The carbon cycle describes the continuous movement of carbon atoms between the Earth’s major spheres. This cycling involves both the abiotic parts of the planet, such as the atmosphere, oceans, and rocks, and the biotic parts of the ecosystem, including all plants, animals, and microorganisms. Carbon, the foundational element of all organic molecules, must be transferred from an inorganic reservoir—primarily carbon dioxide gas in the atmosphere or dissolved in water—into living organisms. The process of incorporating this inorganic carbon into biological structures is known as carbon fixation.
Photosynthesis: The Main Entry Point
Carbon enters the biotic world through photosynthesis performed by primary producers. On land, plants carry out this process, while in aquatic environments, algae and cyanobacteria are the major contributors. These organisms absorb carbon dioxide (\(\text{CO}_2\)) directly from the air or water and use light energy to convert it into organic compounds, making carbon available to virtually all other life forms.
The fixation of carbon occurs within the chloroplasts during the Calvin cycle. An enzyme called RuBisCO catalyzes the initial reaction. RuBisCO binds the carbon atom from a \(\text{CO}_2\) molecule to a five-carbon sugar, ribulose-1,5-bisphosphate (RuBP). This product is unstable and quickly splits into two molecules of a three-carbon compound.
Energy derived from the light-dependent reactions is used to convert these three-carbon molecules into a stable sugar called Glyceraldehyde 3-phosphate (G3P). G3P serves as the building block for larger carbohydrates. Producers use G3P to synthesize glucose, cellulose for structural support, and other complex organic molecules that form their biomass.
Transferring Carbon Through Consumption
Once carbon has been fixed into a producer’s organic molecules, it moves through the ecosystem via consumption. Herbivores, or primary consumers, acquire carbon by feeding on plants or algae. Within the consumer’s body, these large organic molecules are broken down through digestion, and the smaller carbon-containing molecules are absorbed for two main purposes.
A significant portion of the ingested carbon is used as structural material to build and repair the consumer’s cells and tissues, contributing to its growth and overall biomass. The carbon atoms become part of muscle tissue, fats, and DNA. When a carnivore or secondary consumer eats the herbivore, this organic carbon is transferred again, moving up the trophic levels of the food chain.
The carbon is also released back into the abiotic environment through cellular respiration. All organisms, including producers and consumers, break down organic carbon compounds, such as glucose, to release the stored chemical energy needed for life functions. During this metabolic process, which occurs in the cells’ mitochondria, the carbon atoms from the organic fuel are oxidized and released as \(\text{CO}_2\) back into the atmosphere or water.
Chemosynthesis: A Different Approach to Fixing Carbon
While photosynthesis is the predominant method of carbon entry, chemosynthesis supports life in environments where sunlight is absent. This method is utilized by specific types of bacteria and archaea, which are often found in extreme habitats like deep-sea hydrothermal vents and cold seeps on the ocean floor.
Instead of using light energy, these chemosynthetic organisms harness chemical energy released from the oxidation of reduced inorganic molecules. Common energy sources include hydrogen sulfide (\(\text{H}_2\text{S}\)), methane (\(\text{CH}_4\)), or ferrous iron compounds. The energy released from these chemical reactions is then used to convert inorganic carbon, typically carbon dioxide or bicarbonate, into organic matter.
These microbes form the base of unique food webs, supporting dense communities of organisms like giant tube worms and mussels, which often host the chemosynthetic bacteria within their bodies. This process demonstrates a pathway for carbon to enter living systems that is completely independent of solar energy. Although less widespread than photosynthesis, chemosynthesis provides a substantial entry point for carbon into the biotic components of these unique ecosystems.