in the inner mitochondrial membrane then almost all of the inorganic carbon leaves the mitochondria as HCO₃⁻, thus limiting the potential for CO₂ leakage through the plasmalemma. This mechanism could increase inorganic C supply to ribulose bisphosphate carboxylase-oxygenase by some 10% at the energetic expense of less than 1% of the total ATP generation by plastids plus mitochondria.     

Effect of CO2, CO2 and Diamox on photosynthesis and photorespiration in Chlamydomonas reinhardtii (green alga) and Anacystis nidulans (Cyanobacterium, blue-green alga)

Photorespiration by Chlamydomonas reinhardtii and Anacystis nidulans was measured as the oxygen inhibition of CO₂ uptake and the CO₂ compensation points. Net photosynthesis was oxygen dependent in Chlamydomonas grown in 5% CO₂, but CO₂ insensitive in cultures bubbled with air. Anacystis, even when cultured in 5% CO₂, exhibited an CO₂ insensitive net photosynthesis. The CO₂ compensation point of Chlamydomonas grown in cultures bubbled with air and Anacystis grown in 5% CO₂ enriched air, were reached shortly after the measurement was begun and the values were very low, less than 10 μl CO₂ 1^?1; while Chlamydomonas grown in 5% CO₂ enriched air for 4 days showed a high, but temporary CO₂ compensation point (60 μl CO₂ 1^?1). After a two hour adaptation in low CO₂, a stable, low CO₂ compensation point was reached. It seems that photorespiration can only be detected by the methods used in this study when the algae are cultured in high CO₂, but a mechanism exists which blocks photorespiration when the green algae are adapted to low CO₂ concentrations. When Chlamydomonas was treated with Diamox, an inhibitor of carbonic anhydrase, after cultivation in low CO₂ (air), the cells behaved as if they had been grown in high CO₂. They showed an oxygen sensitive net photosynthesis and a high CO₂ compensation point. This indicates that carbonic anhydrase plays an important role in the regulation of a measurable photorespiration in Chlamydomonas. The results are discussed in relation to previous observations of photorespiration measured by enzyme assay, metabolic products and gas exchange properties.     

Impacts of Elevated CO2 Concentration on Biochemical Composition, Carbonic Anhydrase, and Nitrate Reductase Activity of Freshwater Green Algae

To investigate the biochemical response of freshwater green algae to elevated CO2 concentrations, Chlorella pyrenoidosa Chick and Chlamydomonas reinhardtii Dang cells were cultured at different CO2 concentrations within the range 3-186μmol/L and the biochemical composition, carbonic anhydrase (CA),and nitrate reductase activities of the cells were investigated. Chlorophylls (Chl), carotenoids, carbonhydrate,and protein contents were enhanced to varying extents with increasing CO2 concentration from 3-186μmol/L. The CO2 enrichment significantly increased the Chl a/Chl b ratio in ChloreUa pyrenoidosa, but not in Chlamydomonas reinhardtii. The CO2 concentration had significant effects on CA and nitrate reductase activity. Elevating CO2 concentration to 186μmol/L caused a decline in intracellular and extracellullar CA activity. Nitrate reductase activity, under either light or dark conditions, in C. reinhardtii and C. pyrenoidosa was also significantly decreased with CO2 enrichment. From this study, it can be concluded that CO2 enrichment can affect biochemical composition, CA, and nitrate reductase activity, and that the biochemical response was species dependent.     

The CO2concentrating mechanism in cyanobactiria and microalgae

Over the past 10 years it has become clear that cyanobacteria and microalgae possess mechanisms for actively acquiring inorganic carbon from the external medium and are able to use this to elevate the CO₂ concentration around the active site of the primary photosynthetic carboxylating enzyme, ribulose bisphosphate carboxylase-oxygenase (Rubisco). This results in a vastly enhanced photosynthetic affinity for inorganic carbon (C_i) and improved photosynthetic efficiency. The CO₂ concentrating mechanism is dependent on the existence of membrane bound C_i transport systems, and a microenvironment within the cell where the accumulated C_i can be used to elevate CO₂ at the site of Rubisco. Evidence presented in this review suggests that in cyanobacteria this is achieved by the packaging of Rubisco and carbonic anhydrase (CA) into discrete structures, which are termed carboxysomes. Analogous structures in microalgae, termed pyrenoids, may perform a similar function. The recovery and analysis of high-CO₂-requiring mutants has greatly advanced our understanding of the mechanisms and genes underlying these systems, especially in cyanobacteria, and this review places particular emphasis on the contribution made by molecular genetic approaches.     

