Autotrophe

Autotrophe, also called autotroph, refers to an organism that can synthesize its own organic matter from inorganic carbon sources, typically carbon dioxide, using energy captured from light or from chemical reactions. This ability distinguishes autotrophs from heterotrophs, which rely on preformed organic compounds for their carbon needs.

Autotrophs are commonly categorized by energy source into photoautotrophs and chemoautotrophs. Photoautotrophs use light energy to drive carbon fixation. Plants, algae, and cyanobacteria are classic examples, with oxygenic photosynthesis releasing molecular oxygen. Some bacteria perform anoxygenic photosynthesis, using reduced sulfur compounds or hydrogen as electron donors, yet still function as photoautotrophs. Chemoautotrophs derive energy from the oxidation of inorganic molecules, such as ammonia, nitrite, sulfide, ferrous iron, or hydrogen, enabling carbon fixation in environments like deep-sea vents, hot springs, or nutrient-poor soils.

All autotrophs fix carbon dioxide into organic molecules, enabling growth and maintenance. Photoautotrophs commonly employ the Calvin cycle, though several bacteria use alternative carbon fixation pathways, such as the reverse tricarboxylic acid cycle, the reductive acetyl-CoA pathway, or the 3-hydroxypropionate cycle. Chemoautotrophs fix CO2 using energy obtained from inorganic chemical reactions rather than light.

Ecologically, autotrophs act as primary producers, forming the base of most ecosystems by converting inorganic carbon into organic matter that sustains other organisms. Oxygenic photoautotrophs contribute significantly to atmospheric oxygen and global biogeochemical cycles. In environments devoid of sunlight, chemoautotrophs can support life, as seen in hydrothermal vent communities.

Autotrophy is believed to have arisen early in life’s history, enabling carbon fixation before widespread photosynthesis. The endosymbiotic origin of chloroplasts in plants and algae highlights the evolutionary importance of autotrophy in shaping Earth's biosphere.