Cyanobacteria

• 1. Introduction

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  Cyanobacteria include a microbe called Prochlorococcus, and a single millilitre of surface seawater can hold 100,000 of its cells or more. Across the world's oceans, the estimated total is several octillion individuals, a billion billion billion. Cyanobacteria as a whole are probably the most numerous group that has ever existed on Earth. Their informal name, blue-green algae, comes from a bluish green colour, though they are bacteria. Some live as single cells, some in chains, and some in colonies that take many forms. All of them can turn sunlight into energy.

  A group this old and this abundant raises large questions. How did organisms this small change the air of an entire planet? What do they have to do with the green plants that surround us? And why do lakes and reservoirs around the world now close to swimmers when a bluish scum appears on the water?

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• 2. The Oxygen Makers

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  Until about 2.4 billion years ago, the atmosphere held oxygen at around 0.001 percent of today's level, or less. Cyanobacteria are the first organisms known to have produced the gas. Their pigments absorb red and blue light, which leaves the greenish tint, and use that energy to split water molecules into hydrogen ions and oxygen. The hydrogen ions help turn carbon dioxide into carbohydrates, a process called carbon fixation. Oxygen is simply released as a byproduct.

  In the early atmosphere, free oxygen would have been removed almost at once by various chemicals at the surface. Only by releasing it continuously over billions of years are cyanobacteria thought to have turned an oxygen-poor, weakly reducing atmosphere into an oxidizing one. Researchers call the result the Great Oxidation Event. It brought the "rusting of the Earth" in the early Proterozoic and dramatically changed the composition of life on the planet.

  The Huronian glaciation ran from around 2.4-2.1 billion years ago. Rising oxygen may have lowered the concentration of atmospheric methane and triggered that glaciation. On this reasoning, cyanobacteria may have killed off most of the other bacteria of the time.

  Oxygenic photosynthesis evolved only once, in prokaryotic cyanobacteria. Every photosynthetic eukaryote, including all plants and algae, got the ability through endosymbiosis with cyanobacteria or with hosts that already carried them. In other words, the oxygen that makes air breathable for aerobic organisms originally comes from cyanobacteria or their plastid descendants.

  Chromium isotope data point to low atmospheric oxygen in the middle of the Proterozoic, which fits the late arrival of animals. A second and much steeper rise, the Neoproterozoic Oxygenation Event, came around 800-500 million years ago.

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• 3. Fossils And Plastids

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  Shark Bay in Western Australia is one of the few places where layered stromatolites still grow in the sea. Stromatolites form in shallow water when films of microbes, especially cyanobacteria, trap, bind and cement grains of sediment. During the Precambrian they thrived in most marine and non-marine settings that sunlight could reach. After the Cambrian explosion of marine animals, grazing herbivores greatly reduced the mats in the sea. Today they survive mostly in salty waters where grazing invertebrates cannot live.

  Stromatolite fossils date from 3.5 billion years ago. The oldest undisputed evidence of cyanobacteria is 2.1 billion years old, though some evidence reaches back 2.7 billion years. Cyanobacteria might even have emerged 3.5 billion years ago, and they likely first evolved in fresh water. A related structure, the oncolite, grows in roughly spherical layers around a nucleus such as a shell fragment. Oncolites rarely exceed 10 centimetres across.

  In the 19th century, the German botanist Andreas Franz Wilhelm Schimper reported that chloroplasts look like cyanobacteria. In 1905, the Russian biologist Konstantin Mereschkowski proposed that plastids had a symbiogenic origin. Lynn Margulis brought the idea back more than 60 years later, but it was not fully accepted until supporting data piled up. Chloroplasts have a circular chromosome, ribosomes of the prokaryotic type and their own DNA, much like mitochondria.

  Green plants, red algae and glaucophytes are now thought to form one group, the Archaeplastida, which arose from a single endosymbiotic event. A separate and more recent event involved the rhizarian Paulinella chromatophora. Other lineages went further, with a eukaryote engulfing another plastid-bearing eukaryote in a "Matryoshka-like" fashion.

  Green and red algae overtook cyanobacteria as major producers on continental shelves near the end of the Neoproterozoic. Only with the Mesozoic radiations of dinoflagellates, coccolithophorids and diatoms did production in shelf waters take its modern form.

