The concentration of CO₂ in the atmosphere has increased by 30% over the past century. While this may appear beneficial at first, stimulating photosynthesis and increasing plant production, there is unfortunately a downside. The increase in atmospheric CO₂ levels has led to a simultaneous decrease in mineral concentrations in most cultivated plants, including wheat, rice and tomatoes. This has a particularly negative effect on protein quantities and essential microelement content, such as iron and zinc. The altered mineral composition of the plants we consume increases the risk of malnutrition, particularly in countries where plant products are the main source of protein and minerals.

1. Context of the increase in atmospheric CO₂

The Earth’s atmosphere is composed of a mixture of gases dominated by nitrogen (N₂, approximately 78%) and oxygen (O₂, approximately 21%) (see Earth’s atmosphere and gaseous envelope). Carbon dioxide (CO₂) is a minor gas, currently representing less than 0.05% of the atmosphere’s composition. However, it is of major importance because variations in its concentration in the atmosphere have significant and direct consequences on both the functioning of photosynthetic organisms and the climate.

The concentration of CO₂ in the atmosphere remained stable for nearly a million years, fluctuating slightly around 250 ppm depending on glacial cycles [1]. Conversely, a gradual increase in atmospheric CO₂ concentration has been observed since the beginning of the 19^th century (Figure 1). In 2024, the average annual value of CO₂ concentration in the atmosphere, measured at the Mauna Loa reference centre (Hawaii, USA), was 424.61 ppm.

The IPCC’s assessments of the state of scientific knowledge have made it possible to establish projections on the evolution of CO₂ concentrations in the atmosphere until the end of the 21^st century (see From the discovery of the greenhouse effect to the IPCC). Based on the amounts of CO₂ likely to be emitted by human activities in the coming years, various scenarios have been established: these range from stabilisation of CO₂ concentrations in the atmosphere (optimistic scenario) to CO₂ concentrations in the atmosphere of around 1000 ppm by the end of the century (pessimistic scenario), more than twice the level recorded today and nearly four times the level observed before the industrial era (Figure 2). Since these scenarios were established, the measurements taken have followed the most extreme projection, suggesting rapid increase that will lead to high CO₂ concentrations in the atmosphere by the end of this century (Figure 2) [2].

Overlaying the IPCC projections for the 21^st century on existing data from the last 800,000 years shows a sharp break over the last century (Figure 3) [1]. On this timescale, this is a major upheaval, particularly for plants, for which CO₂ plays a decisive role in their physiological processes.

2. Determinants of the nutritional quality of cultivated plants

2.1 Elements involved in the nutritional quality of plants for human health

Nutrients are chemical substances that the body needs to perform its basic functions. There are six main categories of nutrients that are essential to human health:

• Carbohydrates, lipids and proteins are considered macronutrients and serve as sources of energy.
• Water is needed in large quantities but does not provide energy.
• Vitamins and minerals are considered micronutrients and play an essential role in metabolism [3].

Plant foods contain almost all the mineral and organic nutrients established as essential for human nutrition. Humans need more than 22 mineral elements [4]. These are divided into major minerals (macro minerals) and trace elements (micro minerals):

• Major minerals include calcium (Ca), magnesium (Mg), potassium (K), sodium (Na), chloride (Cl), phosphorus (P) and sulphur (S).
• Trace elements are iodine (I), zinc (Zn), selenium (Se), iron (Fe), manganese (Mn), copper (Cu), cobalt (Co), molybdenum (Mo), fluoride (F), chromium (Cr) and boron (B) [5].

Minerals play an essential role in our bodies, particularly in building strong bones, transmitting nerve impulses and regulating our heart rate. Consequently, a deficiency in the intake of some of these elements can have serious consequences for human health (see Focus Minerals: good for our health and that of plants). This is particularly true for iron and zinc, two micronutrients that are essential for human nutrition and are obtained almost exclusively from plant consumption. It is estimated that, at present, nearly one-fifth of the world’s population suffers from zinc deficiency. The global increase in anaemia and immune weakness is directly linked to the decrease in zinc and iron content in crops such as wheat, rice and soya beans [6].

2.2 The importance of mineral nutrition for the nutritional quality of crops

While the nutrients present in plants are essential for human nutrition, the quality of plant production also depends on an adequate supply of nutrients.

Plants are autotrophic organisms and, to ensure their growth and development, they select and extract the elements they need from their surrounding environment:

• Carbon from atmospheric carbon dioxide (CO₂).
• Oxygen (O₂) from the air and soil.
• Water and minerals from the soil.

For optimal mineral nutrition, plants need at least 14 mineral elements:

• Six elements are needed in large quantities: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S).
• At least eight others are required in smaller quantities: chlorine (Cl), boron (B), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), nickel (Ni) and molybdenum (Mo) [7].

