What physiological acclimation supports increased growth at high CO2 conditions?

Chlamydomonas acidophila Negoro is a green algal species abundant in acidic waters (pH 2–3.5), in which inorganic carbon is present only as CO₂. Previous studies have shown that aeration with CO₂ increased its maximum growth rate, suggesting CO₂ limitation under natural conditions. To unravel the underlying physiological mechanisms at high CO₂ conditions that enables increased growth, several physiological characteristics from high- and low-CO₂-acclimated cells were studied: maximum quantum yield, photosynthetic O₂ evolution (P_max), affinity constant for CO₂ by photosynthesis (K_0.5,p), a CO₂-concentrating mechanism (CCM), cellular Rubisco content and the affinity constant of Rubisco for CO₂ (K_0.5,r). The results show that at high CO₂ concentrations, C. acidophila had a higher K_0.5,p, P_max, maximum quantum yield, switched off its CCM and had a lower Rubisco content than at low CO₂ conditions. In contrast, the K_0.5,r was comparable under high and low CO₂ conditions. It is calculated that the higher P_max can already explain the increased growth rate in a high CO₂ environment. From an ecophysiological point of view, the increased maximum growth rate at high CO₂ will likely not be realised in the field because of other population regulating factors and should be seen as an acclimation to CO₂ and not as proof for a CO₂ limitation.     

Biomass Production and Carbohydrate Content of Arabidopsis thaliana at Atmospheric CO2 Concentrations from 390 to 1680 μl l-1

Abstract: The concentration dependency of the impact of elevated atmospheric CO₂ concentrations on Arabidopsis thaliana L. was studied. Plants were exposed to nearly ambient (390), 560, 810, 1240 and 1680 μl I^-1 CO₂ during the vegetative growth phase for 8 days. Shoot biomass production and dry matter content were increased upon exposure to elevated CO₂. Maximal increase in shoot fresh and dry weight was obtained at 560 μl I^-1 CU₂, which was due to a transient stimulation of the relative growth rate for up to 3 days. The shoot starch content increased with increasing CO₂ concentrations up to two-fold at 1680 μl I^-1 CO₂, whereas the contents of soluble sugars and phenolic compounds were hardly affected by elevated CO₂. The chlorophyll and carotenoid contents were not substantially affected at elevated CO₂ and the chlorophyll a/b ratio remained unaltered. There was no acclimation of photosynthesis at elevated CO₂; the photosynthetic capacity of leaves, which had completely developed at elevated CO₂ was similar to that of leaves developed in ambient air. The possible consequences of an elevated atmospheric CO₂ concentration to Arabidopsis thaliana in its natural habitat is discussed.     

Growth, photosynthesis, and respiration of Chlorella sorokiniana after N-starvation. Interactions between light, CO2 and NH4+ supply

N-sufficient cells of Chlorella sorokiniana Shihira and Krauss, strain 211/8k, absorbed NH₄⁺ under light plus CO₂ conditions, when growth occurred, but not in darkness or in the absence of CO₂, when growth was inhibited. N-sufficient cells subjected to conditions of N-starvation for a 24-h period showed a marked loss of photosynthetic activity. Upon supply of NH₄⁺, N-starved cells sufflated with CO₂ air exhibited a time-dependent recovery of photosynthetic activity, both when suspended in light and in darkness. By contrast, growth only occurred in cells suspended in light. N-starved cells absorbed NH₄⁺ in darkness, but at a lower rate than in light. All of these data suggest that dark NH₄⁺ uptake is driven by N assimilation to recover from N-starvation and that the light-dependent NH₄⁺ uptake is driven by growth, being then influenced by conditions that affect recovery or growth. Unlike CO₂ conditions, in a CO₂-free atmosphere, absorption of NH₄⁺ by N-starved cells occurred at a higher rate in darkness than in light. Accordingly, resumption of photosynthetic potential after NH₄⁺ supply occurred in darkened cells, but not in illuminated cells. Respiratory activity of N-starved cells was enhanced up to 3-fold by NH₄⁺ and 2-fold by methylammonium, with different patterns, suggesting that respiratory enzymes were affected by N-metabolism, especially through short-term control mechanisms triggered by the expenditure of metabolic energy involved in N-metabolism.     