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• 4. Masters Of The Ocean

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  Marine phytoplankton supplies almost half of the Earth's total primary production, and cyanobacteria provide about 25 percent of the marine share. Only a few lineages moved into the open ocean. They are Crocosphaera and its relatives, the cyanobacterium UCYN-A, Trichodesmium, Prochlorococcus and Synechococcus. The nitrogen-fixing ones matter most. By converting nitrogen gas into ammonium, which cells then use to build amino acids and proteins, they control how productive the sea is and how much organic carbon sinks to the deep.

  Prochlorococcus was discovered in 1986. At 0.5-0.8 micrometres across, it is the smallest known photosynthetic organism. It is found between latitudes 40 degrees north and 40 degrees south and dominates the nutrient-poor regions of the oceans. It accounts for about 20 percent of the oxygen in the atmosphere, and it performs more than half of the photosynthesis of the open ocean.

  The filamentous cyanobacteria Richelia and Calothrix live in association with diatoms such as Hemiaulus, Rhizosolenia and Chaetoceros. Some planktonic species partner with haptophyte algae, such as coccolithophores. Cyanobacterial partners, called cyanobionts, have also been found in dinoflagellates, tintinnids, radiolarians and amoebae. In some hosts they fix nitrogen rather than carbon. The role of most of them is still unknown.

  Up to 5 percent of marine Synechococcus cells have been reported to hold mature viral particles. The viruses that infect cyanobacteria are called cyanophages. They have icosahedral heads packed with double-stranded DNA, and they find their hosts by colliding with them through Brownian motion. They regulate cyanobacterial populations and may help hold back blooms. The first one, LPP-1, was discovered in 1963.

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• 5. Filaments, Light, And Partners

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  Filamentous cyanobacteria can build three kinds of specialised cells. Akinetes are climate-resistant resting spores, and hormogonia are motile filaments that travel off to start new colonies. To break free, a hormogonium often has to tear apart a weaker cell in the parent filament, called a necridium. Together with the connections between their cells, these features are considered the first signs of multicellularity.

  The third specialised cell is the thick-walled heterocyst, which holds the enzyme nitrogenase. Nitrogenase is sensitive to oxygen, so the heterocyst gives it a shielded home. There, atmospheric nitrogen is fixed into ammonia, nitrites or nitrates that plants can absorb. Because photosynthesis can inhibit this work, fixation commonly runs at night. In rice paddies, Anabaena partners with the aquatic fern Azolla and can serve as a biofertilizer for the crop.

  In 1991, Ganther and others isolated nitrogen-fixing cyanobacteria including Nostoc, Anabaena and Cylindrospermum from plant roots and soil. On wheat seedlings, Anabaena loosely colonises root hairs while Nostoc clings tightly to the root surface. Elsewhere, Peltigera lichens hold cyanobacteria, and some cyanobacteria live in the fur of sloths, where they provide camouflage. In desert soils, Microcoleus vaginatus wraps sand grains in a polysaccharide sheath, which stabilises the ground and helps it hold water.

  A cell's colour depends on the light it receives. In green light, some cyanobacteria accumulate more phycoerythrin, which absorbs green. In red light they make more phycocyanin, which absorbs red. They can therefore shift from brick red to bright blue-green, a process called complementary chromatic adaptation.

  Light can also hurt. Too much of it bleaches cells, and ultraviolet radiation is especially deadly. In the hypersaline mats of Guerrero Negro in Mexico, Oscillatoria and Spirulina subsalsa sink into the lower layers by day and rise at dusk. Microcoleus chthonoplastes in the mats of Camargue, France, does the opposite and moves to the top layer during the day. Many species glide along surfaces, apparently using a "slime jet" of gel that swells rapidly as it hydrates. Synechococcus swims at 25 micrometres per second by a method unlike a bacterial flagellum.

  Anabaena variabilis can steer toward light by bending its trichome. Phormidium uncinatum cannot, and relies on random collisions to line itself up.

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• 6. Blooms And Poisons

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  In 2021, Maeda and colleagues showed that oxygen made by cyanobacteria can be trapped in a web of sticky polysaccharides and cells. Working with the cyanobacterium Synechocystis, they found that this lets the microbes form buoyant blooms. The polysaccharides carry sulphate groups, a feature seen only in cyanobacteria. Type IV pili, which are retractable protein fibres on the cell surface, also seem to play a part. Gas vesicles and carbohydrate ballast give other ways to control buoyancy.

  Freshwater blooms can look like blue-green paint or scum, and officials often close recreational waters when they appear. Calm ponds and lakes favour them. Churning water from streams or fountains disrupts their lifecycles, so blooms seldom occur in rivers unless the flow is slow. Warmth lets Microcystis outcompete diatoms and green algae. A proposed model suggests that when stressed Microcystis cells die, the microcystins they release push survivors to make more polysaccharide and form colonies.

