Although prokaryotes—organisms lacking a nucleus—often appear simple, they share a universal need with all living beings: a source of energy. Spanning the domains Bacteria and Archaea, these microbes display remarkable metabolic diversity, from sulfur‑oxidizing extremophiles thriving in hydrothermal vents at up to 750°F to organisms that harness sunlight.
The term phototroph (Greek for “light nourishment”) describes organisms that derive energy directly from photons. While green plants are the most familiar example, many prokaryotes and eukaryotes also photosynthesize, turning light into chemical energy.
Like plants, phototrophic bacteria use pigments to harvest photons. These pigments—bacteriochlorophylls—reside in the plasma membrane, not in chloroplasts. Seven variants (a, b, c, d, e, cs, g) absorb distinct wavelengths from infrared to far‑red light, allowing each species to occupy a specific niche in the light spectrum.
Bacterial photosynthesis mirrors plant photosynthesis in two stages:
Different bacteria employ distinct carbon‑fixation pathways. Cyanobacteria use the Calvin cycle, while others may rely on the reverse Krebs cycle, which couples electron donors like hydrogen or sulfide to CO₂ reduction.
Photoautotrophic prokaryotes form the foundation of most ecosystems. By converting light energy into sugars, they supply the primary food source for heterotrophs that cannot photosynthesize. Additionally, their CO₂ fixation helps regulate atmospheric carbon levels and, historically, produced the free oxygen that defines Earth’s breathable atmosphere—a process central to the Great Oxygenation Event and the evolution of complex life.