Carbonic anhydrase activity and inorganic carbon fluxes in low- and high-C1 cells of Chlamydomonas reinhardtü and Scenedesmus obliquus

Carbonic anhydrase (CA) activity associated with high- and low-dissolved inorganic carbon (C₁) grown cells was examined in whole cells by measuring ¹⁸O exchange from doubly labeled CO₂ (¹³C¹⁸O¹⁸O). Both algal species showed the presence of extracellular (periplasmic) as well as intracellular CA activity, which were both greatly increased in low-C₁ cells. The periplasmic CA activity was at least 40-fold higher in lowcompared to high-C₁ cells in both C. reinhardtii and S. obliquus. while low-C₁ cells of S. obliquus showed the highest activity of internal CA. The CA inhibitor ethoxyzolamide showed a strong inhibition of the C₁ uptake process in both C. reinhardtii and S. obliquus as in cyanobacteria. which may indicate that the nature of the primary uptake process is similar in both green algae and cyanobacteria. By using a mass spectrometnc disequilibrium technique it was possible to separate the C₁ fluxes of net HCO^?₃-uptake and net CO₂-uptake during steady-state photosynthesis in high- and Sow-C₁ grown cells of Chlamydomonas reinhardtii (WT. 2137+) and Scenedesmus obliquus (WT. D3). It was found that both high- and low-C₁ cells of the two algae can utilize both CO₂ and HCO^?₃ for photosynthesis, although low-C₁ cells have a higher affinity for the uptake of both C₁ species. Induction at low-C₁ causes an increase in the affinity of both species for HCO^?₃ and CO₂; changes in net CO₂-uptake were, however, significantly greater.     

The molecular and biochemical analyses of CO2-concentrating mechanisms in cyanobacteria and microalgae

Abstract. The supply of dissolved inorganic carbon for photosynthesis in the aquatic environment is much more variable than that experienced by terrestrial plants. In response to this variability, cyanobacteria and many species of microalgae acclimate to rapid changes in the availability of dissolved inorganic carbon by the induction of high affinity/high capacity CO₂-concentrating systems (CCMs). Biochemical and molecular analyses of the acclimation response have recently identified several components that are required for efficient operation of the CCMs and CO₂ assimilation. This has been accomplished using in vivo labelling studies, and characterization of high inorganic carbon (C_i) requiring cyanobacterial and algal mutants. The identification and regulation of expression of polypeptides synthesized in response to limiting C_i concentrations, and the proposed role of the carboxysome and the pyrenoid in the functioning of the CCMs are examined.     

Regulation of the induction of bicarbonate uptake by dissolved CO2 in the marine diatom, Phaeodactylum tricornutum