The mineral content of cultivated plants depends, of course, on their availability in the soil, but it is also greatly affected by environmental stresses:

• Plants exposed to high salinity may experience a decrease in potassium (K) transport (see How do plants tolerate a salty diet?).
• Rising temperatures and water stress drastically reduce the transport and assimilation of macronutrients such as nitrogen (N), phosphorus (P) and sulphur (S), which affects plant growth and yield.

These considerations are particularly important in the context of climate change, especially since the increase in atmospheric CO₂ also appears to have a negative impact on the mineral content of most cultivated plants (Figure 4) [8].

3. Elevated atmospheric CO₂ is beneficial for plant growth, but not for their mineral nutrition

3.1 Photosynthesis and biomass production

The increase in atmospheric CO₂ concentration has a direct effect on plant physiology: through photosynthesis, plants are able to assimilate carbon from CO₂ in the air to produce sugars (see The path of carbon in photosynthesis), which they use in particular to synthesise other biomolecules that make up their biomass. Most plant species use C3 photosynthesis, where the first metabolic step in CO₂ assimilation is the production of a 3-carbon sugar, 3-phosphoglycerate, catalysed by the enzyme RUBISCO (ribulose 1,5-bisphosphate carboxylase/oxygenase).

For C3 plants (such as wheat or rice), the current concentration of CO₂ in the air is too low to allow RUBISCO to reach its maximum activity, and an increase in this concentration therefore stimulates photosynthesis (Figure 5).

This effect is not observed in species (such as maize) that use C4 photosynthesis (see The path of carbon in photosynthesis), whose functioning is not limited by the current concentration of CO₂ (Figure 5).

The increase of CO₂ in the atmosphere should therefore cause a “CO₂ fertilisation” effect, leading to an increase in plant biomass and improved agricultural production. This hypothesis is supported by two types of data:

(1) the results obtained from experiments involving the artificial enrichment of the atmosphere with CO_₂.

(2) long-term monitoring of terrestrial plant biomass.

Numerous studies have been conducted to characterise the response of plants to CO₂ enrichment in the air, including experiments conducted outdoors and under natural conditions (FACE experiments for Free-Air CO₂ Enrichment; see Focus The FACE technique). These studies have confirmed that for C3 species, increased CO₂ levels stimulate photosynthesis and growth (Figure 6), with yield increases of around 15 to 20% for cultivated species [9].

Long-term monitoring of terrestrial plant biomass relies on the use of satellite observation images. Data compiled since the late 1970s indicate that vegetation is becoming denser in many regions of the globe (Figure 7) [10], and that terrestrial plant biomass has increased significantly during this period in response to the past rise in atmospheric CO₂ concentration. This increased biomass constitutes a carbon sink that has mitigated the rise in atmospheric CO₂ by approximately 30%.

The effect of CO₂ fertilisation on vegetation is therefore very real and could be an important factor in decarbonising the atmosphere and improving food security.

However, it is often observed that this effect diminishes over time as plants are exposed to high levels of CO₂. This is explained by the phenomenon known as photosynthetic acclimation, which corresponds to a feedback inhibition of the photosynthetic apparatus of C3 plants, with a decrease in the amount of RUBISCO of around 20% on average. As a result, the leaves gradually become less efficient at photosynthesis, and biomass production does not increase as much as expected.

3.2 Impact on mineral nutrition: the paradox

One of the main causes of photosynthetic acclimation to high CO₂ appears to be a deficiency in plant mineral nutrition [11]. Indeed, it is often sufficient to increase the supply of minerals, particularly nitrogen, for C3 plants to fully express the growth potential associated with the CO₂ fertilisation effect.

It has also been observed that cultivation in a CO₂ enriched atmosphere leads to a general decrease in mineral content in the organs of C3 plants (Figure 8) [12]. This decrease is variable but often significant (in the order of 10 to 15%) and is observed worldwide and in the vast majority of C3 species. Thus, the increase of CO₂ in the atmosphere alters the chemical composition of plants, with the notable consequence for human nutrition that harvested organs are richer in sugars and poorer in nitrogen (and therefore protein), iron and zinc.

These observations suggest that plants grown in a CO₂-rich environment become less efficient at taking up or assimilating minerals from the soil:

• Often, the amounts of minerals absorbed by plants increase in response to rising CO₂ levels, but this increase is smaller than that of biomass, leading to a decrease in mineral content (expressed in terms of biomass used as a reference).
• Furthermore, the stimulation of mineral uptake is less significant than that of root growth, indicating reduced uptake efficiency (amount absorbed per unit of root biomass).

Surprisingly, this alteration in mineral absorption efficiency appears to be associated with the stimulation of photosynthesis induced by a high concentration of CO₂. Indeed, the decrease in mineral content is not observed in C4 plants, where photosynthesis is not increased.

