Direct Effects Of CO2 Research Articles

Effects of elevated CO2 associated with maize on multiple generations of the cotton bollworm, Helicoverpa armigera

AbstractUnder elevated environmental carbon dioxide (CO2), leaf chewers tend to compensate for decreased leaf nutritional quality with increased consumption; mortality and development times also increase and cause a reduction in the fitness of leaf chewers. However, the effect of elevated CO2 on multiple successive generations of these and other insects is not well understood. Furthermore, information about the direct effects of increased environmental CO2 on developmental time and consumption of herbivores is lacking. In this paper, we tested the hypothesis that cascade effects of elevated CO2 through plants, rather than the direct effects of elevated CO2, are the main factors decreasing the fitness of cotton bollworm, Helicoverpa armigera Hübner (Lepidoptera: Noctuidae). We used two series of experiments to quantify the growth, development, and consumption of H. armigera fed on an artificial diet or C4 plants (maize) grown under two CO2 levels (ambient vs. double ambient). In the first series of experiments, elevated CO2 had no effect on the population abundance or individual consumption for three successive generations of cotton bollworms fed on an artificial diet. In the second series of experiments, elevated CO2 reduced population abundance of cotton bollworm larvae for two successive generations when they were fed maize milky grains. The specific effects were longer larval duration, lower fecundity, and decreased rm of cotton bollworms. Furthermore, elevated CO2 increased individual consumption when cotton bollworm was fed maize milky grains for two successive generations and decreased the population’s total consumption in the first generation but increased it in the second generation. The results from this study indicate that: (1) The effects of elevated CO2 on three successive generations of cotton bollworm fed on artificial diet were weak, or even non‐existent, and (2) elevated CO2 increased the consumption when cotton bollworm were fed maize. Our study also suggests that the damage inflicted by cotton bollworm on maize (a C4 plant) will be seriously affected by the increases in atmospheric CO2, which is unlike our previous results for spring wheat (a C3 plant).

Central CO2chemoreception in cardiorespiratory control

Central CO₂chemoreception in cardiorespiratory control

The direct effects of increasing CO2 and temperature on non-calcifying organisms: increasing the potential for phase shifts in kelp forests.

Predictions about the ecological consequences of oceanic uptake of CO(2) have been preoccupied with the effects of ocean acidification on calcifying organisms, particularly those critical to the formation of habitats (e.g. coral reefs) or their maintenance (e.g. grazing echinoderms). This focus overlooks the direct effects of CO(2) on non-calcareous taxa, particularly those that play critical roles in ecosystem shifts. We used two experiments to investigate whether increased CO(2) could exacerbate kelp loss by facilitating non-calcareous algae that, we hypothesized, (i) inhibit the recovery of kelp forests on an urbanized coast, and (ii) form more extensive covers and greater biomass under moderate future CO(2) and associated temperature increases. Our experimental removal of turfs from a phase-shifted system (i.e. kelp- to turf-dominated) revealed that the number of kelp recruits increased, thereby indicating that turfs can inhibit kelp recruitment. Future CO(2) and temperature interacted synergistically to have a positive effect on the abundance of algal turfs, whereby they had twice the biomass and occupied over four times more available space than under current conditions. We suggest that the current preoccupation with the negative effects of ocean acidification on marine calcifiers overlooks potentially profound effects of increasing CO(2) and temperature on non-calcifying organisms.

Living in a high CO2 world: impacts of global climate change on marine phytoplankton

The planet is currently going through a period of global climate change, the pace of which is unprecedented in geological history. Marine phytoplankton contribute approximately 50% of the total global primary productivity and play a vital role in global carbon cycling. Consequently it is extremely important to understand the impact that global climate change will have on the ecological performance of these organisms. In this review we summarise current understanding of the influence that global climate change has on the physiological properties, productivity and assemblage composition of marine phytoplankton. While most phytoplankton are likely to show little direct effect of elevated CO2 on photosynthetic rates, some, including the ecologically important coccolithophorids, are likely to show significant stimulation of growth. The rise in temperature consequent upon the elevated atmospheric levels of CO2 and other greenhouse gases will stimulate growth of some species and increase ocean temperatures in some areas beyond their current optima. However, more importantly, increasing global temperatures will stimulate stratification of the water column which, in tropical and mid-latitudes will exacerbate nutrient limitation in surface waters. This in turn will lead to changes in phytoplankton assemblage composition, primary productivity and sensitivity to UVB radiation.

