photoautotrophy

Photoautotrophy is a metabolic strategy in which organisms capture light energy and use it to synthesize organic carbon from carbon dioxide, effectively converting inorganic carbon into biomass. Photoautotrophs derive their energy from light and their carbon from CO2, distinguishing them from chemoautotrophs that use chemical energy. Major groups include land plants, algae, and cyanobacteria, as well as many of the protists.

Most photoautotrophs perform photosynthesis, a two-stage process. In oxygenic photosynthesis, light-dependent reactions drive electron transport, splitting water to release O2 and generate ATP and NADPH, which power the Calvin cycle that fixes CO2 into sugars. Some bacteria perform anoxygenic photosynthesis, using electron donors other than water and producing little or no oxygen.

Key pigments include chlorophylls and carotenoids; photosynthetic membranes host the photosystems I and II. In eukaryotes, these processes occur largely within chloroplasts and their thylakoid membranes, while photosynthetic bacteria use specialized membranes. The pigment composition shapes the wavelengths absorbed and the ecological niches these organisms occupy.

Ecological and evolutionary significance: Photoautotrophy underpins most terrestrial and aquatic food webs and plays a central role in the global carbon cycle. Oxygenic photosynthesis by cyanobacteria and the subsequent endosymbiotic acquisition of chloroplasts by early eukaryotes helped shape Earth's atmosphere and ecosystems. Rates of photoautotrophic productivity are influenced by light, temperature, nutrient availability, and CO2 concentration.