Cyanobacteria: photosynthetic bacteria that transformed Earth's atmosphere

Key characteristics

• Photosynthesis: Cyanobacteria use chlorophyll a and accessory pigments to capture light and generate oxygen, a process that shaped Earth's atmosphere.
• Cell structure: As prokaryotes they lack membrane‑bound organelles, but many have internal thylakoid membranes where photosynthesis occurs.
• Nitrogen fixation: Several cyanobacteria can convert atmospheric nitrogen to bioavailable forms, sometimes in specialized cells called heterocysts.
• Reproduction and form: They reproduce by binary fission, fragmentation or akinetes (resting cells); forms include unicellular, filamentous and colonial types.

Fossil history and the oxygenation of Earth

Cyanobacteria have one of the longest fossil records of any life form, with stromatolite fossils dating back at least 3.5 billion years. Their capacity for oxygenic photosynthesis gradually introduced free oxygen into the previously reducing atmosphere. This long-term change culminated in the Great Oxygenation Event, roughly 2.4 billion years ago, which transformed global chemistry, eliminated many anaerobic lineages and set the stage for aerobic life. The widely accepted endosymbiotic theory holds that the chloroplasts of plants and algae descended from an ancestral cyanobacterium that became an intracellular symbiont; molecular evidence in modern genomes supports this relationship.

Ecology, benefits and hazards

Cyanobacteria play essential roles in ecosystems by producing oxygen and supporting food webs, especially in aquatic environments. Nitrogen‑fixing cyanobacteria supply nutrients to other organisms and contribute to soil formation and primary productivity. However, under nutrient enrichment and warm conditions some species form harmful algal blooms. These blooms can produce cyanotoxins (for example, microcystins) that pose risks to humans, livestock and wildlife and can impair water supplies and recreation.

Human uses and scientific importance

Cyanobacteria are studied for their ecological role, evolutionary significance and practical applications. They are used in research on photosynthesis, microbial ecology and the origin of eukaryotic plastids. In applied science, cyanobacteria are explored for wastewater treatment, biofertilizers, carbon capture and as potential platforms for sustainable production of biofuels and valuable biochemicals. Their simplicity and photosynthetic efficiency make them attractive model organisms for synthetic biology, though scaling such technologies remains an active area of research.

Distinctions and notable facts

Though superficially similar to algae, cyanobacteria are prokaryotic; their pigments and photosynthetic pathways are ancestral to those of chloroplasts. They created some of the earliest large microbial ecosystems—stromatolites—that persist in a few places today. While many cyanobacteria are harmless or beneficial, management of eutrophication and bloom events is a major environmental challenge in freshwater and coastal systems.

Further reading and resources

• Taxonomic overview of cyanobacteria
• Bacterial cell biology and classification
• Principles of oxygenic photosynthesis
• Why cyanobacteria are not algae
• Phylum-level descriptions and diversity
• Species lists and databases
• Molecular evidence linking cyanobacteria and chloroplasts
• Ancient fossils and stromatolite records
• Stromatolites: formation and significance
• Archaean biosphere studies
• Proterozoic microbial ecosystems
• Geological eons and deep time
• Early Earth's reducing atmosphere
• Oxygen as a biospheric agent
• Timescales in Earth's early history
• The Great Oxygenation Event and its consequences