Physiological properties of photosynthesis were determined in the marine diatom, Phaeodactylum tricornutum UTEX640, during acclimation from 5% CO₂ to air and related to H₂CO₃ dissociation kinetics and equilibria in artificial seawater. The concentration of dissolved inorganic carbon at half maximum rate of photosynthesis (K_0·5[DIC]) value in high CO₂‐grown cells was 1009 mmol m ^{? 3} but was reduced three‐fold by the addition of bovine carbonic anhydrase (CA), whereas in air‐grown cells K_0·5[DIC] was 71 mmol m ^{? 3}, irrespective of the presence of CA. The maximum rate of photosynthesis (P_max) values varied between 300 and 500 μ mol O₂ mg Chl ^{? 1} h ^{? 1} regardless of growth pCO₂. Bicarbonate dehydration kinetics in artificial seawater were re‐examined to evaluate the direct HCO₃ ^? uptake as a substrate for photosynthesis. The uncatalysed CO₂ formation rate in artificial seawater of 31·65°/_oo of salinity at pH 8·2 and 25 °C was found to be 0·6 mmol m ^{? 3} min ^{? 1} at 100 mmol m ^{? 3} DIC, which is 53·5 and 7·3 times slower than the rates of photosynthesis exhibited in air‐ and high CO₂‐grown cells, respectively. These data indicate that even high CO₂‐grown cells of P. tricornutum can take up both CO₂ and HCO₃ ^? as substrates for photosynthesis and HCO₃ ^? use improves dramatically when the cells are grown in air. Detailed time courses were obtained of changes in affinity for DIC during the acclimation of high CO₂‐grown cells to air. The development of high‐affinity photosynthesis started after a 2–5 h lag period, followed by a steady increase over the next 15 h. This acclimation time course is the slowest to be described so far. High CO₂‐grown cells were transferred to controlled DIC conditions, at which the concentrations of each DIC species could be defined, and were allowed to acclimate for more than 36 h. The K_0·5[DIC] values in acclimated cells appeared to be correlated only with [CO_2(aq)] in the medium but not to HCO₃ ^? , CO₃^{2 ?} , total [DIC] or the pH of the medium and indicate that the critical signal regulating the affinity of cells for DIC in the marine diatom, P. tricornutum, is [CO_2(aq)] in the medium.     

HCO3− and CO2 leakage from Synechococcus UTEX 625

The leakage of various inorganic carbon species from air-grown cells of Synechococcus UTEX 625 was investigated after a light to dark transition or during a light period using a mass spectrometer under a wide variety of experimental conditions. Total inorganic carbon efflux and CO₂ efflux during the initial period of darkness were measured with or without carbonic anhydrase in the reaction medium respectively. The HCO₃^? efflux after a light to dark transition was estimated by difference. Carbon dioxide efflux in the light was measured by inhibiting CO₂ transport with either Na₂S or COS₃ or quenching the ¹³C inorganic carbon transport by the addition of ¹²C inorganic carbon in excess. In cells in which CO₂ fixation was inhibited, when only the HCO₃^? transport system was fully operative, CO₂ effluxed continuously during the light period at a rate equal to about 25% of that in darkness. When only the CO₂ transport system was operative, HCO₃^? effluxed during the light period. The difference between the light and dark efflux rates was consistent with a 0.6 unit decrease in the intracellular pH upon darkening the cells. The permeabilities of the cell for CO₂ (2.94 ± 0.14 ± 10^?8ms^?1; mean ± SE, n=137) and HCO₃^? (1.4–1.7 ± 10^?9 ms^?1) were calculated.     

The regulation of photosynthetic rate and activation of extracellular carbonic anhydrase under CO2-limiting conditions in the marine diatom Skeletonema costatum

In the marine diatom Skeletonema costatum , carbonic anhydrase activity exterior to the plasma membrane (CA_ext) was detected only when the available CO₂ concentration was less than 5·0 mmol m^–3, this activity being unaffected by the total dissolved inorganic carbon concentration. The inhibition of CA_ext by dextran bound sulphonamide (DBS) demonstrated the key role of this enzyme in maintaining photosynthetic rate under CO₂-limited conditions. Treatment with trypsin followed by affinity chromatography on p-aminomethylbenzene-sulphamide agarose and subsequent SDS-PAGE analysis revealed a polypeptide from carbon-replete cells of identical molecular mass to the CA_ext released by trypsin from CO₂-limited cells. Redox activity in the plasma membrane of intact cells was measured by following the light-dependent reduction of ferricyanide or NADP, the greatest activity being shown by CO₂-limited cells. Overall the results suggest that high rates of redox activity under conditions of CO₂-limitation were required for the activation of CA_ext.     

Blue, red and blue plus red light control of chlorophyll content and CO2 gas exchange in barley leaves: Quantitative description of the effects of light quality and fluence rate

The dependence of the Chl content and the rate of CO₂ gas exchange (RGE) on both blue and red quanta fluence rates have been studied in primary leaves of barley ( Hordeum vulgare L. cv. Viner). Empirical equations connecting the two photosynthetic indices with fluence rates of blue or red light were developed. These equations consist of 3 (Chl content) or 2 (RGE) terms, each reflecting the involvement of a specific reaction in the long-term light control of the development of the photosynthetic apparatus. On the basis of the equations the effects of mixed blue plus red light on both the Chl content and RGE were calculated. An additive mode of the co-action of blue and red light in the range of high PFDs (10–170 μmol m⁻² s⁻¹) becomes evident from the comparison of the experimental results and calculated data. The results indicate the involvement of phytochrome, cryptochrome and chlorophyll in the long-term regulation of the Chl content and RGE.     