This is a profound paradox, as it was previously established that photosynthesis had a positive effect on mineral absorption. This positive effect results from the action of regulatory mechanisms that coordinate carbon uptake by the leaves and mineral uptake by the roots. An illustration of this regulation is the fact that plants absorb nitrates more quickly during the day than at night, because the efficiency of root uptake is stimulated within a few hours by the illumination of the aerial parts (Figure 9A).

However, long-term stimulation (several weeks) of photosynthesis by high CO₂ levels causes the opposite effect, with a decrease in this efficiency (Figure 9B).

4. Why does a high CO₂ level impair plant mineral nutrition?

The causes of mineral depletion in plants grown in a CO₂-rich environment have not yet been clearly identified: none of the hypotheses put forward fully explain the phenomenon.

4.1 Immobilisation of minerals in underground microbial biomass

The first hypothesis is that the bioavailability of minerals in the soil could decrease due to their immobilisation in underground microbial biomass.

Indeed, the stimulation of photosynthesis leads to a significant transfer of organic carbon into the soil. This carbon provides the energy needed by microorganisms living in the soil, whose growth is stimulated in response to high CO₂ concentrations. Competition between plants and microorganisms for soil resources may therefore increase, thereby reducing the amount of minerals available to plants. However, this hypothesis alone cannot explain the mineral depletion in plants grown under high CO₂ levels, as it is also observed in plants grown hydroponically in liquid nutrient solutions and therefore in the presence of optimal mineral concentrations.

4.2 Dilution of minerals in biomass

In a second hypothesis, mineral depletion could be the result of dilution in biomass. We know that the mineral content in the aerial parts of plants does not remain constant but decreases as they grow. Because they compete for light, plants concentrate their mineral resources in the upper part of their foliage, which is well lit and where most photosynthesis takes place, to the detriment of the lower parts that are less exposed to sunlight. This inevitably leads to an overall decrease in the nitrogen content of the aerial parts as the plant grows taller. The growth stimulation induced by high CO₂ levels can amplify this effect by increasing the height of the aerial parts and thus the proportion of their biomass that is poor in minerals because it is not very active in photosynthesis. However, this explanation is contradicted by the fact that the decrease in mineral content in response to high CO₂ levels is observed even in the absence of increased growth.

4.3 Impact of stomatal closure

The increase in CO₂ concentration in the air causes the stomata in the leaves to close. Stomata are the pores through which plants transpire (see The tireless-quest for water by plants). The resulting decrease in transpiration reduces the flow of water from the soil to the aerial parts. This can have two effects on mineral nutrition:

• on the one hand, by reducing the convection of minerals in the soil to the roots.
• on the other hand, by slowing down their rise in the xylem sap towards the leaves.

These two effects may combine to limit the accumulation of minerals in the aerial parts. As with the other hypotheses, this one does not explain everything, in particular why the negative effect of high CO₂ concentrations on mineral content is not observed in C4 plants, even though CO₂ also reduces transpiration in these plants as it does in C3 plants.

4.4 A specific effect for nitrogen?

The nitrogen nutrition of plants seems to be more affected by high CO₂ concentrations than that of other minerals. Indeed, it is often nitrogen that shows the greatest decrease in content in response to high CO₂ (Figure 8). Another observation confirms that the physiology of nitrogen nutrition is a determining factor in this decrease. Curiously, legumes are among the plant species whose growth is most stimulated by high CO₂ concentrations and whose nitrogen content is least affected. This atypical response is most likely due to the unique ability of legumes to establish nitrogen-fixing symbioses (see Plants that live on air). This therefore indicates that high CO₂ levels have a particular impact on the nitrogen content of plants when they extract this element from the soil.

Several studies have highlighted a specific effect of high CO₂ concentrations on plants’ use of nitrate (NO₃⁻), which is the main source of nitrogen in the soil [11]. Both the root absorption of this ion and its assimilation into the metabolic pathways for amino acid synthesis are suppressed. The physiological causes of this negative effect are still unclear, but they could be associated with a change in the redox status of plants in response to high CO₂ levels (see next paragraph).

4.5 Role of redox status and the oxidative pentose phosphate pathway (OPPP)

Increased CO₂ affects many key enzymes involved in the redox status of plants, such as catalases, peroxidases and oxidases. This could have an impact on the accumulation of reactive oxygen species (ROS) in plants and lead to changes in the expression of genes involved in mineral transport. Reactive oxygen species are oxygenated chemical species such as free radicals, oxygen ions and peroxides, which become highly chemically reactive due to the presence of unpaired valence electrons. Examples include the superoxide anion O₂⁻, singlet oxygen O₂^., hydrogen peroxide H₂O₂ and ozone O₃. Reactive oxygen species are formed in cells in response to many environmental factors (Figure 10) and have long been considered solely toxic and involved in cell death (see Environmental constraints and oxidative stress in plants).

