Soil microbial responses to increased concentrations of atmospheric CO2

Terrestrial ecosystems respond to an increased concentration of atmospheric CO₂. While elevated atmospheric CO₂ has been shown to alter plant growth and productivity, it also affects ecosystem structure and function by changing below-ground processes. Knowledge of how soil microbiota respond to elevated atmospheric CO₂ is of paramount importance for understanding global carbon and nutrient cycling and for predicting changes at the ecosystem-level. An increase in the atmospheric CO₂ concentration not only alters the weight, length, and architecture of plant roots, but also affects the biotic and abiotic environment of the root system. Since the concentration of CO₂ in soil is already 10–50 times higher than that in the atmosphere, it is unlikely that increasing atmospheric CO₂ will directly influence the rhizosphere. Rather, it is more likely that elevated atmospheric CO₂ will affect the microbe–soil–plant root system indirectly by increasing root growth and rhizodeposition rates, and decreasing soil water deficit. Consequently, the increased amounts and altered composition of rhizosphere-released materials will have the potential to alter both population and community structure, and activity of soil- and rhizosphere-associated microorganisms. This occurrence could in turn affect plant health and productivity and plant community structure. This review covers current knowledge about the response of soil microbes to elevated concentrations of atmospheric CO₂.     

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.     

Species richness enhances both algal biomass and rates of oxygen production in aquatic microcosms

Accelerating rates of species extinction have generated much recent interest in understanding how biodiversity affects the functioning of ecosystems. Experiments to date have shown communities composed of fewer species generally capture a smaller fraction of available resources, and achieve lower standing stock biomass than more diverse communities. However, it is uncertain how changes in biodiversity and the resulting alterations in biomass affect the rates of important ecological processes like primary production, which regulates fluxes of CO₂ and O₂ between the biotic and abiotic components of the environment. Here we show that species richness influences not only the standing stock biomass of primary producers, but also rates of gross primary production measured by changes in O₂ concentrations in aquatic systems. We manipulated the richness of five widespread species of algae in laboratory microcosms and then quantified how richness impacts algal biomass, rates of gross primary production (GPP), and the ratio of production to respiration. Algal biomass increased by a factor of 1.82 for each level of species richness, and GPP by a factor of 1.20, for each additional species. Production to respiration ratios increased about 10% for each additional species, indicating that systems with more species were increasingly autotrophic – that is, they produced more O₂ than they consumed, and accumulated CO₂ faster than they released it. These trends were driven by two highly productive species that became co-dominant in species rich polycultures at the expense of other taxa. Our experiment suggests that changes in biodiversity may influence not only the rates at which O₂ and CO₂ are produced and released in ecosystems, but also the total amount of carbon that is sequestered and stored as biomass.     

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.     

Elevated CO2 and conifer roots: effects on growth, life span and turnover

Elevated CO₂ increases root growth and fine (diam. 2 mm) root growth across a range of species and experimental conditions. However, there is no clear evidence that elevated CO₂ changes the proportion of C allocated to root biomass, measured as either the root:shoot ratio or the fine root:needle ratio. Elevated CO₂ tends to increase mycorrhizal infection, colonization and the amount of extramatrical hyphae, supporting their key role in aiding the plant to more intensively exploit soil resources, providing a route for increased C sequestration. Only two studies have determined the effects of elevated CO₂ on conifer fine-root life span, and there is no clear trend. Elevated CO₂ increases the absolute fine-root turnover rates; however, the standing crop root biomass is also greater, and the effect of elevated CO₂ on relative turnover rates (turnover:biomass) ranges from an increase to a decrease. At the ecosystem level these changes could lead to increased C storage in roots. Increased fine-root production coupled with increased absolute turnover rates could also lead to increases in soil organic C as greater amounts of fine roots die and decompose. Although CO₂ can stimulate fine-root growth, it is not known if this stimulation persists over time. Modeling studies suggest that a doubling of the atmospheric CO₂ concentration initially increases biomass, but this stimulation declines with the response to elevated CO₂ because increases in assimilation are not matched by increases in nutrient supply.     

