The role of organisms in the carbon cycle is central and multifaceted. They move carbon through ecosystems, link atmospheric carbon to the biosphere, and influence both short-term fluxes and long-term storage. Key roles and processes

• Photosynthesis by autotrophs: Plants, algae, and some microbes capture carbon dioxide from the atmosphere or dissolved in water and convert it into organic carbon compounds (like glucose) using sunlight. This constitutes the primary entry point of carbon into living systems.

• Respiration by all organisms: Autotrophs, heterotrophs (animals, fungi, many microbes) release CO2 back to the atmosphere or water during cellular respiration as they convert organic carbon into usable energy. This returns carbon to inorganic forms and closes much of the short-term carbon loop.

• Decomposition: When organisms die, decomposers such as bacteria and fungi break down dead matter, releasing CO2 (and methane under anaerobic conditions) and other nutrients back into the environment. This process recycles carbon within ecosystems and connects the carbon in litter and soils to the atmosphere and oceans.

• Soil and sediment storage: Carbon is stored temporarily in soils as organic matter (humus, leaf litter, roots) and in sediments (including marine sediments). Microbial activity and physical processes regulate how long carbon stays stored before returning to CO2 or being buried for longer timescales.

• Fossil carbon and long-term storage: Geological processes bury carbon over millions of years, forming fossil fuels. When burned, these storehouses are released back into the atmosphere as CO2, driving long-term atmospheric carbon changes. Human activities accelerate this flux, altering the natural balance.

• Microbial mediation: Microorganisms are instrumental at several points—fixing carbon through photosynthesis, decomposing organic matter, and influencing soil respiration and carbon turnover. Their metabolic activities determine the rates and pathways of carbon exchange among air, water, and land.

Interconnected outcomes

• Balance between fixation and release: The net effect on atmospheric CO2 depends on the relative strengths of photosynthetic carbon uptake and respiration/decomposition. In healthy ecosystems with abundant vegetation and soil carbon, uptake can offset part of the CO2 released by respiration and decay, though not always completely.

• Human impacts: Deforestation reduces the land’s capacity to remove CO2 via photosynthesis, while burning fossil fuels adds extra CO2 to the atmosphere. These changes perturb the natural carbon balance, contributing to climate change.

Common misunderstandings clarified

• All organisms contribute to carbon exchange, but the dominant path for atmospheric CO2 uptake is photosynthesis by plants, algae, and photosynthetic microbes. Respiration and decay continually recycle carbon back to CO2, completing the cycle.

• Carbon storage in soils and sediments can act as a long-term sink, but its duration varies and is influenced by climate, soil type, land use, and microbial activity.

If you’d like, I can tailor this overview to a specific level (high school, undergraduate, or layperson) or focus on a particular ecosystem (forest, ocean, tundra) and provide a concise step-by-step diagrammatic description of the carbon fluxes involved.