Fractionation of carbon isotopes in oxygenic photosynthesis

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Oxygenic photosynthesis is a process used by plants, algae, and cyanobacteria to convert carbon dioxide and water into carbohydrates using energy from light and releasing molecular oxygen as a product. The chemical pathway of photosynthesis fixes carbon in two stages: the light-dependent reactions and the light-independent reactions.

The light-dependent reactions capture light energy to generate the energy-storage molecules NADPH and ATP, following the overall general equation:

2 H₂O + 2 NADP⁺ + 3 ADP + 3 P_i + light -> 2 NADPH + 2 H⁺ + 3 ATP + O₂

The light-independent reactions undergo the Calvin-Benson cycle, in which the energy from NADPH and ATP is used to convert carbon dioxide and water into organic compounds via the enzyme RuBisCO. The overall general equation for the light-independent reactions is the following:

3 CO₂ + 9 ATP + 6 NADPH + 6 H⁺ -> C₃H₆O₃-phosphate + 9 ADP + 8 P_i + 6 NADP⁺ + 3 H₂O

The 3-carbon products of the Calvin cycle are later converted to glucose or other carbohydrates such as starch, sucrose, and cellulose.

Carbon on Earth naturally occurs in two stable isotopes, with 98.9% in the form of ¹²C and 1.1% in ¹³C. The ratio between these isotopes varies in biological organisms due to metabolic processes that transform, or "fractionate" carbon through kinetic or thermodynamic effects. Oxygenic photosynthesis takes place in plants and microorganisms through different chemical pathways, which causes varying forms of organic material to reflect different ratios of ¹³C isotopes. Understanding these variations in carbon fractionation across species is applied in isotope geochemistry and ecological isotope studies, to understand biochemical processes, establish food chains, or reconstruct past climates with geological samples.

Carbon isotope fractionations are expressed in the measure ?¹³C ("delta thirteen C"), which is reported in parts per thousand (per mil, ?). ?¹³C is defined in relation to the Pee Dee Belemnite (PDB) as an established reference standard, as below:

${\displaystyle \delta ^{13}C=\left({\frac {\left({\frac {^{13}C}{^{12}C}}\right)_{sample}}{\left({\frac {^{13}C}{^{12}C}}\right)_{standard}}}-1\right)\times 1000}$

Phototrophic organisms uptake inorganic CO₂ from the atmosphere or water and then use photosynthesis to fix organic carbon in the form of carbohydrates. Organic carbon contains less ¹³C (ie, is depleted) relative to inorganic carbon because photosynthetic carbon fixation involves several fractionating processes. The following sections will outline different photosynthetic pathways and their associated delta values.

Video Fractionation of carbon isotopes in oxygenic photosynthesis

Rubisco

A large fractionation of ¹³C in photosynthesis is due to the carboxylation reaction, which is carried out by the enzyme ribulose-1,5-bisphosphate carboxylase oxygenase, or Rubisco. Rubisco catalyzes a five-carbon molecule, ribulose-1,5-bisphosphate (abbreviated as RuBP) with CO₂ to form two molecules of 3-phosphoglyceric acid (abbreviated as PGA). PGA reacts with NADPH to produce 3-phosphoglyceraldehyde. Isotope fractionation due to carboxylation alone is predicted to be a 28? depletion. Rubisco causes a kinetic isotope effect because ¹²C and ¹³C compete for the same active site and ¹³C has an intrinsically lower reaction rate.

Maps Fractionation of carbon isotopes in oxygenic photosynthesis

Modeling Fractionation

In addition to the discriminating effects of enzymatic reactions, the diffusion of CO₂ gas to a plant cell carboxylation site also influences isotopic fractionation. Depending on the type of plant (see sections below), external CO₂ must be transported through the boundary layer and stomata and into the internal gas space of a plant cell, where it dissolves and diffuses to the chloroplast. The diffusitivity of a gas is inversely proportional to the square root of its molecular reduced mass, causing ¹³CO₂ to be 4.4% less diffusive than ¹²CO₂.

A prevailing model for fractionation combines the isotope effects of the carboxylation reaction with the isotope effects from gas diffusion in the following equation:

${\displaystyle \delta ^{13}CO_{2\ sample}=\delta ^{13}CO_{2\ atm}-a-(b-a)(c_{i}/c_{a})}$

Where:

• the atmospheric ?¹³CO_{2 atm} = -7.8?
• the discrimination due to diffusion a = 4.4%
• the carboxylation discrimination b = 30?
• c_a is the partial pressure of CO₂ in the external atmosphere, and
• c_i is the partial pressure of CO₂ in the intercellular spaces.