Application of the chloroform fumigation-incubation method to the estimation of soil microbial biomass in burned and unburned Japanese red pine forests

Abstract Estimation of soil microbial biomass in burned and unburned Japanese red pine forests was attempted using the chloroform fumigation-incubation method. As the amount of CO₂-C evolved from the fumigated soil for 10–20 days after fumigation (designated as F') was always lower than that from the unfumigated soil during the same period (UF'), the formula, microbial biomass-C(M) = the amount of CO₂-C evolved from the fumigated soil for 0–10 days after fumigation, F) − F'/ k _c, was proposed instead of Jenkinson's conventional formula, M = (F − UF')/ k _c. The k _c value was also determined as 0.30 using 3 fungal and 3 bacterial cultured species as internal standards. Microbial biomass-C calculated by (F − F')/0.30 decreased with soil depth at both the burned (Nenoura, 3.5 years after fire) and unburned (Ato) sites, showing the significant correlation with the decrease of soil respiration and organic C content along soil depth. Microbial biomass-C in the 0–2 cm soil layer at the burned site at Nenoura was 130 mg/100 g dry soil and those in the HF horizon and 0–2 cm soil layer at the unburned site at Ato were 686 and 146 mg/100 g dry soil, respectively.     

Stimulation of r- vs. K-selected microorganisms by elevated atmospheric CO2 depends on soil aggregate size

Increased root exudation under elevated atmospheric CO₂ and the contrasting environments in soil macro- and microaggregates could affect microbial growth strategies. We investigated the effect of elevated CO₂ on the contribution of fast- ( r -strategists) and slow-growing ( K -strategists) microorganisms in soil macro- and microaggregates. We fractionated the bulk soil from the ambient and elevated (for 5 years) CO₂ treatments of FACE-Hohenheim (Stuttgart) into large macro- (>2 mm), small macro- (0.25–2.00 mm), and microaggregates (<0.25 mm) using 'optimal moist' sieving. Microbial biomass (C_mic), the maximum specific growth rate (μ), growing microbial biomass (GMB) and lag-period ( t _lag) were estimated by the kinetics of CO₂ emission from bulk soil and aggregates amended with glucose and nutrients. Although C_org and C_mic were unaffected by elevated CO₂, μ values were significantly higher under elevated than ambient CO₂ for bulk soil, small macroaggregates, and microaggregates. Substrate-induced respiratory response increased with decreasing aggregate size under both CO₂ treatments. Based on changes in μ, GMB and lag period, we conclude that elevated atmospheric CO₂ stimulated the r- selected microorganisms, especially in soil microaggregates. Such an increase in r -selected microorganisms indicates acceleration of available C mineralization in soil, which may counterbalance the additional C input by roots in soils in a future elevated atmospheric CO₂ environment.     

Effect of elevated CO2 on carbon partitioning and exudate release from Plantago lanceolata seedlings

Plantago lanceolata L. seedlings were grown in sand microcosm units over a 43‐day experimental period under two CO₂ regimes (800 or 400 µmol mol⁻¹) to investigate the effect of elevated atmospheric CO₂ concentration on carbon partitioning and exudate release. Total organic carbon (TOC) content of the collected exudate material was measured throughout the experimental period. After 42 days growth the seedlings were labelled with [¹⁴C]‐CO₂ and the fate of the label within the plant and its release by the roots monitored. Elevated CO₂ significantly (P ≤ 0.001) enhanced shoot, root and total dry matter production although the R:S ratio was unaltered, suggesting no alteration in gross carbon partitioning. The cumulative release of TOC (in mg C) over 0‐42 days was unaltered by CO₂ treatment however, when expressed as a percentage of net assimilated C, ambient‐grown plants released a significantly (P≤ 0.001) higher percentage from their roots compared to elevated CO₂‐grown plants (i.e. 8 vs 3%). The distribution of ¹⁴C‐label was markedly altered by CO₂ treatment with significantly (P≤ 0.001) greater per cent label partitioned to the roots under elevated CO₂. This indicates increased partitioning of recent assimilate below‐ground under elevated CO₂ treatment although there was no significant difference in the percentage of ¹⁴C‐label released by the roots. Comparison of plant C budgets based on ¹⁴C‐pulse‐chase methodology and TOC measurements is discussed.     

