Cyanobacteria

Lore & Background

Cyanobacteria are the first organisms known to have produced oxygen. Their photopigments absorb red- and blue-spectrum sunlight to split water molecules into hydrogen ions and oxygen. The hydrogen ions react with carbon dioxide to produce carbohydrates, and oxygen is released as a byproduct. Over billions of years, this process converted Earth's anoxic atmosphere into an oxidizing one, leading to the Great Oxidation Event and the 'rusting of the Earth' during the early Proterozoic. This dramatically changed life on Earth and likely led to the evolution of eukaryotes via endosymbiosis.

Cyanobacteria use photosynthetic pigments such as chlorophyll, carotenoids, and phycobilins. Unlike heterotrophic prokaryotes, they have internal membranes called thylakoids where photosynthesis occurs. Photoautotrophic eukaryotes like red algae, green algae, and plants perform photosynthesis in organelles thought to have ancestry in cyanobacteria, acquired via endosymbiosis. These endosymbionts evolved into plastids such as chloroplasts.

Some cyanobacteria are nitrogen-fixing and live in moist soils and water, either freely or in symbiosis with plants or lichen-forming fungi. They are globally widespread and major contributors to biogeochemical cycles. Planktonic cyanobacteria are fundamental to marine food webs and contribute about 25% of global marine primary production. Marine picocyanobacteria such as Prochlorococcus and Synechococcus dominate phytoplankton assemblages, with Prochlorococcus possibly the most plentiful genus on Earth, accounting for about 20% of the oxygen in the atmosphere.

Reader's Guide

Cyanobacteria are among the oldest organisms on Earth, with fossil records dating back at least 2.1 billion years. They are the only oxygenic photosynthetic prokaryotes and have been essential players in Earth's ecosystems. Their production of oxygen transformed the planet's atmosphere and enabled the evolution of aerobic life. Today, they are major contributors to global carbon and nitrogen fluxes. However, some cyanobacteria produce cyanotoxins that cause harmful algal blooms, disrupting aquatic ecosystems and posing health risks to humans and animals. These blooms are increasing in frequency and magnitude due to agricultural and industrial pollution. Cyanobacteria also have biotechnological applications, including bioethanol production, food colorings, dietary supplements, and raw materials. Model organisms like Synechocystis and Cyanothece are studied for these purposes. Their endosymbiotic origin of plastids in eukaryotes remains a key concept in evolutionary biology.

Did You Know?

• Cyanobacteria are probably the most numerous taxon to have ever existed on Earth.
• The smallest known photosynthetic organism, Prochlorococcus, is just 0.5 to 0.8 micrometres across.
• About 25% of global marine primary production is contributed by cyanobacteria.
• Cyanobacteria produce toxins called cyanotoxins, including microcystins, saxitoxin, and cylindrospermopsin.

The Rank of Kingdom in Biological Hierarchy

When Carl Linnaeus formalized his rank-based naming system in 1735, he placed kingdom at the very top of the hierarchy, followed by class, order, genus, and species. Over time, two additional principal ranks—phylum (or division) and family—were inserted into the sequence, and in 1990 the rank of domain was added above kingdom, making kingdom the second-highest tier in the taxonomic ladder. Below kingdom, two subordinate ranks exist: subkingdom (subregnum) and infrakingdom (infraregnum). Some systems also recognize superkingdom, which may function as a synonym for domain or occupy a position between kingdom and domain. In certain classification frameworks, an extra rank called branch (Latin: ramus) can be slotted between subkingdom and infrakingdom; for instance, Cavalier-Smith's scheme uses this rank to separate Protostomia from Deuterostomia. Kingdoms themselves are subdivided into smaller groups known as phyla, and the entire structure provides the scaffolding within which organisms such as those in the phylum Cyanobacteria find their taxonomic home.

From Ancient Dichotomy to the Third Kingdom

The division of living things into animals and plants stretches back to antiquity. Aristotle catalogued animal species in his History of Animals, while his student Theophrastus produced a companion volume, the Historia Plantarum, devoted to the plant world. When Linnaeus established his two-kingdom scheme in 1735—Regnum Animale and Regnum Vegetabile—he also set aside a third kingdom, Regnum Lapideum, for minerals, yet he left all microscopic creatures out of his taxonomy entirely. This omission persisted even after Antonie van Leeuwenhoek, widely regarded as the father of microscopy, sent his first observations of single-celled organisms to the Royal Society of London in 1674. By the mid-nineteenth century, the rigid plant-animal split had grown increasingly untenable, as the boundaries between the two groups blurred. In 1860, John Hogg proposed a third kingdom, Protoctista, to house what he called the lower creatures. Two years later, Ernst Haeckel independently introduced Protista for organisms that were neither animal nor plant. Haeckel revised this grouping several times before settling on a distinction between unicellular and multicellular life.

The Prokaryote-Eukaryote Divide and Cyanobacteria's Taxonomic Journey

Advances in microscopy eventually revealed a fundamental split between organisms whose cells lack a distinct nucleus and those whose cells contain one. In 1937, Édouard Chatton coined the terms prokaryote and eukaryote to capture this distinction. The following year, Herbert F. Copeland built on this insight to propose a four-kingdom system that created a new Kingdom Monera for all prokaryotic organisms, encompassing what we now recognize as Bacteria and Archaea. Notably, Haeckel had already placed the blue-green algae—then called Phycochromacea—within Monera in his 1904 work The Wonders of Life, and over time this grouping gained wider acceptance until the blue-green algae were formally classified as bacteria within the phylum Cyanobacteria. In the 1960s, Roger Stanier and C. B. van Niel championed Chatton's earlier framework, and their 1962 paper established, for the first time, a rank above kingdom: a superkingdom or empire. Their two-empire system of prokaryotes and eukaryotes was later expanded into the three-domain model of Archaea, Bacteria, and Eukaryota.

Five, Six, or No Kingdoms at All?

The five-kingdom model, proposed by Robert Whittaker in 1969, became a widely adopted standard in biology education. Whittaker's scheme—Animalia, Plantae, Fungi, Protista, and Monera—was grounded primarily in differences in nutritional strategy: multicellular autotrophs, multicellular heterotrophs, and multicellular saprotrophs, with the remaining two kingdoms covering unicellular and simple colonial organisms. Textbooks in the United States and parts of Canada later split Monera into two separate kingdoms, Archaea and Bacteria, yielding a six-kingdom system, while educators in countries including Bangladesh, Brazil, Greece, India, Pakistan, Spain, and the United Kingdom retained the five-kingdom format. Yet some modern classifications rooted in cladistics have moved to discard the term kingdom altogether, arguing that several traditional kingdoms fail the test of monophyly—they do not encompass all descendants of a single common ancestor. In everyday usage, the words flora, fauna, and more recently funga continue to describe the life present in a particular region or era.

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