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In C3 plants

A C3 plant uses C3 carbon fixation, one of the three metabolic photosynthesis pathways. This pathway is the first step of the Calvin-Benson cycle, which converts CO₂ and RuBP into 3-phosphoglycerate. C3 plants are the most common type of plant, and typically thrive under moderate sunlight intensity and temperatures, CO₂ concentrations above 200 ppm, and abundant groundwater. C3 plants do not grow well in very hot or arid regions, for which C4 and CAM plants are better adapted.

The isotope fractionation in C3 carbon fixation arise from the combined effects of diffusion of CO₂ gas through the stomata of the plant, and the carboxylation via Rubisco. Stomatal conductance discriminates against the heavier ¹³C by 4.4?. RuBisCO carboxylation has a larger discrimination of 27?.

The enzyme catalyzes the carboxylation of of CO₂ and the 5-carbon sugar, RuBP, into 3-phosphoglycerate, a 3-carbon compound. through the following reaction:

CO₂ + H₂O + RuBP ->_Rubisco 2(3-phosphoglycerate)

The product 3-phosphoglycerate is depleted in ¹³C due to the kinetic isotope effect of the above reaction. The overall ¹³C fractionation for C3 photosynthesis ranges between -20 to -37?.

The wide range variation of delta values expressed in C3 plants is modulated by the stomatal conductance, or the rate of CO₂ entering, or water vapor exiting, the small pores in the epidermis of a leaf. The ?¹³C of C3 plants depends on the relationship between stomatal conductance and photosynthetic rate, which is a good proxy of water use efficiency in the leaf. C3 plants with high water-use efficiency tend to be less fractionated in ¹³C (ie, ?¹³C is relatively less negative) compared to C3 plants with low water-use efficiency.

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In C4 plants

C4 plants rely on C4 carbon fixation and are more prevalent in hot, sunny, and dry climates. These plants differ from C3 plants because the Calvin Cycle in C4 plants produces a four-carbon molecule called malate, rather than 3-phosphoglycerate. This difference allows C4 photosynthesis to efficiently shuttle CO₂ to the Rubisco enzyme and maintain high concentrations of CO₂ within bundle sheath cells. These cells are part of the characteristic kranz leaf anatomy, which spatially separates photosynthetic cell-types in a concentric arrangement to accumulate CO₂ near Rubisco. These chemical and anatomical mechanisms improve the ability of Rubisco to fix carbon, rather than perform its wasteful oxygenase activity. The Rubisco oxygenase activity, called photorespiration, causes the RuBP substrate to be lost to oxidation, and consumes energy to do so. The adaptation of C4 plants provides an advantage over the C3 pathway, which loses efficiency due to photorespiration. The ratio of photorespiration to photosynthesis in a plant varies with environmental conditions, as decreased CO₂ and elevated O₂ concentrations would increase the efficiency of photorespiration. Atmospheric CO₂ on Earth decreased abruptly at a point between 32-25 million years ago. This gave a selective advantage to the evolution of the C4 pathway, which can limit photorespiration rate despite the reduced ambient CO₂. Today, C4 plants represent roughly 5% of plant biomass on Earth, but about 23% of terrestrial carbon fixation. Types of plants which use C4 photosynthesis include grasses and economically important crops, such as maize, sugar cane, millet, and sorghum.

Isotopic fractionation is different in C4 carbon fixation compared to C3 plants due to the spatial separation of CO₂ capture in the mesophyll cells and the Calvin cycle in the bundle sheath cells. In C4 plants, carbon converted to bicarbonate, fixed into oxaloacetate via the enzyme phosphoenolpyruvate (PEP) carboxylase, which is then converted to malate, a four-carbon (C-4) compound. The malate is transported from the mesophyll to bundle sheath cells, which are impermeable to CO₂. The internal CO₂ is concentrated in these cells as malate is reoxidized then decarboxylated back into CO₂ and pyruvate. This enables Rubisco to perform catalysis while internal CO₂ is sufficiently high to avoid the competing photorespiration reaction. The delta value in the C4 pathway is -12 to -16? depleted in ¹³C due to the combined effects of PEP carboxylase and Rubisco.

The isotopic discrimination in the C4 pathway varies relative to the C3 pathway due the additional chemical conversion to steps and activity of PEP carboxylase. After diffusion into the stomata, the conversion of CO₂ to bicarbonate concentrates the heavier ¹³C. The subsequent fixation via PEP carboxylase is thereby less depleted in ¹³C than that from Rubsico: about 2? in PEP carboxylase versus 29? in Rubisco. However, a portion of the isotopically-heavy carbon which was fixed by PEP carboxylase leaks out of the bundle sheath cells. This limits the carbon available to Rubisco, which in turn lowers its fractionation effect. The overall delta value in C4 plants is -12 to -16 ?.

