Phototrophs

Phototrophs are organisms that obtain energy from light, typically through photosynthesis, and use that energy to drive cellular processes. They are not limited to a single group and include plants, algae, cyanobacteria, and certain bacteria and archaea. Phototrophs are categorized by their carbon source: photoautotrophs fix inorganic carbon (carbon dioxide or bicarbonate) into organic compounds, while photoheterotrophs obtain carbon from organic compounds but still harvest light for energy.

In oxygenic phototrophs, such as plants, algae, and cyanobacteria, chlorophyll-based photosystems I and II capture light energy, drive an electron transport chain, split water to release oxygen, and generate ATP and NADPH. These energy-rich molecules power carbon fixation via the Calvin cycle, producing sugars that build biomass. Anoxygenic phototrophs, including purple sulfur bacteria, green sulfur bacteria, purple non-sulfur bacteria, and heliobacteria, use bacteriochlorophylls and harvest light to transfer electrons from inorganic or organic donors. They do not produce oxygen and may fix carbon through alternative pathways such as the reverse TCA cycle.

Pigments vary among phototrophs, enabling absorption of different light wavelengths and adaptation to diverse environments. Phototrophs inhabit soils, freshwater, oceans, leaf surfaces, hot springs, and extreme habitats. Some archaea and bacteria use retinal-based phototrophy, such as bacteriorhodopsin or proteorhodopsin, to harvest light energy without traditional photosynthesis. Together, phototrophs are primary producers that form the base of many ecosystems and play a central role in global carbon and energy flows.