Potential impacts of climate change and climate variability on China’s rice yield and production

We assessed the effect of greenhouse gas-induced climate change, as well as the direct fertilization effect of CO2, on rice yields and production in China. Our methodology coupled the regional climate model PRECIS (Providing Regional Climates for Impacts Studies) with the CERES (Crop Environment Resources Synthesis) rice crop model to simulate current (1961-1990) and future (2011-2100) rice yields and production under A2 and B2 climate change scenarios. The movement of rice-producing areas to more favorable climatic conditions was also considered. Results demonstrate that simulated future rice yields decrease without the CO2 fertilization effect and increase in some areas and time slices with it. Yield variability measured by the coefficient of variation exhibits increases in both scenarios, with the largest increases under A2. Due to favorable climatic conditions, single rice cropping may expand further north in China, and double rice cropping may move to the northern portion of the Yangtze River basin. China's potential rice production is projected to increase in the future, due to the CO2 fertilization effect and shifts in rice-producing areas. Three rice cultiva- tion regions (Sichuan basin, Yangtze River basin, and Huang-huaihai plain) are identified as highly sensitive to future climate change if no adaptation measures are implemented, due to yield decreases and large increases in production variability.

Growth and nutritive value of cassava (Manihot esculenta Cranz.) are reduced when grown in elevated CO2

Global food security in a changing climate depends on both the nutritive value of staple crops as well as their yields. Here, we examined the direct effect of atmospheric CO(2) on cassava (Manihot esculenta Cranz., manioc), a staple for 750 million people worldwide. Cassava is poor in nutrients and contains high levels of cyanogenic glycosides that break down to release toxic hydrogen cyanide when damaged. We grew cassava at three concentrations of CO(2) (C(a): 360, 550 and 710 ppm) supplied together with nutrient solution containing either 1 mM or 12 mM nitrogen. We found that total plant biomass and tuber yield (number and mass) decreased linearly with increasing C(a). In the worst-case scenario, tuber mass was reduced by an order of magnitude in plants grown at 710 ppm compared with 360 ppm CO(2). Photosynthetic parameters were consistent with the whole plant biomass data. It is proposed that since cassava stomata are highly sensitive to other environmental variables, the decrease in assimilation observed here might, in part, be a direct effect of CO(2) on stomata. Total N (used here as a proxy for protein content) and cyanogenic glycoside concentrations of the tubers were not significantly different in the plants grown at elevated CO(2). By contrast, the concentration of cyanogenic glycosides in the edible leaves nearly doubled in the highest C(a). If leaves continue to be used as a protein supplement, they will need to be more thoroughly processed in the future. With increasing population density, declining soil fertility, expansion into marginal farmland, together with the predicted increase in extreme climatic events, reliance on robust crops such as cassava will increase. The responses to CO(2) shown here point to the possibility that there could be severe food shortages in the coming decades unless CO(2) emissions are dramatically reduced, or alternative cultivars or crops are developed.

CO2 activates orexin-containing neurons in mice

Accumulating evidence shows possible participation of orexin, a hypothalamic neuropeptide also known as hypocretin, in regulation of breathing. In this study we examined whether hypercapnic stimulation in vivo activates orexin-containing neurons in the hypothalamus. A hypercapnic gas mixture of 10% CO(2), 21% O(2), and residual N(2) or room air was inhaled to awake and freely moving mice for 3h. Their brains were examined by double immunohistochemical staining for orexin and c-Fos. We found that inhalation of CO(2) significantly increased c-Fos expression in the orexin-containing cells that were located in the perifornical region and the dorsomedial hypothalamus (15.8+/-1.6% for room air vs. 28.4+/-2.7% for CO(2), p=0.011) but not in the lateral hypothalamic area (12.3+/-1.2% for room air vs. 12.4+/-3.0% for CO(2)). Although current methodology cannot differentiate between direct effect of CO(2) on the orexin-containing neurons and indirect one through other neurons, this is the first report showing that inhalation of CO(2) did activate the orexin-containing neurons in vivo.