Influence of soil O2 and CO2 on root respiration for Agave deserti

Respiration measured as CO₂ efflux was determined at various soil O₂ and CO₂ concentrations for individual, attached roots of a succulent perennial from the Sonoran Desert, Agave deserti Engelm. The respiration rate increased with increasing O₂ concentration up to about 16% O₂ for established roots and 5% O₂ for rain roots (fine branch roots on established roots induced by wetting of the soil) and then remained fairly constant up to 21% O₂. When O₂ was decreased from 21 to 0%, the respiration rates were similar to those obtained with increasing O₂ concentration. The CO₂ concentration in the root zone, which for the shallow-rooted A. deserti in the field was about 1 000 μl l^-1, did not affect root respiration at concentrations up to 2 000 μl l^-1, but higher concentrations reduced it, respiration being abolished at 20 000 μl l^-1 (2%) CO₂ for both established and rain roots. Upon lowering CO₂ to 1 000 μl l^-1 after exposure to concentrations up to 10000 μl l^-1 CO₂, inhibition of respiration was reversible. Uptake of the vital stain neutral red by root cortical cells was reduced to zero, indicating cell death, in about 4 h at 2% CO₂, substantiating the detrimental effects of high soil CO₂ concentrations on roots of A. deserti . This CO₂ response may explain why roots of desert succulents tend to occur in porous, well-aerated soils.     

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 .     

Relationship between photosystem II activity and CO2 fixation in leaves

There is now potential to estimate photosystem II (PSII) activity in vivo from chlorophyll fluorescence measurements and thus gauge PSII activity per CO₂ fixed. A measure of the quantum yield of photosystem II, Φ_II (electron/photon absorbed by PSII), can be obtained in leaves under steady-state conditions in the light using a modulated fluorescence system. The rate of electron transport from PSII equals Φ_II times incident light intensity times the fraction of incident light absorbed by PSII. In C₄ plants, there is a linear relationship between PSII activity and CO₂ fixation, since there are no other major sinks for electrons; thus measurements of quantum yield of PSII may be used to estimate rates of photosynthesis in C₄ species. In C₃ plants, both CO₂ fixation and photorespiration are major sinks for electrons from PSII (a minimum of 4 electrons are required per CO₂, or per O₂ reacting with RuBP). The rates of PSII activity associated with photosynthesis in C₃ plants, based on estimates of the rates of carboxylation (v_o) and oxygenation (v_o) at various levels of CO₂ and O₂, largely account for the PSII activity determined from fluorescence measurements. Thus, in C₃ plants, the partitioning of electron flow between photosynthesis and photorespiration can be evaluated from analysis of fluorescence and CO₂ fixation.     

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.     

Sensing of atmospheric CO2 by plants

Abstract. Despite recent interest in the effects of high CO₂ on plant growth and physiology, very little is known about the mechanisms by which plants sense changes in the concentration of this gas. Because atmospheric CO₂ concentration is relatively constant and because the conductance of the cuticle to CO₂ is low, sensory mechanisms are likely to exist only for intercellular CO₂ concentration. Therefore, responses of plants to changes in atmospheric CO₂ will depend on the effect of these changes on intercellular CO₂ concentration. Although a variety of plant responses to atmospheric CO₂ concentration have been reported, most of these can be attributed to the effects of intercellular CO₂ on photosynthesis or stomatal conductance. Short-term and long-term effects of CO₂ on photosynthesis and stomatal conductance are discussed as sensory mechanisms for responses of plants to atmospheric CO₂. Available data suggest that plants do not fully realize the potential increases in productivity associated with increased atmospheric CO₂. This may be because of genetic and environmental limitations to productivity or because plant responses to CO₂ have evolved to cope with variations in intercellular CO₂ caused by factors other than changes in atmospheric CO₂.     

Effect of darkness and CO2 starvation on NH4+ and NO3− assimilation in the unicellular alga Cyanidium caldarium

Cyanidium caldarium (Tilden) Geitler, a non-vacuolate unicellular alga, resuspended in medium flushed with air enriched with 5% CO₂, assimilated NH₄⁺ at high rates both in the light and in the dark. The assimilation of NO₃⁻, by contrast, was inhibited by 63% in the dark. In cell suspensions flushed with CO₂-free air, NH₄⁺ assimilation decreased with time both in the light and in the dark and ceased almost completely after 90 min. The addition of CO₂ completely restored the capacity of the alga to assimilate NH₄⁺. NO₃⁻ assimilation, by contrast, was 33% higher in the absence of CO₂ and was linear with time. It is suggested that NO₃⁻ and NH₄⁺ metabolism in C. caldarium are differently controlled in response to the light and carbon conditions of the cell.     

