CO2 Concentrating Mechanisms in CO2 Fixation Pathways | Photosynthesis

The below mentioned article provides a summary on Co₂ Concentrating Mechanisms in CO₂ Fixation Pathways.

When the terrestrial plants first appeared in the ancient geologic time, the atmospheric CO₂ concentration was several fold higher than today. Ultimately, the CO₂ concentration became reduced to the present low values. It is believed that the relative abundance of land plants was mainly responsible for the substantial reduction in CO₂ concentration by photosynthetic assimilation.

The enzyme Rubisco is the entry point for inorganic carbon into the photosynthetic carbon reduction cycle.

Rubisco exerts the main regulatory control to CO₂ assimilation in C₃ plants, which constitute about 95% of the plant species. Atmospheric O₂ also interacts with Rubisco as a competitive inhibitor with respect to CO₂ and as a substrate for this bi-functional enzyme to produce phosphoglycolate, which is partially metabolized to CO₂ in the photo respiratory carbon oxidation cycle.

Consequently, Rubisco is not saturated by CO₂ in the present atmosphere. Thus the present day plants have developed several photosynthetic adaptations, which facilitate their survival in a CO₂-depleted atmosphere. These adaptations include the operation of a CO₂ concentrating mechanism (CCM).

Two atmospheric gases, CO₂ and O₂ act as competitive inhibitor for each other for the same site on the large subunit of rubisco (8 large subunits encoded by chloroplast genome and 8 small subunits of rubisco encoded by nuclear genome). Oxygenase action of rubisco can be reduced by raising CO₂ concentration in the vicinity of rubisco in order to suppress photo respiratory loss of photo synthetically fixed carbon.

It is quite apparent that different CO₂ fixation pathways can efficiently serve this function and each of these has evolved a mechanism by which the diversion of carbon from photosynthesis to photorespiration because of oxygenase action of rubisco has been largely prevented.

It is through this mechanism, photorespiration is greatly reduced in C₄ plants, CAM plants, C₁ plants (microalgae, cyanobacteria) and submerged aquatic macrophytes (called SAM plants), all of which possess CO₂ concentrating mechanisms (CCMs) to overcome CO₂ deficient situation. There are three different mechanisms of concentrating CO₂ at the site of carboxylation of rubisco.

These are as follows:

1. C₄-Mode:

The first CCM is exhibited by C₄ plants. It is characterized by primary carboxylation in mesophyll cells and formation of C-4 dicarboxylic acids. This is followed by intercellular transport of C-4 acids from mesophyll cells to adjacent bundle sheath cells through the symplasts and their subsequent decarboxylation, thereby enriching rubisco environment with CO₂.

Thus in C₄ plants, CCM is based on spatial separation of two carboxylation reactions and O₂ inhibition of photosynthesis is largely prevented. The success of this mechanism lies in the fact that PEP-case of mesophyll is not inhibited by O₂. Thus, as a result of CO₂ concentrating mechanism operating in bundle sheath cells where rubisco resides, C₄ photosynthesis does not become sensitive to O₂.

Furthermore, PEP-case activity of mesophyll is very high with relatively greater affinity towards CO₂ as compared to rubisco. So, practically no CO₂ can escape through mesophyll cells into the atmosphere, because it is immediately re-fixed and returned to bundle sheath.

2. CAM Mode:

The second CCM is shown by CAM plants, i.e., plants showing Crassulacean acid metabolism. This is characterized by primary carboxylation at night leading to the formation of dicarboxylic acids and their storage in vacuoles.

Then these are released in the cytoplasm during day, followed by decarboxylation of C-4 acids, which enrich the vicinity of rubisco with CO₂. Thus in CAM plants, CCM is based on temporal separation of two carboxylation reactions.

3. C₁ Photosynthetic Cycle:

The third type of CCM has been discovered recently and is exhibited by aquatic algae (green microalgae and cyanobacteria), where dissolved inorganic carbon (C_i) is transported into the cell across the plasma membrane in the form of either CO₂ or HCO₃^– but stored there as HCO₃^– ion.

Its decarboxylation, i.e., formation of CO₂ and H₂O by the enzyme carbonic anhydrase (CA) leads to enrichment with CO₂ around the active site of rubisco under extreme CO₂ limiting condition. This results in enhanced affinity for CO₂ and improved photosynthetic efficiency.

Mechanism:

There are four major components of CO₂ concentrating mechanism (CCM) in the organisms showing C_i photosynthesis as follows:

(a) A pumping mechanism that actively transports inorganic carbon (C_i) into the cell and chloroplast. This is situated in the plasma membrane.