Carbon acquisition by Trichodesmium: the effect of pCO2 and diurnal changes

We investigated carbon acquisition by the N2‐fixing cyanobacterium Trichodesmium IMS101 in response to CO2 levels of 15.1, 37.5, and 101.3 Pa (equivalent to 150, 370, and 1000 ppm). In these acclimations, growth rates as well as cellular C and N contents were measured. In vivo activities of carbonic anhydrase (CA), photosynthetic O2 evolution, and CO2 and HCO3‐ fluxes were measured using membrane inlet mass spectrometry and the 14C disequilibrium technique. While no differences in growth rates were observed, elevated CO2 levels caused higher C and N quotas and stimulated photosynthesis and N2 fixation. Minimal extracellular CA (eCA) activity was observed, indicating a minor role in carbon acquisition. Rates of CO2 uptake were small relative to total inorganic carbon (Ci) fixation, whereas HCO{3 contributed more than 90% and varied only slightly over the light period and between CO2 treatments. The low eCA activity and preference for HCO3‐ were verified by the 14C disequilibrium technique. Regarding apparent affinities, half‐saturation concentrations (K1/2) for photosynthetic O2 evolution and HCO3‐ uptake changed markedly over the day and with CO2 concentration. Leakage (CO2 efflux : Ci uptake) showed pronounced diurnal changes. Our findings do not support a direct CO2 effect on the carboxylation efficiency of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RubisCO) but point to a shift in resource allocation among photosynthesis, carbon acquisition, and N2 fixation under elevated CO2 levels. The observed increase in photosynthesis and N2 fixation could have potential biogeochemical implications, as it may stimulate productivity in N‐limited oligotrophic regions and thus provide a negative feedback in rising atmospheric CO2 levels.

Effects of soluble cobalt and cobalt incorporated into calcium phosphate layers on osteoclast differentiation and activation

Metal ions originating from mechanical debris and corrosive wear of prosthetic implant alloys accumulate in peri-implant soft tissues, bone mineral, and body fluids. Eventually, metal ions such as cobalt (II) (Co 2+), which is a major component of cobalt–chromium-based implant alloys and a known activator of osteolysis, are incorporated into the mineral phase of bone. We hypothesize that the accumulation of Co 2+ in the mineral could directly activate osteolysis by targeting osteoclasts. To test this hypothesis, we coated tissue culture plastic with a thin layer of calcium phosphate (CaP) containing added traces of Co 2+, thereby mimicking the bone mineral accumulation of Co 2+. Murine bone marrow osteoclasts formed in the presence of M-CSF and RANKL were cultured on these surfaces to examine the effects of Co 2+ on osteoclast formation and resorptive activity. Treatment conditions with Co 2+ involved incorporation into the CaP layer, adsorption to the mineral surface, or addition to culture media. Micromolar concentrations of Co 2+ delivered to developing osteoclast precursors by all 3 routes increased both osteoclast differentiation and resorptive function. Compared to CaP layers without Co 2+, we observed a maximal 75% increase in osteoclast numbers and a 2.3- to 2.7-fold increase in mineral resorption from the tissue culture wells containing 0.1 μ m Co 2+ and 0.1–10 μ m Co 2+, respectively. These concentrations are well within the range found in peri-implant tissues in vivo. This direct effect of Co 2+ on osteoclasts appears to act independently of the particulate phagocytosis/inflammation-mediated pathways, thus enhancing osteolysis and aseptic implant loosening.

Regardless of Whether Rising Atmospheric Carbon Dioxide Levels Increase Air Temperature, Flowering Phenology Will Be Affected

Many species, particularly those that flower in spring, are flowering 3–5 d earlier than they did in the first half of the twentieth century. This has been interpreted as a consequence of rising temperatures. However, correlated with this rise in temperature is an increase in atmospheric CO2, which may also modify phenology. The effects of elevated CO2 and temperature on time of flowering were examined in 22 Asteraceae species. Plants were exposed to three treatments, ambient CO2 with ambient temperature, elevated CO2 (700 μL L−1) with ambient temperature, and elevated CO2 with elevated temperature ($$\mathrm{ambient}\,+1.5^{\circ }\mathrm{C}\,$$). On average, elevated CO2 by itself advanced flowering by 4 d; increasing temperature as well as CO2 advanced flowering by an additional 3 d. There were marked differences among species in response to treatment. In elevated‐CO2 conditions, this was related to photoperiodic response: CO2 was more likely to hasten phenology in long‐ than in short‐day species. Early‐ and late‐flowering species did not differ in response to elevated CO2, but the combined effect of elevated CO2 and temperature hastened flowering more in early‐ than in late‐flowering species. The direct effect of CO2 on phenology may be as important as its indirect effect through climate change.