  Cyanobacteria can produce neurotoxins, cytotoxins, endotoxins and hepatotoxins, collectively called cyanotoxins. They include microcystins, saxitoxin and cylindrospermopsin. Extreme blooms can use up the oxygen in water and cut the light that reaches below the surface. That compromises the feeding and mating of species that depend on sight. In water treatment, cyanobacteria can plug filters, and their toxins can cause serious illness if swallowed.

  People living within half a mile of cyanobacterially contaminated lakes have shown a 2.3 times greater risk of developing ALS. Around Lake Mascoma in New Hampshire, the risk was up to 25 times the expected rate. The suspected culprit is the toxin BMAA. It has also been found in desert crusts in Qatar, where it might have contributed to higher ALS rates among Gulf War veterans. Linda Lawton of Robert Gordon University in Aberdeen and her collaborators have spent 30 years studying the problem and ways to make water safer.

  Toxic blooms have caused major water quality problems in Lake Taihu in China, Lake Erie and Lake Okeechobee in the United States, Lake Victoria in Africa and the Baltic Sea. Climate change is likely to make blooms more frequent, intense and long-lasting. Warmer, more strongly layered lakes let buoyant cyanobacteria float up into the light. Long summer droughts leave water standing in reservoirs, rivers and estuaries.

  For small systems such as swimming pools, the Department of Agriculture in Australia says 12 grams of 70 percent calcium hypochlorite in 1000 litres of water is often effective. It no longer recommends copper sulfate, because it kills livestock, crustaceans and fish.

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• 7. Harvest And Technology

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  The cyanobacterium Synechocystis sp. PCC6803 was the third prokaryote, and the first photosynthetic organism, to have its genome completely sequenced. It remains an important model organism. Genome sizes vary widely. Prochlorococcus has the smallest known genome of a photosynthetic organism, at 1.7 megabases. Nostoc punctiforme has 9 megabases, and Calothrix species are estimated at 12-15 megabases, as large as yeast.

  Research into renewable energy has looked at cyanobacteria as a way to convert sunlight directly into electricity. Their internal photosynthetic pathways can be coupled to chemical mediators that pass electrons to external electrodes. Efforts are also underway to commercialise algae-based diesel, gasoline and jet fuel. Cyanobacteria have been engineered to make ethanol, and yields can rise further when one or two genes of the Calvin cycle are over-expressed.

  Spirulina, sold as the species Limnospira platensis, is a familiar cyanobacterial food. So is Aphanizomenon flos-aquae from Klamath Lake. The blue colour extracted from spirulina is used as a natural food coloring. Some microalgae also carry fatty acids, amino acids, proteins, vitamins and minerals. Cyanobacteria may also yield substances that could one day serve as anti-inflammatory agents or fight bacterial infections. In rats that had suffered heart attacks, their output of sugar and oxygen showed therapeutic value.

  Researchers from several space agencies argue that cyanobacteria could produce goods for people living at future crewed outposts on Mars, by transforming materials available on that planet.

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• 8. A Naming Dispute

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  The year 1753 is the starting point of naming under the Botanical Code, while the Prokaryotic Code starts in 1980. Most cyanobacterial names are published under the botanical system, which does not require live cultures. It relies on preserved herbarium specimens, which has slowed the arrival of molecular evidence for individual species. Proposals to start over under the Prokaryotic Code have mostly failed, because few researchers want the clerical work of re-documenting almost every taxon.

  In 1866, Haeckel placed bacteria in the phylum Monera, within the kingdom Protista. Nostoc and other cyanobacteria were left out, and classified with algae. Later, cyanobacteria were sorted by shape into five sections, numbered I-V. Phylogenetic studies do not support the first three. The last two, Nostocales and Stigonematales, form a single group of heterocyst-bearing species. Beggiatoa was formerly thought to be a colourless cyanobacterium.

  Two recent overhauls are the work of Komarek and colleagues in 2014 and of Strunecký and colleagues in 2023. In 2022, the Prokaryotic Code was amended to accept names from the botanical side. Bacteriologists prefer the name Cyanobacteriota, while botanists favour Cyanobacteria and include the non-photosynthetic Vampirovibrionophyceae. Some bacteriologists call that group Melainabacteria. The search for non-photosynthetic relatives of cyanobacteria is still very active, and the treatment of these groups may change.

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