Carbon dioxide fixation, also known as carbon assimilation, is a fundamental biological process where living organisms convert inorganic carbon dioxide into organic compounds. These organic compounds are then utilized for energy storage and as building blocks for other biomolecules. This conversion forms the basis of nearly all food chains on Earth, sustaining life.
The Fundamental Process
Carbon dioxide fixation involves transforming atmospheric carbon dioxide, an inorganic gas, into usable organic molecules. This transformation typically requires an energy input, often from sunlight, as in photosynthesis. Some organisms, however, use chemical energy for this conversion, a process known as chemosynthesis. This process captures carbon from its gaseous form, incorporating it into organic matter. This makes carbon available for organisms to build their structures and fuel metabolic activities. The initial reactants include carbon dioxide, water, and an energy source, which are converted into sugars and other organic compounds.
Key Biological Pathways
The primary mechanism for carbon dioxide fixation is the Calvin Cycle, also known as the Calvin-Benson cycle or C3 pathway, widespread among plants, algae, and cyanobacteria. This cycle occurs in the stroma of chloroplasts in eukaryotes and in the cytoplasm or carboxysomes of prokaryotes. It consumes adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH), products of photosynthesis’ light-dependent reactions.
The Calvin cycle proceeds in three main stages. The first, carbon fixation, involves the enzyme RuBisCO combining carbon dioxide with a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP). This reaction creates an unstable six-carbon compound that quickly splits into two molecules of a three-carbon compound called 3-phosphoglycerate (3-PGA).
In the second stage, reduction, 3-PGA molecules convert into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar. This conversion requires energy from ATP and reducing power from NADPH. Some G3P molecules then synthesize glucose and other carbohydrates, serving as the plant’s food source.
The final stage is RuBP regeneration. Remaining G3P molecules are rearranged and, with additional ATP, convert back into RuBP. This regeneration ensures the cycle continues to fix more carbon dioxide. Six turns of the Calvin cycle produce one glucose molecule, utilizing 6 CO2, 18 ATP, and 12 NADPH.
Some plants evolved alternative pathways to fix carbon dioxide more efficiently, especially in hot and dry conditions, minimizing photorespiration. The C4 pathway spatially separates initial carbon fixation from the Calvin cycle. In C4 plants like maize and sugarcane, carbon dioxide is first fixed in mesophyll cells into oxaloacetate, a four-carbon compound, by the enzyme PEP carboxylase.
This four-carbon compound is then transported to specialized bundle-sheath cells, where carbon dioxide is released at a higher concentration for the Calvin cycle. This helps RuBisCO operate more efficiently by reducing its exposure to oxygen.
Similarly, CAM (Crassulacean Acid Metabolism) plants, such as cacti and pineapples, separate these processes temporally. CAM plants open stomata at night to absorb carbon dioxide, fixed into malic acid and stored in vacuoles. During the day, with stomata closed to conserve water, stored malic acid releases carbon dioxide for the Calvin cycle. Both C4 and CAM pathways are adaptations that enhance water-use efficiency and allow plants to thrive in challenging environments.
Who Performs Carbon Dioxide Fixation
Carbon dioxide fixation is carried out by a diverse array of organisms, collectively known as autotrophs, meaning they can produce their own food. Photoautotrophs, the most recognized, use light energy to drive the process. This group includes green plants, various algae, and cyanobacteria.
These photosynthetic organisms are responsible for the vast majority of carbon fixation on Earth, forming the base of nearly all terrestrial and aquatic food webs. Their conversion of atmospheric carbon dioxide into organic matter makes them primary producers, supporting heterotrophs, which consume fixed carbon.
Beyond photoautotrophs, certain bacteria and archaea, termed chemoautotrophs, also fix carbon dioxide. Instead of sunlight, they derive energy from the oxidation of inorganic chemical compounds like hydrogen sulfide, ammonia, or ferrous iron. Chemoautotrophs are often found in unique and extreme environments, including deep-sea hydrothermal vents, hot springs, and soil.
Many chemoautotrophs utilize the Calvin cycle for carbon fixation, similar to photoautotrophs, though some employ alternative pathways like the reductive acetyl-CoA pathway or the reverse Krebs cycle. Their existence highlights diverse strategies life has evolved to assimilate carbon in the absence of light, contributing to localized ecosystems and the broader carbon cycle.
Global Significance
Carbon dioxide fixation plays a major role in the global carbon cycle, acting as the primary mechanism for removing carbon dioxide from the atmosphere. This process is instrumental in regulating atmospheric carbon dioxide levels, directly influencing Earth’s climate. Without continuous removal, atmospheric carbon dioxide concentrations would rise unchecked, leading to significant changes in global temperatures.
Photosynthesis, the main form of carbon fixation, releases oxygen as a byproduct, a gas indispensable for most living organisms’ respiration. This constant replenishment of atmospheric oxygen has shaped Earth’s atmosphere over geological timescales, making the planet habitable for complex life.
Carbon fixation forms the very foundation of biomass production across all ecosystems. The organic compounds generated represent stored chemical energy, transferred through food webs as organisms consume one another. This energy flow underpins virtually all life on Earth, from microscopic bacteria to large animals.
Over geological timescales, the burial of fixed carbon in sediments has led to fossil fuel formation (coal, oil, natural gas), effectively sequestering carbon from the active cycle for millions of years. This long-term carbon storage, driven by past carbon fixation, has profoundly influenced Earth’s climate balance and carbon availability in various reservoirs.