Little evidence for respiratory acclimation by microbial communities to short-term shifts in temperature in red pine (Pinus resinosa) litter

Forest litter is a large reservoir of organic compounds that adds CO₂ to the atmospheric carbon pool when it decomposes. Predicting CO₂ efflux from litter decomposition is difficult because litter can undergo significant diurnal and day-to-day shifts in temperature. Moreover, the relationship between temperature and respiration may change if the decomposer microorganisms acclimate to short-term temperature changes. Therefore, we studied the relationship between temperature and respiration by litter decomposer microorganisms in a Pinus resinosa (Ait.) system and tested the hypothesis that their respiration acclimates to temperature. We found only limited evidence for acclimation following 6 °C shifts for 7 days. This suggests that increase in respiratory CO₂ loss associated with increased temperature would not be greatly ameliorated by physiological acclimation for periods of up to a week.     

Degradation activity of adhered and suspended Pseudomonas cells cultured on 2,4,6-trichlorophenol, measured by indirect conductimetry

The degradation activity (expressed as specific CO₂ production rates) of adhered and suspended Pseudomonas cells, strains SP1 and SP2, during the degradation of 2,4,6-trichlorophenol (2,4,6-TCP), was compared using indirect conductimetry technique. This technique is defined as the measurement of CO₂ ionization in an alkaline solution and expressed as the negative conductance change values of such solution. The attachment surfaces were porous glass and silicone rubber. The 2,4,6-TCP concentrations ranged from 10 to 500 mg 1⁻¹. Specific respiration rates were determined from CO₂ evolution rates and biomass yields of both suspended and adhered cell cultures. CO₂ evolution rates were determined after conversion of conductance change values into CO₂ produced values. Results indicate that glass-adhered cells reached a higher maximum specific CO₂ evolution rate ( Q _CO2max) than both suspended and silicone rubber-adhered cells. However, suspended cells showed a lower saturation constant ( K_s ) than the adhered cells. These results suggest that depending on support nature the respiration activity of adhered cells could be higher than of suspended cells. Moreover, the indirect conductimetry technique could efficiently be used by measurements of respiration activities of both attached or suspended xenobiotic-degrading micro-organisms.     

Growth, respiration and survival of Legionella pneumophila at high temperatures

The optimum temperature for multiplication of legionella strains in culture media is around 37°C. The effect of high temperatures on the growth of strains isolated from various environments is poorly known. We studied the growth (cell multiplication, respiration) of clinical and environmental Legionella pneumophila strains in liquid media at intervals of 0.5°C in the temperature range from 41.6 to 51.6°C using a temperature gradient incubator. Cell multiplication and CO₂ production decreased markedly with all the strains at temperatures above 44–45°C. CO₂ continued to be produced up to 51.6C even if cell multiplication generally stopped at around 48.4–50.0C. Thus, legionella retained its metabolic activity beyond the maximum temperature for cell multiplication. The CO₂ production per bacterial cell (metabolic quotient, qCO₂) increased with increasing temperature up to 45°C, whereafter it decreased, the turning point being almost at the same at which the rate of cell multiplication decreased. The difference in qCO₂ between the strains may reflect their different physiological capacities for tolerating high temperatures.     