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In CAM plants

Plants that use crassulacean acid metabolism, also known as CAM photosynthesis, temporally separate their chemical reactions between day and night. This strategy is well-adapted to increase water-use efficiency in arid climates because it modulates stomatal conductance. During the night, CAM plants open stomata to allow CO₂ to enter and become fixed to organic acids stored in vacuoles. This carbon is released to the Calvin cycle during the day, when stomata are closed to prevent water loss, and the light reactions provide necessary ATP and NADPH. This pathway differs from C4 photosynthesis because, in CAM plants, fixed carbon is stored in the vesicle at night for later use during the day. Thus, CAM plants temporally concentrates CO₂ to improve Rubsico efficiency, whereas C4 plants spatially concentrate CO₂ in bundle sheath cells. The distribution of plants which use CAM photosynthesis includes epiphytes (e.g., orchids, bromeliads) and succulents xerophytes (e.g., cacti).

In Crassulacean acid metabolism (CAM), isotopic fractionation combines the effects of C3 and C4 pathways between daytime and nighttime. At night, when temperature and water loss are lower, the CO₂ diffuses through the stomata and produce malate via phosphenolpyruvate carboxylase. During the following day, stomata are closed, malate is decarboxylated, and CO₂ is fixed by Rubisco. This process alone is similar to that of C4 plants and yields characteristic C4 fractionation values of approximately -11?. However, in the afternoon, CAM plants may open their stomata and perform C3 photosynthesis. In daytime alone, CAM plants have approximation -28? fractionation, characteristic of C4 plants. These combined effects provide ?¹³C values for CAM plants in the range of -10 to -20?.

The ¹³C to ¹²C ratio can indicate the temporal separation of CO₂ fixation, which is the extent of biomass derived from nocturnal CO₂ fixation relative to diurnal CO₂ fixation. This distinction can be made because PEP carboxylase, the enzyme responsible for net CO₂ uptake at night, discriminates ¹³C less than Rubisco, which is responsible to daytime CO₂ uptake. CAM plants which fix CO₂ primarily at night would be predicted to show ?¹³C values similar to C4 plants, whereas daytime CO₂ fixation would show ?¹³C values more similar to C3 plants.

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In phytoplankton

In contrast to terrestrial plants, where CO₂ diffusion in air is relatively fast and not normally limiting, diffusion of dissolved CO₂ in water is considerably slower and can often limit carbon fixation by phytoplankton. As gaseous CO_2(g) is dissolved into aquatic CO_2(aq), it is fractionated by both kinetic and equilibrium effects that are temperature-dependent. Relative to plants, the dissolved CO₂ source for phytoplankton can be enriched in ¹³C by about 8? from atmospheric CO₂.

Isotope fractionation of ¹³C by phytoplankton photosynthesis is affected by the diffusion of extracellular aqueous CO₂ into the cell, the Rubisco-dependent cell growth rate, and the cell geometry and surface area. In phytoplankton, the use of bicarbonate and carbon concentrating mechanisms distinguish the isotopic fractionation from plant photosynthetic pathways.

The difference between intracellular and extracellular CO₂ concentrations reflects the CO₂ demand of the cell, which is determined by its growth rate. The ratio of carbon demand to supply governs the diffusion of CO₂ into the cell, and is negatively correlated with the carbon fractionation by phytoplankton. This allows the ?¹³C values between CO_2(aq) and phytoplankton biomass to be used to estimate the phytoplankton growth rates.

However, growth rate alone does not account for observed fractionation. The flux of CO_2(aq) into and out of a cell is roughly proportional to the cell surface area, and the cell carbon biomass varies as a function of cell volume. Phytoplankton geometry that maximizes surface area to volume should have larger isotopic fractionation due to photosynthesis.

The biochemical characteristics of phytoplankton are similar to C3 plants, while their gas exchange characteristics can more closely resemble the C4 strategy. More specifically, phytoplankton improve the efficiency of their primary carbon-fixing enzyme, Rubisco, with carbon concentrating mechanisms (CCM), just as C4 plants accumulate CO₂ in the bundle sheath cells. The CCM activity may include the active uptake of bicarbonate and CO₂ through the cell membrane, active transport of inorganic carbon from the cell membrane to the chloroplasts, and active conversion of CO₂ to bicarbonate. Combined, the parameters affecting ¹³C fractionation in phytoplankton contribute to ?¹³C values between -18 to -25?.

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See also

• Isotope geochemistry
• Paleoclimatology
• Carbon dioxide in Earth's atmosphere

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References

Source of the article : Wikipedia