(b) An energy supply system to drive CO₂ and HCO₃^– across the plasma membrane which is linked with plasma membrane-bound electron transport system and ATPase driven proton pump.

(c) A CO₂ leakage control device to reduce the efflux of CO₂ out of the cell to the surrounding medium. Excess CO₂ may be maintained as HCO₃^– pool inside the cell.

For this purpose, a ‘microenvironment’ has been developed in algae in the form of carboxysomes in cyanobacteria and pyrenoids in chloroplast of green microalgae where CO₂ is liberated from HCO₃^– by carbonic anhydrase (CA) enzyme activity, thus enrichirig the vicinity of rubisco with CO₂. Both rubisco and CA remain in these structures.

Protein cover in carboxysomes and starch sheath in pyrenoids are more permeable to HCO₃^– and less permeable to CO₂, thereby preventing leakage of CO₂ out of these microenvironments.

(d) A mechanism operates to provide inter-conversion between CO₂ and HCO₃^– so that both forms of C_i may serve as substrate for the transport and fixation process. CA can serve this purpose.

The active transport of CO₂/ HCO₃^– occurs via base catalysed reaction and can be mediated by a basic metal Zn^{2 +} at the active site of the enzyme CA located in the plasma membrane. C_i transport is linked to an energy-driven process in the form of electron transport and ATPase pump.

A single transporter model has been proposed instead of two different ones in which the transporter may use either CO₂ or HCO₃^– as substrate.

In whatever form of C_i is taken up, it is accumulated only as HCO₃^– in the cytosol. In this model, the transporter has CA-like properties, which accepts either CO₂ or HCO₃^– or both on the outside surface, but HCO₃^– appears to be the only species of C_i delivered to the interior of the cell.

Although both forms of C_i are actively taken up, CO₂ is the preferred molecule. It is postulated that HCO₃^– which accumulates in the cytosol penetrates into the discrete spherical polyhedral protein coated structures termed carboxysomes where it is decarboxylated by carbonic anhydrase (CA) and thus rubisco is enriched with CO₂ for efficient fixation.

It is interesting to note that CA is absent in cytosol. So this will prevent the formation of CO₂ from HCO₃^– through action of CA and will minimize the wasteful leakage of CO₂ out of the cell. The plants showing C_i photosynthesis may be called C₁ plants, where 1-C compound HCO₃^– donates CO₂ for photosynthetic fixation. This is comparable with C₄ plants where 4-C compound donates CO₂.

Besides the above three different CCMs in photosynthetic organisms, there are many submerged aquatic macrophytes (SAM), which show mixed pathways. They exhibit photosynthetic characteristics related to their submerged habitat.

The SAM mode of CO₂ fixation pathway in these plants does not exactly fit into the three major CCMs found in terrestrial plants and algae, and hence, Holaday et al., (1983) termed this as SAM mode of CO₂ fixation which in fact involves all the known CCMs depending on the prevailing growth conditions.

Examples of SAM plants are Hydrilla, Myriophyllum, Ceratophyllum, Potamogeton, etc. In summer season, low CO₂ concentration and high O₂ concentration prevail in the aquatic environment during daytime.

This is created by high light intensity and temperature favouring high rate of photosynthesis. Such a condition also favours photorespiration, particularly when CO₂ is very depleted in the afternoon by photosynthetic activity.

In this situation, these plants shift to C₄ mode of photosynthesis for cutting down photorespiration, which is required for greater productivity and survival. In winter, however, SAM plants revert back to C₃ mode of photosynthesis when the environmental conditions do not favour photorespiration.

When the atmosphere is extremely hot, stomata may remain closed during daytime, and CO₂ will be fixed during night. This is definitely a CAM adaptation where carbon uptake in dark not only reduces the loss of carbon in dark respiration, possibly through re-fixation by PEPcase, but a higher CO₂ concentration prevails during the night like CAM plants. SAM plant also show C₁ character as they take up CO₂ and HCO₃^– ions from the external medium.

These plants make use of HCO₃^– pool which serves as the source of CO₂ similar to green algae and cyanobacteria. Basically, all SAM plants are, however, C₃ type plants under normal environmental conditions with diurnal variation in photo respiratory activity in response to external CO₂ and O₂ concentration.

Warburg effect, i.e., O₂ inhibition of photosynthesis, is shown by SAM plants, which is a common feature in C₃ species. Thus, the SAM plants exhibit all the known types of CCM. These submerged aquatic macrophytes show mixed pathways and hence they should constitute a distinct photosynthetic group.