Elevated atmospheric CO2 influences the interaction between the parasitic angiosperm Orobanche minor and its host Trifolium repens

The influence of the root holoparasitic angiosperm Orobanche minor Sm. on the biomass, photosynthesis, carbohydrate and nitrogen content of Trifolium repens L. was determined for plants grown at two CO₂ concentrations (350 and 550 μmol mol⁻¹). Infected plants accumulated less biomass than their uninfected counterparts, although early in the association there was a transient stimulation of growth. Infection also influenced biomass allocation both between tissues (infected plants had lower root:shoot ratios) and within tissues:infected roots were considerably thicker before the point of parasite attachment and thinner below. Higher concentrations of starch were also found in roots above the point of attachment, particularly for plants grown in elevated CO₂. Elevated CO₂ stimulated the growth of T. repens only during the early stages of development. There was a significant interaction between infection and CO₂ on growth, with infected plants showing a greater response, such that elevated CO₂ partly alleviated the effects of the parasite on host growth. Elevated CO₂ did not affect total O. minor biomass per host, the number of individual parasites supported by each host, or their time of attachment to the host root system. Photosynthesis was stimulated by elevated CO₂ but was unaffected by O. minor . There was no evidence of down-regulation of photosynthesis in T. repens grown at elevated CO₂ in either infected or uninfected plants. The data are discussed with regard to the influence of elevated CO₂ on other parasitic angiosperm-host associations and factors which control plant responses to elevated CO₂.     

Impact of elevated carbon dioxide on the rhizosphere communities of Carex arenaria and Festuca rubra

The increase in atmospheric carbon dioxide (CO₂) levels is predicted to stimulate plant carbon (C) fixation, potentially influencing the size, structure and function of micro- and mesofaunal communities inhabiting the rhizosphere. To assess the effects of increased atmospheric CO₂ on bacterial, fungal and nematode communities in the rhizosphere, Carex arenaria (a nonmycorrhizal plant species) and Festuca rubra (a mycorrhizal plant species) were grown in three dune soils under controlled soil temperature and moisture conditions, while subjecting the aboveground compartment to defined atmospheric conditions differing in CO₂ concentrations (350 and 700 μL L⁻¹). Real-time polymerase chain reaction (PCR) and PCR-denaturing gradient gel electrophoresis methods were used to examine effects on the size and structure of rhizosphere communities. Multivariate analysis of community profiles showed that bacteria were most affected by elevated CO₂, and fungi and nematodes to a lesser extent. The influence of elevated CO₂ was plant dependent, with the mycorrhizal plant ( F. rubra ) exerting a greater influence on bacterial and fungal communities. Biomarker data indicated that arbuscular mycorrhizal fungi (AMF) may play an important role in the observed soil community responses. Effects of elevated CO₂ were also soil dependent, with greater influence observed in the more organic-rich soils, which also supported higher levels of AMF colonization. These results indicate that responses of soil-borne communities to elevated CO₂ are different for bacteria, fungi and nematodes and dependent on the plant type and soil nutrient availability.     

Water vapour fluxes and their impact under elevated CO2 in a C4-tallgrass prairie

We measured leaf-level stomatal conductance, xylem pressure potential, and stomate number and size as well as whole plant sap flow and canopy-level water vapour fluxes in a C4-tallgrass prairie in Kansas exposed to ambient and elevated CO₂. Stomatal conductance was reduced by as much as 50% under elevated CO₂ compared to ambient. In addition, there was a reduction in stomate number of the C4 grass, Andropogon gerardii Vitman, and the C3 dicot herb, Salvia pitcheri Torr., under elevated CO₂ compared to ambient. The result was an improved water status for plants exposed to elevated CO₂ which was reflected by a less negative xylem pressure potential compared to plants exposed to ambient CO₂. Sap flow rates were 20 to 30% lower for plants exposed to elevated CO₂ than for those exposed to ambient CO₂. At the canopy level, evapotranspiration was reduced by 22% under elevated CO₂. The reduced water use by the plant canopy under elevated CO₂ extended the photosynthetically-active period when water became limiting in the ecosystem. The result was an increased above- and belowground biomass production in years when water stress was frequent.     