C2H2 and Ar induce rapid declines in nitrogenase activity and CO2 evolution in nodules of Datisca glomerata

Preliminary studies have indicated that after addition of C₂H₂ there is a rapid decline in nitrogenase activity in the nodules of Datisca glomerata . The present work was undertaken to determine whether (1) there is also a decline in respiration and (2) the decline is associated with the cessation of ammonia production. The rates of C₂H₄ and CO₂ evolution by nodulated root systems of Datisca were measured as a function of time after exposure to C₂H₂. The peak rate of C₂H₄ evolution occurred at 30 s after C₂H₂ exposure, while the rate of CO₂ evolution started to decline at 60 s after exposure to C₂H₂. Incubation of nodules in a gas mixture containing Ar also caused a decline in CO₂ evolution. Further, pretreatment with Ar eliminated most of the C₂H₂-induced decline in nitrogenase activity and CO₂ evolution. These C₂H₂- and Ar-induced declines in Datisca nodules are more rapid than those reported in any other nodules. They are evidence that continued ammonia formation is essential for maintenance of normal nitrogenase activity in Datisca nodules.     

Soil CO2 efflux in a boreal pine forest under atmospheric CO2 enrichment and air warming

The response of forest soil CO₂ efflux to the elevation of two climatic factors, the atmospheric concentration of CO₂ (↑CO₂ of 700 μmol mol⁻¹) and air temperature (↑ T with average annual increase of 5°C), and their combination (↑CO₂+↑ T ) was investigated in a 4-year, full-factorial field experiment consisting of closed chambers built around 20-year-old Scots pines ( Pinus sylvestris L.) in the boreal zone of Finland. Mean soil CO₂ efflux in May–October increased with elevated CO₂ by 23–37%, with elevated temperature by 27–43%, and with the combined treatment by 35–59%. Temperature elevation was a significant factor in the combined 4-year efflux data, whereas the effect of elevated CO₂ was not as evident. Elevated temperature had the most pronounced impact early and late in the season, while the influence of elevated CO₂ alone was especially notable late in the season. Needle area was found to be a significant predictor of soil CO₂ efflux, particularly in August, a month of high root growth, thus supporting the assumption of a close link between whole-tree physiology and soil CO₂ emissions. The decrease in the temperature sensitivity of soil CO₂ efflux observed in the elevated temperature treatments in the second year nevertheless suggests the existence of soil response mechanisms that may be independent of the assimilating component of the forest ecosystem. In conclusion, elevated atmospheric CO₂ and air temperature consistently increased forest soil CO₂ efflux over the 4-year period, their combined effect being additive, with no apparent interaction.     

Drought-induced shifts in daily CO2 uptake patterns for leafy cacti

For cacti with persistent, relatively large leaves, most shoot CO₂ uptake under well-watered conditions occurs by the leaves using the C₃ pathway. For three species in the primitive subfamily Pereskioideae, droughts of 7 or 14 days decreased leaf daytime net CO₂ uptake by an average of 49 and 88%, respectively; these species always had a net CO₂ release at night by the leaves and both at night and during the day by the stems. For three leafy species in subfamily Opuntioideae, 7 and 14 days of drought reduced leaf daytime net CO₂ uptake by 90 and 100%, respectively. Although drought reduced the total CO₂ uptake over 24 h, the average percentage occurring at night by the leaves of these species increased from 5% under wet conditions to 71% after 7 days of drought to 99% after 14 days of drought. For two of the three species of Opuntioideae, 7 days of drought caused the small net CO₂ uptake by the sterns to shift from the daytime to the nighttime, while for the third species drought caused a reduction of its stem nocturnal net CO₂ uptake. Thus, shifts from predominantly daytime to predominantly nighttime net CO₂ uptake can be induced by drought for the leaves and the stems of leafy cacti in subfamily Opuntioideae, indicating a high degree of biochemical versatility.     