Growth, photosynthetic properties and Rubisco activities and amounts of marine macroalgae grown under current and elevated seawater CO2 concentrations

Growth rates, photosynthetic responses and the activity, amount and CO₂ affinity of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) were determined for common marine macroalgae grown in seawater (containing 14.5 ± 2.1 µM CO₂) or CO₂‐enriched seawater (averaging 52.8 ± 19.2 µM CO₂). The algae were grown in 40 L fiberglass tanks (outdoor) for 4–15 weeks and in a field experimental setup for 5 days. Growth rates of the species studied (representing the three major divisions, i.e. Chlorophyta, Rhodophyta and Phaeophyta) were generally not significantly affected by the increased CO₂ concentrations in the seawater medium. Rubisco characteristics of algae cultivated in CO₂‐enriched seawater were similar to those of algae grown in nonenriched seawater. The lack of response of photosynthetic traits in these aquatic plants is likely to be because of the presence of CO₂ concentrating mechanisms (CCMs) which rely on HCO₃^– utilization, the inorganic carbon (Ci) form that dominates the total Ci pool available in seawater. Significant changes on the productivity of these particular marine algae species would not be anticipated when facing future increasing atmospheric CO₂ levels.     

An integrated portable apparatus for the simultaneous field measurement of photosynthetic CO2 and water vapour exchange, light absorption and chlorophyll fluorescence emission of attached leaves

Abstract. A portable apparatus has been constructed to measure simultaneously the quantum yield of CO₂ assimilation, light absorption, chlorophyll fluorescence emission and water vapour exchange of attached intact leaves in the field. The core of the instrument is a light-integrating spherical leaf chamber which includes ports for a light source, photosynthetically active radiation sensor, fluorescence probes and gas inlet and outlet manifolds. Measurement of the quantum flux inside the empty chamber and with a leaf present allows determination of leaf absorptance. An open gas exchange system is employed using an infra-red analyser to measure leaf CO₂ exchange. Using a DC white light source the quantum yield of CO₂ assimilation based on absorbed light (φ_abs) may be determined rapidly in either ambient air or artificial gas mixtures. Inclusion of capacitance humidity probes into the gas inlet and outlet ports allows simultaneous determination of water vapour exchange and subsequent estimation of stomatal conductance to CO₂ and intercellular CO₂ concentration. Measurement of fluorescence emission by the sample leaf exposed to white light is achieved by a modulated fluorescence detection system. In addition to determination of the minimal, maximal and variable fluorescence levels, a further analysis allows the photochemical and non-photochemical components of fluorescence quenching, to be estimated. The theory and design of this apparatus is described in detail. The use of the apparatus in the field is demonstrated through a study of the photosynthetic performance of a maize and bean crop during the growing season and by analysis of the photosynthetic performance of crops subjected to nitrogen-stress and a herbicide treatment.     

Effects of long-term exposure to elevated CO2 and N fertilization on the development of photosynthetic capacity and biomass accumulation in Quercus suber L.