Carbon dioxide and ethylene interactions in tulip bulbs

The effect of CO₂ on ethylene-induced gummosis (secretion of polysaccharides), weight loss and respiration in tulip bulbs ( Tulipa gesneriana L.) was investigated. A pretreatment with 1-MCP prevented these ethylene-induced effects, indicating that ethylene action must have been directed via the ethylene receptor. Treatment with 0.3 Pa ethylene for 2 days caused gummosis on 50% of the total number of bulbs of cultivar Apeldoorn, known to be sensitive for gummosis. Addition of CO₂ (10 kPa) reduced the ethylene-induced gummosis to 18%. In a second experiment the influence of ethylene and CO₂ on respiration and FW loss of bulbs of the cultivar Leen van der Mark was studied. A range of ethylene partial pressures (0.003–0.3 Pa) was applied continuously for 29 days. Ethylene caused a transient peak in O₂ consumption rate during the first days after the start of application. The relation between O₂ consumption rate and ethylene partial pressure could be described by Michaelis-Menten kinetics. Respiratory peaks were reduced by CO₂. This inhibition by CO₂ could not totally be due to competition with ethylene at the receptor binding-site, as was indicated by the use of an O₂ consumption model. Pre-treatment of bulbs with 1-MCP and subsequent exposure to CO₂ showed that CO₂ could influence respiration irrespective of any interaction with ethylene. Ethylene and CO₂ both stimulated weight loss. The effect of combined treatments of ethylene and CO₂ on weight loss was at least as strong as the sum of the separate effects, which implies that competition between ethylene and CO₂ at the receptor binding-site was unlikely.     

An open-top chamber for measuring soil respiration and the influence of pressure difference on CO2 efflux measurement

1. A new open-top chamber for measuring CO₂ efflux from the soil is reported here. The new design enables measurement of the equilibrium CO₂ efflux, when there is no detectable pressure difference between the chamber and outside nor leakage of CO₂ into or out of the chamber.
2. In previous dynamic-chamber techniques, the measured CO₂ efflux is dependent on the pressure difference between the inside and outside of the chamber, and a negative pressure difference of –1Pa may cause an order of magnitude increase in measured CO₂ efflux. Although the measured CO₂ efflux is less sensitive to a positive pressure difference than to a negative one, a positive pressure difference of even a few tenths of a Pa will lead to a considerable underestimation in soil CO₂ evolution.
3. The influence of pressure difference on measured CO₂ efflux is negligible in the new design and the estimated CO₂ efflux is close to the undisturbed soil respiration rate. Flow rates up to 8lmin^–1, or air movement over the soil surface up to 55cmmin^–1, will not affect CO₂ evolution from the soil. The influence of pressure difference is related to the type of soil being measured and this has also been reported here for the new design.     

Disturbance, rainfall and contrasting species responses mediated aboveground biomass response to 11 years of CO2 enrichment in a Florida scrub-oak ecosystem

This study reports the aboveground biomass response of a fire-regenerated Florida scrub-oak ecosystem exposed to elevated CO₂ (1996–2007), from emergence after fire through canopy closure. Eleven years exposure to elevated CO₂ caused a 67% increase in aboveground shoot biomass. Growth stimulation was sustained throughout the experiment; although there was significant variability between years. The absolute stimulation of aboveground biomass generally declined over time, reflecting increasing environmental limitations to long-term growth response. Extensive defoliation caused by hurricanes in September 2004 was followed by a strong increase in shoot density in 2005 that may have resulted from reopening the canopy and relocating nitrogen from leaves to the nutrient-poor soil. Biomass response to elevated CO₂ was driven primarily by stimulation of growth of the dominant species, Quercus myrtifolia , while Quercus geminata , the other co-dominant oak, displayed no significant CO₂ response. Aboveground growth also displayed interannual variation, which was correlated with total annual rainfall. The rainfall × CO₂ interaction was partially masked at the community level by species-specific responses: elevated CO₂ had an ameliorating effect on Q. myrtifolia growth under water stress. The results of this long-term study not only show that atmospheric CO₂ concentration had a consistent stimulating effect on aboveground biomass production, but also showed that available water is the primary driver of interannual variation in shoot growth and that the long-term response to elevated CO₂ may have been caused by other factors such as nutrient limitation and disturbance.     