The Effect of Elevated [CO2] on Uptake and Allocation of 13C and 15N in Beech (Fagus sylvatica L.) during Leafing

Abstract: A continuous dual ¹³CO₂ and ¹⁵NH₄¹⁵NO₃ labelling experiment was undertaken to determine the effects of ambient (350μmol mol^-1) or elevated (700μmol mol^-1) atmospheric CO₂ concentrations on C and N uptake and allocation within 3-year-old beech ( Fagus sylvatica L.) during leafing. After six weeks of growth, total carbon uptake was increased by 63 % (calculated on total C content) under elevated CO₂ but the carbon partitioning was not altered. 56 % of the new carbon was found in the leaves. On a dry weight basis was the content of structural biomass in leaves 10 % lower and the lignin content remained unaffected under elevated as compared to ambient [CO₂]. Under ambient [CO₂] 37 %, and under elevated [CO₂] 51 %, of the lignin C of the leaves derived from new assimilates. For both treatments, internal N pools provided more than 90 % of the nitrogen used for leaf-growth and the partitioning of nitrogen was not altered under elevated [CO₂]. The C/N ratio was unaffected by elevated [CO₂] at the whole plant level, but the C/N ratio of the new C and N uptake was increased by 32 % under elevated [CO₂].     

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.     

Affinity for CO2 in relation to the ability of freshwater macrophytes to use HCO–3

1. The affinity of photosynthesis for CO₂ is calculated here as the initial slope of net-photosynthetic rate against concentration of CO₂. The affinity for CO₂ for pairs of freshwater macrophytes with similar leaf morphology but able or unable to use HCO^–₃ as a carbon source was compared.
2. Species restricted to CO₂ had a higher affinity for CO₂ than species that were also able to use HCO^–₃ when rates were expressed on the basis of area, dry mass and content of chlorophyll a .
3. Published values for the affinity for CO₂ and the concentration of CO₂ which half-saturated rate of photosynthesis were compiled and compared. Despite a large range of values, affinity for CO₂ was greater for species restricted to CO₂ than for those also able to use HCO^–₃ and statistically different when the slope was expressed on the basis of dry mass and chlorophyll a content.
4. The difference in affinity is consistent with predicted benefits of a high permeability to CO₂ for species relying on passive diffusion of CO₂ and a lower permeability for species able to use HCO^–₃ in order to reduce efflux of CO₂ from a high internal concentration generated by active transport.
5. The implications of the different affinities are discussed in terms of species distribution.     

Leaf isoprene emission rate as a function of atmospheric CO2 concentration

There is considerable interest in modeling isoprene emissions from terrestrial vegetation, because these emissions exert a principal control over the oxidative capacity of the troposphere. We used a unique field experiment that employs a continuous gradient in CO₂ concentration from 240 to 520 ppmv to demonstrate that isoprene emissions in Eucalyptus globulus were enhanced at the lowest CO₂ concentration, which was similar to the estimated CO₂ concentrations during the last Glacial Maximum, compared with 380 ppmv, the current CO₂ concentration. Leaves of Liquidambar styraciflua did not show an increase in isoprene emission at the lowest CO₂ concentration. However, isoprene emission rates from both species were lower for trees grown at 520 ppmv CO₂ compared with trees grown at 380 ppmv CO₂. When grown in environmentally controlled chambers, trees of Populus deltoides and Populus tremuloides exhibited a 30–40% reduction in isoprene emission rate when grown at 800 ppmv CO₂, compared with 400 ppmv CO₂. P. tremuloides exhibited a 33% reduction when grown at 1200 ppmv CO₂, compared with 600 ppmv CO₂. We used current models of leaf isoprene emission to demonstrate that significant errors occur if the CO₂ inhibition of isoprene is not taken into account. In order to alleviate these errors, we present a new model of isoprene emission that describes its response to changes in atmospheric CO₂ concentration. The model logic is based on assumed competition between cytosolic and chloroplastic processes for pyruvate, one of the principal substrates of isoprene biosynthesis.     

Canopy photosynthesis of crops and native plant communities exposed to long-term elevated CO2

Abstract. There have been seven studies of canopy photosynthesis of plants grown in elevated atmospheric CO₂: three of seed crops, two of forage crops and two of native plant ecosystems. Growth in elevated CO₂ increased canopy photosynthesis in all cases. The relative effect of CO₂ was correlated with increasing temperature: the least stimulation occurred in tundra vegetation grown at an average temperature near 10°C and the greatest in rice grown at 43°C. In soybean, effects of CO₂ were greater during leaf expansion and pod fill than at other stages of crop maturation. In the longest running experiment with elevated CO₂ treatment to date, monospecific stands of a C₃ sedge, Scirpus olneyi (Grey), and a C₄ grass, Spartina patens (Ait.) Muhl., have been exposed to twice normal ambient CO₂ concentrations for four growing seasons, in open top chambers on a Chesapeake Bay salt marsh. Net ecosystem CO₂ exchange per unit green biomass (NCE_b) increased by an average of 48% throughout the growing season of 1988, the second year of treatment. Elevated CO₂ increased net ecosystem carbon assimilation by 88% in the Scirpus olneyi community and 40% in the Spartina patens community.     