Increased mercury in forest soils under elevated carbon dioxide

Fossil fuel combustion is the primary anthropogenic source of both CO2 and Hg to the atmosphere. On a global scale, most Hg that enters ecosystems is derived from atmospheric Hg that deposits onto the land surface. Increasing concentrations of atmospheric CO2 may affect Hg deposition to terrestrial systems and storage in soils through CO(2)-mediated changes in plant and soil properties. We show, using free-air CO2 enrichment (FACE) experiments, that soil Hg concentrations are almost 30% greater under elevated atmospheric CO2 in two temperate forests. There were no direct CO2 effects, however, on litterfall, throughfall or stemflow Hg inputs. Soil Hg was positively correlated with percent soil organic matter (SOM), suggesting that CO(2)-mediated changes in SOM have influenced soil Hg concentrations. Through its impacts on SOM, elevated atmospheric CO2 may increase the Hg storage capacity of soils and modulate the movement of Hg through the biosphere. Such effects of rising CO2, ones that transcend the typically studied effects on C and nutrient cycling, are an important next phase for research on global environmental change.

Effects of climate change on crop production in Cameroon

This study involves an assessment of the potential effects of greenhouse gas climate change, as well as the direct fertilization effect of CO2 on crop yields in Cameroon. The methodology involves coupling the transient diagnostics of 2 atmosphere-ocean general circulation models, namely NASA/Goddard Institute GISS and the Hadley Centre's HadCM3, to the CropSyst crop model to simulate current and future (2020, 2080) crop yields (bambara nut, groundnut, maize, sorghum and soybean) in 8 agricultural regions of Cameroon. For the future we estimate substantial yield increases for bambara groundnut, soybean and groundnut, and little or no change and even decreases of maize and sorghum yields, varying according to the climate scenario and the agricul- tural region. Maize and sorghum (both C4 crops) yields are expected to decrease by 14.6 and 39.9%, respectively, across the whole country under GISS 2080 scenarios. The results also show that the effect of temperature patterns on climate change is much more important than that of precipitation. Findings call for monitoring of climate change/variability and dissemination of information to farmers, to encourage adaptation to climate change.

The use of branch enclosures to assess direct and indirect effects of elevated CO2 on photosynthesis, respiration and isoprene emission of Populus alba leaves

We used a novel system of branch enclosures to study the impact of elevated CO2 (900 ppm) on the gas-exchange characteristics of developed and developing leaves of white poplar (Populus alba L.), as well as of leaves subsequently developing at ambient CO2, outside the enclosures in which the CO2 concentration was raised. We found no significant effect of elevated CO2 on photosynthesis, respiration and isoprene emission, as the rates of developed and developing leaves inside the enclosures, and of leaves developing outside the enclosures, were similar to those recorded using enclosures maintained at ambient CO2. The enclosure system, however, largely influenced the rates of gas-exchange. In fact, leaves already developed inside the enclosures showed rates of photosynthesis, stomatal conductance, and isoprene emission higher than leaves developing inside the enclosures, and also higher than leaves developing outside the enclosure. These differences were caused by a higher efficiency in the light use and by a higher Ribulose 1.5 bisphosphate carboxylase (Rubisco) activity in leaves fully developed inside enclosures than in the other leaf classes. The experiment overall suggests that branch enclosures may alter the physiology of the plants, reducing or counteracting the impact of elevated CO2, which we predicted to stimulate photosynthesis and uncouple isoprene emission from photosynthesis. This may be an important bias against the use of enclosure systems for studies of the impact of environmental constraints and global change factors on physiological features.

Insects take a bigger bite out of plants in a warmer, higher carbon dioxide world.

Carbon dioxide is a potent “greenhouse” gas. The dramatic increase in its concentration in the atmosphere as a result of human activities, beginning with accelerated fossil fuels combustion in the late 18th century, and perhaps even earlier, with modern agricultural expansion 8,000 years ago (1, 2), is driving a striking rise in global temperature (3). For the past 650,000 years, until relatively recently, the concentration of CO2 in the atmosphere was 280 ppm or less; however, the current concentration exceeds 380 ppm and, on its present trajectory, will surpass 550 ppm by 2050 (3). The accumulation of CO2 and other greenhouse gases in the atmosphere is forcing an elevation of global mean temperature; during the lifetime of child born today, the average temperature of the earth will increase by as much as ≈6°C (3). Working in concert, elevated temperature and CO2 are redistributing plant and animal communities on the surface of the earth (4). Because of the direct effect of CO2 and temperature on global food supplies, the influence of these changes on plant physiology and ecology is being actively studied (4–7). How these elements of global change may alter the interactions between plants and the insects that feed on them is relatively unknown. By bringing to light secrets contained in the fossil record, Currano et al. (8), published in this issue of PNAS, found that the amount and diversity of insect damage to plants increased in association with an abrupt rise in atmospheric CO2 and global temperature that occurred >55 million years ago. If the past is indeed a window to the future, their findings suggest that increased insect herbivory will be one more unpleasant surprise arising from anthropogenic climate change.