However, for several years now, moderate production of reactive oxygen species has been recognised as an essential mechanism of cell signalling. Redox status and the accumulation of reactive oxygen species are particularly involved in the regulation of genes encoding nitrate transporters such as NRT2.1 [13]. The impact of increased atmospheric CO₂ on the redox status of plants could thus contribute to the decrease in nitrogen content observed in plants grown under conditions of high _CO₂ concentration. In addition, increased CO₂ also represses several genes involved in the oxidative pentose phosphate pathway (OPPP) (see Focus The oxidative pentose phosphate pathway). This metabolic pathway produces reducing power in the form of NADPH, which enables the plant to detoxify reactive oxygen species. Given that the OPPP pathway is involved in the regulation of genes encoding NO₃⁻ transporters [14], the impact of CO₂ on this pathway could also contribute to a decrease in nitrogen uptake by roots in response to increased atmospheric CO₂. More generally, as the OPPP pathway and ROS production are involved in the regulation of other ion transporters [14],[15], this mechanism could explain the more general consequences of increased atmospheric CO₂ on the decline in mineral content in cultivated plants.

5. Consequences for human health and agriculture

5.1 Impact on food security and risks to public health

Given the negative effect of high CO₂ levels on the mineral content of plants, several studies have examined the potential impact of this phenomenon on food security. The consumption of products derived from cultivated plants (cereals in particular) is essential for the intake of protein and minerals, and in some regions of the world represents a very large proportion of the diet [16].

Several studies have therefore used nutritional composition data from plants grown in fields under an artificially CO₂ enriched atmosphere (an experimental approach called FACE for Free Air CO₂ Enrichment – see focus The FACE technique) and integrated this data with data on the consumption of plant products around the world. These analyses show that in regions of the world where nutrient intake is mainly sustained by plant-based foods, the increase in atmospheric CO₂ will lead to an increased risk of nutritional deficiencies, particularly those associated with protein, essential micronutrients such as iron and zinc, and certain vitamins [2],[10].

In terms of numbers, this could represent several hundred million people affected, mainly located in regions already facing high food insecurity (Figure 11).

These major effects of rising CO₂ levels on food security must be viewed in the context of global change. Numerous predictions show that global change, including changes in CO₂ levels, temperature, drought and extreme weather events, will have a negative impact on crop production [17].

5.2 Potential solutions for agriculture

There are solutions available to agriculture to mitigate this negative effect of increased atmospheric CO₂ on the mineral content of plants. On the one hand, it has been shown that greater nutrient availability can reduce the negative impact of high CO₂ concentrations on mineral nutrition [18]. Increased use of fertilisers in agriculture could therefore partially offset this negative effect, but this solution would have very harmful consequences for the environment and for producers’ profitability. It is therefore clearly neither desirable nor credible in a context where agriculture must become more sustainable, more environmentally friendly and more resource efficient.

Conversely, several factors suggest that it is possible to develop or select plant varieties that are resilient to increased atmospheric CO₂ or even benefit from it. Firstly, advances in our understanding of the mechanisms associated with plant response to high CO₂  levels now make it possible to consider their use in plant breeding. Indeed, several key physiological mechanisms targeted by high CO₂ have recently been studied, and modifying the genes associated with these mechanisms can reduce or reverse the negative effect of CO₂ on mineral nutrition [19]. Secondly, several studies have demonstrated the potential of natural genetic diversity in response to elevated atmospheric CO₂. A wide range of responses to high CO₂ levels has been observed in panels representative of the genetic diversity of several plant species. Identifying the genes associated with this diversity of response could improve mineral content under high CO₂ conditions [19].

These studies, carried out under controlled laboratory conditions, are still far from being integrated into varietal selection schemes. The relatively long time required to develop innovative lines means that efforts must be made now to integrate climate change, and particularly the increase in atmospheric CO₂ levels, into current selection schemes.

6. Messages to remember

• The increase in atmospheric CO₂ levels leads to a decrease in the overall mineral content of plants, which increases the risk of malnutrition and affects human health. This could affect several hundred million people, mainly in regions already affected by high food insecurity.
• The decrease in mineral content appears to be associated with the stimulation of photosynthesis induced by high CO₂ concentrations, which is a profound paradox because it was previously established that photosynthesis had a positive effect on mineral absorption.
• The mechanisms behind this phenomenon remain to be discovered, but the accumulation of reactive oxygen species (ROS) in response to increased atmospheric CO₂ could explain all or part of the decrease in mineral uptake by plants.
• Reintroducing genetic diversity into cultivated species is essential to obtain plants that are less sensitive to climate change and to the increase in atmospheric CO₂.
  

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  Notes & references

  Cover image. [Photo © Roger Meireles, under CC0 License, via pxhere]

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