Cyanobacteria are autotrophs. Specifically, they are photoautotrophs, meaning they use sunlight to convert carbon dioxide and water into energy-rich organic compounds through photosynthesis. They fix roughly 25 gigatons of carbon from atmospheric CO₂ every year and account for about half of all primary productivity on Earth. But their metabolic story is more interesting than a simple yes/no label suggests.
How Cyanobacteria Power Themselves
Like plants, cyanobacteria run on oxygenic photosynthesis: they capture light energy, split water molecules, release oxygen, and use the resulting chemical energy to build organic matter from CO₂. They are the only bacteria on Earth that photosynthesize this way. Inside each cell, stacked internal membranes called thylakoids house the molecular machinery that drives the process, arranged in concentric layers around the cell’s outer edge.
Cyanobacteria harvest light using two systems working together. One consists of chlorophyll-containing proteins embedded in the thylakoid membranes, similar to what you’d find in a plant leaf. The other is a structure unique to most cyanobacteria: large antenna complexes called phycobilisomes that sit on the membrane surface and capture wavelengths of light that chlorophyll alone would miss, particularly in the green-to-orange range (550 to 650 nm). This dual system lets cyanobacteria absorb sunlight across a wide spectrum, with solar energy capture efficiencies between 3% and 9%.
The chemical energy produced by photosynthesis then powers the uptake of other inorganic nutrients: nitrogen, sulfur, iron, phosphorus, and a range of essential metals and minerals. Everything the cell needs to grow comes from simple, nonliving ingredients and sunlight. That is the definition of autotrophy.
They Can Also Use Organic Food
While photoautotrophy is their default mode, cyanobacteria are more metabolically flexible than most people realize. They can grow in three different nutritional modes: phototrophic (light only), mixotrophic (light plus organic carbon), and heterotrophic (organic carbon only, no light). In mixotrophic growth, a cyanobacterium simultaneously fixes CO₂ through photosynthesis and absorbs organic compounds like sugars or acetate from its surroundings. Once the available organic carbon runs out, the cell shifts back to purely phototrophic growth.
This flexibility matters in natural environments where light availability fluctuates or dissolved organic compounds are present. It also matters in biotechnology, where researchers grow cyanobacteria in controlled conditions to produce biofuels, bioplastics, and vitamins. Feeding them supplemental organic carbon can boost growth rates significantly.
Nitrogen Fixation: Another Autotrophic Trick
Many cyanobacteria go a step beyond carbon autotrophy. They also fix atmospheric nitrogen, converting N₂ gas into ammonia that cells can use to build proteins and DNA. This makes them independent of external nitrogen sources, a rare and ecologically powerful ability.
The challenge is that the enzyme responsible for nitrogen fixation, nitrogenase, is destroyed by oxygen. Since cyanobacteria produce oxygen through photosynthesis, they face a fundamental conflict. They solve it in two ways. Some species use a biological clock to separate the two processes in time, photosynthesizing during the day and fixing nitrogen at night. Others develop specialized cells called heterocysts, thick-walled cells spaced along the filament like beads on a string, that provide a low-oxygen interior where nitrogenase can work safely. Heterocysts stop doing photosynthesis entirely and devote themselves to nitrogen fixation, feeding ammonia to neighboring vegetative cells in exchange for sugars.
They Made Earth’s Atmosphere Breathable
Cyanobacteria are among the oldest organisms on the planet, dating back 2.5 to 3.5 billion years. Their most consequential achievement was the Great Oxidation Event, roughly 2.45 to 2.32 billion years ago, when cyanobacterial photosynthesis pumped enough oxygen into the atmosphere to transform Earth’s chemistry permanently. Before this event, atmospheric oxygen was essentially zero. Afterward, it climbed to levels that eventually allowed oxygen-dependent life, including animals, to evolve.
Research published in PNAS suggests that the evolution of multicellular cyanobacteria coincided with the onset of the Great Oxidation Event. Multicellular forms may have spread more widely and produced oxygen at higher rates, tipping the atmospheric balance. This single group of autotrophic bacteria reshaped the entire planet.
The Ancestors of All Plant Photosynthesis
Chloroplasts, the structures inside plant and algae cells that carry out photosynthesis, originated from ancient cyanobacteria. More than a billion years ago, a non-photosynthetic cell engulfed a cyanobacterium and, instead of digesting it, kept it as an internal partner. Over evolutionary time, the cyanobacterium became the chloroplast.
The evidence for this is extensive. Chloroplasts and cyanobacteria share the same type of oxygen-producing photosynthesis, the same key pigments (chlorophyll a and carotenoids), and a similar internal architecture of thylakoid membranes. Chloroplast genomes contain genes arranged in the same order as their cyanobacterial counterparts, and phylogenetic analysis consistently places chloroplast genes within the cyanobacterial branch of the tree of life. Chloroplasts even reproduce by splitting in two, just like free-living bacteria. Every salad you eat, every forest you walk through, runs on molecular machinery inherited from cyanobacteria.
How Much Oxygen They Still Produce
Cyanobacteria remain enormously productive today. NOAA estimates that roughly half of Earth’s oxygen comes from the ocean, and the majority of that is produced by photosynthetic plankton, including cyanobacteria. One species alone, Prochlorococcus, is the smallest photosynthetic organism on Earth yet generates up to 20% of the oxygen in the entire biosphere. Its global population is estimated at 2.9 × 10²⁷ cells, a number so large it’s essentially meaningless without context: there are more Prochlorococcus cells in the ocean than there are stars in the observable universe.
Prochlorococcus thrives in warm, nutrient-poor tropical and subtropical waters, reaching concentrations of 250,000 cells per milliliter in the Indian and western Pacific Ocean. As ocean temperatures rise, models project its population could increase by 29% by the end of this century.
Surviving Everywhere on Earth
Cyanobacteria live in virtually every habitat: oceans, freshwater lakes, soil, desert crusts, hot springs, and polar ice. Their ability to thrive under extreme conditions earns them the label “extremophiles.” In hot environments, they produce heat-shock proteins that stabilize their cellular machinery. Under high salinity, they synthesize protective sugars like trehalose that shield membranes and proteins from damage. In polar regions, they tolerate freezing, desiccation, and extreme swings in light intensity across seasons.
They also produce a suite of secondary metabolites, including UV-absorbing compounds and other protective molecules, that help them survive intense radiation and biological competition. This adaptability, paired with their self-sufficient autotrophic metabolism, explains why cyanobacteria have persisted for billions of years and colonized every corner of the planet.