Different carbon support for respiration and secondary production in unproductive lakes

This study investigates the allocation of allochthonous organic carbon (AlloOC) to pelagic respiration and biomass production in unproductive lakes. Metabolic process rates and stable isotopic composition (δ¹³C) of crustacean zooplankton and respired CO₂ were measured in the epilimnion of 13 forest lakes in northern Sweden. The δ¹³C of zooplankton was low (−31.2 to −38.0‰) compared to that of respired CO₂ (−28.4 to −30.6‰), implying that the relative importance of AlloOC was lower for zooplankton (ca 40%) than for respiration (ca 80%). Combining δ¹³C and carbon flux data revealed that a large amount of metabolized AlloOC was lost in respiration, compared to the amount transferred to zooplankton (<3%). Thus, despite large respiratory losses, AlloOC was still important for zooplankton growth, implying a high supply of AlloOC in comparison to phytoplankton generated organic carbon in the lakes.     

Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide

Interactive effects of elevated atmospheric CO₂ and arbuscular mycorrhizal (AM) fungi on biomass production and N₂ fixation were investigated using black locust ( Robinia pseudoacacia ). Seedlings were grown in growth chambers maintained at either 350 μmol mol⁻¹ or 710 μmol mol⁻¹ CO₂. Seedlings were inoculated with Rhizobium spp. and were grown with or without AM fungi. The ¹⁵N isotope dilution method was used to determine N source partitioning between N₂ fixation and inorganic fertilizer uptake. Elevated atmospheric CO₂ significantly increased the percentage of fine roots that were colonized by AM fungi. Mycorrhizal seedlings grown under elevated CO₂ had the greatest overall plant biomass production, nodulation, N and P content, and root N absorption. Additionally, elevated CO₂ levels enhanced nodule and root mass production, as well as N₂ fixation rates, of non- mycorrhizal seedlings. However, the relative response of biomass production to CO₂ enrichment was greater in non-mycorrhizal seedlings than in mycorrhizal seedlings. This study provides strong evidence that arbuscular mycorrhizal fungi play an important role in the extent to which plant nutrition of symbiotic N₂-fixing tree species is affected by enriched atmospheric CO₂.     

Bioenergetic changes in the microalgal photosynthetic apparatus by extremely high CO2 concentrations induce an intense biomass production

Unicellular green alga Chlorella minutissima , grown under extreme carbon dioxide concentrations (0.036–100%), natural temperature and light intensities (Mediterranean conditions), strongly increase the microalgal biomass through photochemical and non-photochemical changes in the photosynthetic apparatus. Especially, CO₂ concentrations up to 10% enhance the density of active reaction centers (RC/CS_o), decrease the antenna size per active reaction center (ABS/RC), decrease the dissipation energy (DI_o/RC) and enhance the quantum yield of primary photochemistry (F_v/F_m). Higher CO₂ concentrations (20–25%) combine the above-mentioned photochemical changes with enhanced non-photochemical quenching of surplus energy, which leads to an enhanced steady-state fraction of 'open' (oxidized) PSII reaction centers (q_p), and minimize the excitation pressure of PSII (1 − q_p) under very high light intensities (approximately 1700 μmol m⁻² s⁻¹ maximal value), avoiding the photoinhibition and leading to an enormous biomass production (approximately 2500%). In conclusion, these extreme CO₂ concentrations – about 1000 times higher than the ambient one – can be easily metabolized from the unicellular green alga to biomass and can be used, on a local scale at least, for the future development of microalgal photobioreactors for the mitigation of the factory-produced carbon dioxide.