C₄ Carbon Fixation - The Evolution and Advantages of The C₄ Pathway

The Evolution and Advantages of The C₄ Pathway

C₄ plants have a competitive advantage over plants possessing the more common C₃ carbon fixation pathway under conditions of drought, high temperatures, and nitrogen or CO₂ limitation. When grown in the same environment, at 30°C, C₃ grasses lose approximately 833 molecules of water per CO₂ molecule that is fixed, whereas C₄ grasses lose only 277 water molecules per CO₂ molecule fixed. This increased water use efficiency of C₄ grasses means that soil moisture is conserved, allowing them to grow for longer in arid environments.

C₄ carbon fixation has evolved on up to 40 independent occasions in different families of plants, making it a prime example of convergent evolution. C₄ plants arose around 25 to 32 million years ago during the Oligocene (precisely when is difficult to determine) and did not become ecologically significant until around 6 to 7 million years ago, in the Miocene Period. C₄ metabolism originated when grasses migrated from the shady forest undercanopy to more open environments, where the high sunlight gave it an advantage over the C₃ pathway. Drought was not necessary for its innovation; rather, the increased resistance to water stress was a by-product of the pathway and allowed C₄ plants to more readily colonise arid environments.

Today, C₄ plants represent about 5% of Earth's plant biomass and 3% of its known plant species. Despite this scarcity, they account for about 30% of terrestrial carbon fixation. Increasing the proportion of C4 plants on earth could assist biosequestration of CO₂ and represent an important climate change avoidance strategy. Present-day C₄ plants are concentrated in the tropics and subtropics (below latitudes of 45°) where the high air temperature contributes to higher possible levels of oxygenase activity by RuBisCO, which increases rates of photorespiration in C₃ plants.