Carbon dioxide directly suppresses spontaneous migration, chemotaxis, and free radical production of human neutrophils

Carbon dioxide (CO(2)) insufflation during laparoscopy has been shown to dampen the systemic stress response to surgery. This is related to a suppression of peritoneal macrophage functions. In vivo data suggest that CO(2) can also affect neutrophils (polymorphonuclear cells, PMNs), the most abundant cell type in the inflamed peritoneal cavity. Nonetheless, the direct effects of CO(2) on PMNs have not yet been investigated. PMNs were isolated from peripheral blood of healthy volunteers and incubated with (1) CO(2) (100% CO(2), pH 6.2), (2) hypoxic control (95% helium/5% CO(2), pH 7.4), and (3) control (95% air/5% CO(2), pH 7.4). Spontaneous and IL-8-induced migrations (chemokinesis and chemotaxis) during 2 h of exposure to different gases were measured with a transwell chamber system. The release of reactive oxygen species (ROS, luminometry) was determined after 15-min and 2-h exposures. In other sets of experiments, PMNs were exposed for 2 h or 4 h and kept under normal conditions for 18 h with lipopolysaccharide (LPS) stimulation thereafter. Final viability and apoptosis were assessed with fluorometry. Exposure to 100% CO(2) completely blocked spontaneous and IL-8 induced migration of PMNs (p < 0.001 vs. controls). Neutrophil migration was slightly diminished in the hypoxic control group. PMA-stimulated ROS production was reduced even after short exposure to 100% CO(2)(p < 0.05). We observed a slight increase of caspase-3/7 activity after exposure to 100% CO(2) and/or hypoxia; however, total viability was not affected. CO(2) incubation directly and temporarily suppresses the proinflammatory functions of PMNs; this is caused only partially by the concomitant hypoxia. This effect will contribute to the dampened inflammatory response to laparoscopic surgery. Further studies are needed to investigate whether the temporary suppression of neutrophil functions could affect the clearance of bacterial contaminations.

Effects of climate change and CO2 increase on potential agricultural production in Southern Québec, Canada

We assessed the effect of greenhouse gas-induced climate change, as well as the direct fertilization effect of CO2, on crop yields in Quebec, Canada. Our methodology coupled the transient diagnostics of 2 Atmosphere–Ocean General Circulation Models (CGCM1 and HadCM3) to the DSSAT 3.5 crop simulation system to simulate current (1961–1990) and future (2040–2069) crop yields for spring wheat, maize, soybean and potato grown in 8 agricultural regions of Quebec. For the future (2040–2069), we predict significant yield increases for soybean, lesser increases for wheat, no significant change for maize, and yield decreases for potato. These yields, especially for soybean, are further increased when incorporating the CO2 fertilization effect, but vary according to the crop, climate scenario and agricultural region. Similar trends have been found in comparable agricultural regions in the Northeastern USA and in Southern Finland. These results are useful for designing appropriate crop and farm management adaptation strategies in response to future climate change.

Methods and uncertainties in bioclimatic envelope modelling under climate change

Potential impacts of projected climate change on biodiversity are often assessed using single-species bioclimatic ‘envelope’models. Such models are a special case of species distribution models in which the current geographical distribution of species is related to climatic variables so to enable projections of distributions under future climate change scenarios. This work reviews a number of critical methodological issues that may lead to uncertainty in predictions from bioclimatic modelling. Particular attention is paid to recent developments of bioclimatic modelling that address some of these issues as well as to the topics where more progress needs to be made. Developing and applying bioclimatic models in a informative way requires good understanding of a wide range of methodologies, including the choice of modelling technique, model validation, collinearity, autocorrelation, biased sampling of explanatory variables, scaling and impacts of non-climatic factors. A key challenge for future research is integrating factors such as land cover, direct CO2effects, biotic interactions and dispersal mechanisms into species-climate models. We conclude that, although bioclimatic envelope models have a number of important advantages, they need to be applied only when users of models have a thorough understanding of their limitations and uncertainties.