The effects of long‐term (4 year) CO₂ enrichment (70 Pa versus 35 Pa) and nitrogen nutrition (8 mm versus 1 mm NO₃^–) on biomass accumulation and the development of photosynthetic capacity in leaves of cork oak (Quercus suber L., a Mediterranean evergreen tree) were studied. The evolution of photosynthetic parameters with leaf development was estimated by fitting the biochemical model of Farquhar et al. (Planta 149, 78–90, 1980) with modifications by Sharkey (Botanical Review 78, 71–75, 1985) to A–C_i response curves. CO₂ enrichment had a small reduction effect on the development of the maximum CO₂ fixation capacity by Rubisco (V_Cmax), and no effect over maximum electron transport capacity (J_max), day‐time respiration (R_d) and Triose‐P utilization (TPU). However, there was a statistically significant effect of N fertilization and the interaction CO₂ × N over the evolution of V_Cmax, J_max and TPU. Relative stomatal limitation (estimated from A–C_i curves) was higher (+20%) for plants grown under ambient CO₂ than for plants grown under elevated CO₂. There was a significant effect of CO₂ and N fertilization over total biomass accumulation as well as leaf area. Plants grown at elevated CO₂ had 27% more biomass than plants grown at ambient CO₂ when given high N. However, for plants grown under low N there was no significant effect of CO₂ enrichment on biomass accumulation. Plants grown under low N also had significantly higher root : shoot ratios whereas there were no differences between CO₂ treatments. The larger biomass accumulation of Q. suber under elevated CO₂ is attributable to a higher availability of CO₂ coupled to a larger leaf area, with no significant decrease in photosynthetic capacity under CO₂ enrichment and elevated N fertilization. For low N fertilization, the effects of CO₂ enrichment over leaf area and biomass accumulation are lost, suggesting that in native ecosystems with low N availability, the effects of CO₂ enrichment may be insignificant.     

Stomatal responses of C3, C3-C4 and C4Flaveria species to light and intercellular CO2 concentration: implications for the evolution of stomatal behaviour

Stomatal function mediates physiological trade‐offs associated with maintaining a favourable H₂O balance in leaf tissues while acquiring CO₂ as a photosynthetic substrate. The C₃ and C₄ species appear to have different patterns of stomatal response to changing light conditions, and variation in this behaviour may have played a role in the functional diversification of the different photosynthetic pathways. In the current study, we used gain analysis theory to characterize the stomatal conductance response to light intensity in nine different C₃, C₄ and C₃‐C₄ intermediate species Flaveria species. The response of stomatal conductance (g_s) to a change in light intensity represents both a direct (related to a change in incident light intensity, I) and indirect (related to a change in intercellular CO₂ concentration, C_i) response. The slope of the line relating the change in g_s to C_i was steeper in C₄ species, compared with C₃ species, with C₃‐C₄ species having an intermediate response. This response reflects the greater relative contribution of the indirect versus direct component of the g_s versus I response in the C₄ species. The C₃‐C₄ species, Flaveria floridana, exhibited a C₄‐like response whereas the C₃‐C₄ species, Flaveria sonorensis and Flaveria chloraefolia, exhibited C₃‐like responses, similar to their hypothesized position along the evolutionary trajectory of the development of C₄ photosynthesis. There was a positive correlation between the relative contribution of the indirect component of the g_s versus I response and water use efficiency when evaluated across all species. Assuming that the C₃‐C₄ intermediate species reflect an evolutionary progression from fully expressed C₃ ancestors, the results of the current study demonstrate an increase in the contribution of the indirect component of the g_s versus I response as taxa evolve toward the C₄ extreme. The greater relative contribution of the indirect component of the stomatal response occurs through both increases in the indirect stomatal components and through decreases in the direct. Increases in the magnitude of the indirect component may be related to the maintenance of higher water use efficiencies in the intermediate evolutionary stages, before the appearance of fully integrated C₄ photosynthesis.     

Effects of long-term elevated [CO2] from natural CO2 springs on Nardus stricta: photosynthesis, biochemistry, growth and phenology

Plants of Nardus stricta growing near a cold, naturally emitting CO₂ spring in Iceland were used to investigate the long-term (> 100 years) effects of elevated [CO₂] on photosynthesis, biochemistry, growth and phenology in a northern grassland ecosystem. Comparisons were made between plants growing in an atmosphere naturally enriched with CO₂ (≈ 790 μ mol mol^–1) near the CO₂ spring and plants of the same species growing in adjacent areas exposed to ambient CO₂ concentrations (≈360 μ mol mol^–1). Nardus stricta growing near the spring exhibited earlier senescence and reductions in photosynthetic capacity (≈25%), Rubisco content (≈26%), Rubisco activity (≈40%), Rubisco activation state (≈23%), chlorophyll content (≈33%) and leaf area index (≈22%) compared with plants growing away from the spring. The potential positive effects of elevated [CO₂] on grassland ecosystems in Iceland are likely to be reduced by strong down-regulation in the photosynthetic apparatus of the abundant N. stricta species.