Origin, fate and significance of CO2 in tree stems

Although some CO₂ released by respiring cells in tree stems diffuses directly to the atmosphere, on a daily basis 15–55% can remain within the tree. High concentrations of CO₂ build up in stems because of barriers to diffusion in the inner bark and xylem. In contrast with atmospheric [CO₂] of c.  0.04%, the [CO₂] in tree stems is often between 3 and 10%, and sometimes exceeds 20%. The [CO₂] in stems varies diurnally and seasonally. Some respired CO₂ remaining in the stem dissolves in xylem sap and is transported toward the leaves. A portion can be fixed by photosynthetic cells in woody tissues, and a portion diffuses out of the stem into the atmosphere remote from the site of origin. It is now evident that measurements of CO₂ efflux to the atmosphere, which have been commonly used to estimate the rate of woody tissue respiration, do not adequately account for the internal fluxes of CO₂. New approaches to quantify both internal and external fluxes of CO₂ have been developed to estimate the rate of woody tissue respiration. A more complete assessment of internal fluxes of CO₂ in stems will improve our understanding of the carbon balance of trees.     

Low vapour pressure deficit reduces the beneficial effect of elevated CO2 on growth of N2-fixing alfalfa plants

Plant responses to elevated CO₂ can be modified by many environmental factors, but very little attention has been paid to the interaction between CO₂ and changes in vapour pressure deficit (VPD). Thirty-day-old alfalfa plants ( Medicago sativa L. cv. Aragón), which were inoculated with Sinorhizobium meliloti 102F78 strain, were grown for 1 month in controlled environment chambers at 25/15°C, 14 h photoperiod, and 600 µmol m⁻² s⁻¹ photosynthetic photon flux (PPF), using a factorial combination of CO₂ concentration (400 µmol mol⁻¹ or 700 µmol mol⁻¹) and vapour pressure deficit (0.48 kPa or 1.74 kPa, which corresponded to relative humidities of 85% and 45% at 25°C, respectively). Elevated CO₂ strongly stimulated plant growth under high VPD conditions, but this beneficial effect was not observed under low VPD. Under low VPD, elevated CO₂ also did not enhance plant photosynthesis, and plant water stress was greatest for plants grown at elevated CO₂ and low VPD. Moreover, plants grown under elevated CO₂ and low VPD had a lower leaf soluble protein and photosynthetic activity (photosynthetic rate and carboxylation efficiency) than plants grown under elevated CO₂ and high VPD. Elevated CO₂ significantly increased leaf adaxial and abaxial temperatures. Because the effects of elevated CO₂ were dependent on vapour pressure deficit, VPD needs to be controlled in experiments studying the effect of elevated CO₂ as well as considered in the extrapolations of results to a warmer, high-CO₂ world.     

In situ responses to elevated CO2 in tropical forest understorey plants

1. Plants growing in deep shade and high temperature, such as in the understorey of humid tropical forests, have been predicted to be particularly sensitive to rising atmospheric CO₂. We tested this hypothesis in five species whose microhabitat quantum flux density (QFD) was documented as a covariable. After 7 (tree seedlings of Tachigalia versicolor and Beilschmiedia pendula ) and 18 months (shrubs Piper cordulatum and Psychotria limonensis, and grass Pharus latifolius ) of elevated CO₂ treatment ( c. 700 μl litre^–1) under mean QFD of less than 11 μmol m^–2 s^–1, all species produced more biomass (25–76%) under elevated CO₂.
2. Total plant biomass tended to increase with microhabitat QFD (daytime means varying from 5 to 11μmol m^–2 s^–1) but the relative stimulation by elevated CO₂ was higher at low QFD except in Pharus .
3. Non-structural carbohydrate concentrations in leaves increased significantly in Pharus (+ 27%) and Tachigalia (+ 40%).
4. The data support the hypothesis that tropical plants growing near the photosynthetic light compensation point are responsive to elevated CO₂. An improved plant carbon balance in deep shade is likely to influence understorey plant recruitment and competition as atmospheric CO₂ continues to rise.