Every breath you take owes something to blue-green bacteria
Cyanobacteria were the first living things known to release oxygen, and over billions of years they turned Earth's air from oxygen-free to breathable, literally rusting the planet. Their descendants live on inside every plant cell as chloroplasts, and one tiny ocean genus still supplies about a fifth of the oxygen in our atmosphere.
Despite the nickname blue-green algae, cyanobacteria are bacteria: single-celled prokaryotes that photosynthesise using chlorophyll, carotenoids and pigments called phycobilins. Unusually for bacteria, they have internal membranes, flattened sacs called thylakoids, where light splits water, the hydrogen is used to fix carbon dioxide into sugars, and oxygen escapes as waste. They arose in fresh water or on land in the middle Archean, and fossils go back at least 2.1 billion years. They may be the most numerous group of organisms ever to live.
Their oxygen transformed the planet. For eons, free oxygen had been swallowed by reactive minerals at the surface; cyanobacteria kept producing it until the atmosphere became oxidising in the Great Oxidation Event of the early Proterozoic. Life had to adapt, and partnerships between oxygen-breathing and oxygen-free cells likely gave rise to complex eukaryotic cells. Later, some eukaryote engulfed a cyanobacterium that became the ancestor of chloroplasts and other plastids in algae and plants.
Today they are everywhere, from hot springs and salt flats to deserts and polar regions, often forming the upper layers of microbial mats. At sea they provide about a quarter of all marine primary production. Prochlorococcus, just 0.5 to 0.8 micrometres across, is the smallest known photosynthesiser and possibly the most abundant genus on Earth. Some filamentous species build specialised cells: heterocysts that shelter the oxygen-sensitive enzyme nitrogenase so they can convert nitrogen gas into usable ammonium, hardy resting spores called akinetes, and gliding filaments called hormogonia that drift off to start new colonies.
Their darker side is growing. Blooms fuelled by farm and industrial pollution release toxins such as microcystins and saxitoxin that poison wildlife and people, and these blooms are becoming larger and more frequent worldwide. Meanwhile, model species like Synechocystis are studied for making bioethanol, food colourings and dietary supplements.
Source: Cyanobacteria