Microbial Community Responses to Atmospheric Carbon Dioxide Enrichment in a Warm-Temperate Forest

Forest productivity depends on nutrient supply, and sustained increases in forest productivity under elevated carbon dioxide (CO2) may ultimately depend on the response of microbial communities to changes in the quantity and chemistry of plant-derived substrates, We investigated microbial responses to elevated CO2 in a warm-temperate forest under free-air CO2 enrichment for 5 years (1997–2001). The experiment was conducted on three 30 m diameter plots under ambient CO2 and three plots under elevated CO2 (200 ppm above ambient). To understand how microbial processes changed under elevated CO2, we assayed the activity of nine extracellular enzymes responsible for the decomposition of labile and recalcitrant carbon (C) substrates and the release of nitrogen (N) and phosphorus (P) from soil organic matter. Enzyme activities were measured three times per year in a surface organic horizon and in the top 15 cm of mineral soil. Initially, we found significant increases in the decomposition of labile C substrates in the mineral soil horizon under elevated CO2; this overall pattern was present but much weaker in the O horizon. Beginning in the 4th year of this study, enzyme activities in the O horizon declined under elevated CO2, whereas they continued to be stimulated in the mineral soil horizon. By year 5, the degradation of recalcitrant C substrates in mineral soils was significantly higher under elevated CO2. Although there was little direct effect of elevated CO2 on the activity of N- and P-releasing enzymes, the activity of nutrient-releasing enzymes relative to those responsible for C metabolism suggest that nutrient limitation is increasingly regulating microbial activity in the O horizon. Our results show that the metabolism of microbial communities is significantly altered by the response of primary producers to elevated CO2. We hypothesize that ecosystem responses to elevated CO2 are shifting from primary production to decomposition as a result of increasing nutrient limitation.

Detection of a direct carbon dioxide effect in continental river runoff records

Continental runoff has increased through the twentieth century despite more intensive human water consumption. Possible reasons for the increase include: climate change and variability, deforestation, solar dimming, and direct atmospheric carbon dioxide (CO2) effects on plant transpiration. All of these mechanisms have the potential to affect precipitation and/or evaporation and thereby modify runoff. Here we use a mechanistic land-surface model and optimal fingerprinting statistical techniques to attribute observational runoff changes into contributions due to these factors. The model successfully captures the climate-driven inter-annual runoff variability, but twentieth-century climate alone is insufficient to explain the runoff trends. Instead we find that the trends are consistent with a suppression of plant transpiration due to CO2-induced stomatal closure. This result will affect projections of freshwater availability, and also represents the detection of a direct CO2 effect on the functioning of the terrestrial biosphere.

Carbon dioxide differentially affects the cytokine release of macrophage subpopulations exclusively via alteration of extracellular pH

The improved outcome after endoscopic surgery has been attributed to less surgical trauma. However, the underlying mechanisms are not fully understood, and direct effects of CO2 used for pneumoperitoneum, cellular acidification, and/or the lack of air contamination have been postulated to additionally modulate immune functions during endoscopic surgery. We investigated the effects of CO2 incubation, extracellular acidification, and air contamination on the inflammatory response of two distinct macrophage populations. R2 and NR 8383 rat macrophage cell lines were used. Interleukin-6 (IL-6) and nitric oxide after lipopolysaccharide (LPS) stimulation were determined in these sets of experiments: incubation in 100% CO2, 5% CO2, and room air for 2h; incubation at pH 7.4, 6.5, and 5.5 for 2 h in 5% CO2; and incubation in 100% CO2, 5% CO2 and room air in fixed pH 6.3. The extracellular pH was monitored during incubation. We determined the alteration of intracellular pH in cells subjected to extracellular acidification by fluorescence microscopy. Extracellular pH decreased to 6.3 during 100% CO2 incubation. IL-6 release was reduced after CO2 incubation in NR 8383 cells and increased in R2 cells (p < 0.05). It was not altered by air incubation. Decreasing the extracellular pH to 6.5 mimicked the effects of CO2 and a decrease to 5.5 suppressed IL-6 release in both cell lines. In fixed pH at 6.3, CO2 and air incubation had no effect. CO2 and pH had no impact on nitric oxide release and vitality. Intracellular pH decreased with extracellular acidification without significant difference between the two cell lines. A decrease in extracellular pH during incubation in CO2 differentially affects IL-6 release in macrophage subpopulations. This may explain contradictory results in the literature. Moreover, we demonstrated that air contamination does not affect macrophage cytokine release. The decrease in extracellular pH is the primary underlying mechanism of the alteration of macrophage cytokine release after CO2 incubation, and it appears that the ability to maintain intracellular pH is not determined by the effects of CO2 or extracellular acidification.