Cyanobacteria

Cyanobacteria are a group of autotrophic gram-negative bacteria of the phylum Cyanobacteriota that can obtain biological energy via oxygenic photosynthesis. Cyanobacteria apparently originated in a freshwater or terrestrial environment, and first appeared in the middle Archean eon. They are probably the most numerous taxon to have ever existed on Earth. The name "cyanobacteria" refers to their bluish green color, which forms the basis of cyanobacteria's informal common name, blue-green algae.
Cyanobacteria are the first organisms known to have produced oxygen. Their photopigments can absorb the red- and blue-spectrum frequencies of sunlight to split water molecules into hydrogen ions and oxygen. The hydrogen ions are used to react with carbon dioxide to produce complex organic compounds such as carbohydrates, and the oxygen is released as a byproduct. By continuously producing and releasing oxygen over billions of years, cyanobacteria are thought to have converted the early Earth's anoxic, weakly reducing prebiotic atmosphere, into an oxidizing one with free gaseous oxygen, resulting in the Great Oxidation Event and the "rusting of the Earth" during the early Proterozoic, dramatically changing the composition of life forms on Earth. The subsequent adaptation of early single-celled organisms to survive in oxygenous environments likely led to endosymbiosis between anaerobes and aerobes, and hence the evolution of eukaryotes during the Paleoproterozoic.
Cyanobacteria use photosynthetic pigments such as various forms of chlorophyll, carotenoids and phycobilins to convert the photonic energy in sunlight to chemical energy. Unlike heterotrophic prokaryotes, cyanobacteria have internal membranes. These are flattened sacs called thylakoids where photosynthesis is performed. Photoautotrophic eukaryotes such as red algae, green algae and plants perform photosynthesis in chlorophyllic organelles that are thought to have their ancestry in cyanobacteria, acquired long ago via endosymbiosis. These endosymbiont cyanobacteria in eukaryotes then evolved and differentiated into specialized organelles such as chloroplasts, chromoplasts, etioplasts, and leucoplasts, collectively known as plastids.
The cyanobacteria Synechocystis and Cyanothece are important model organisms with potential applications in biotechnology for bioethanol production, food colorings, as a source of human and animal food, dietary supplements and raw materials. Cyanobacteria produce a range of toxins known as cyanotoxins that can cause harmful health effects in humans and animals. Sericytochromatia, the proposed name of the paraphyletic and most basal group, is the ancestor of both the non-photosynthetic group Melainabacteria and the photosynthetic cyanobacteria, also called Oxyphotobacteria.

Overview

Cyanobacteria are a large and diverse phylum of photosynthetic prokaryotes. They are defined by their unique combination of pigments and their ability to perform oxygenic photosynthesis. They often live in colonial aggregates that can take on a multitude of forms. Of particular interest are the filamentous species, which often dominate the upper layers of microbial mats found in extreme environments such as hot springs, hypersaline water, deserts and the polar regions, but are also widely distributed in more mundane environments as well. They are evolutionarily optimized for environmental conditions of low oxygen. Some species are nitrogen-fixing and live in a wide variety of moist soils and water, either freely or in a symbiotic relationship with plants or lichen-forming fungi.
Cyanobacteria are globally widespread photosynthetic prokaryotes and are major contributors to global biogeochemical cycles. They are the only oxygenic photosynthetic prokaryotes, and prosper in diverse and extreme habitats. They are among the oldest organisms on Earth with fossil records dating back at least 2.1 billion years. Since then, cyanobacteria have been essential players in the Earth's ecosystems. Planktonic cyanobacteria are a fundamental component of marine food webs and are major contributors to global carbon and nitrogen fluxes. Some cyanobacteria form harmful algal blooms causing the disruption of aquatic ecosystem services and intoxication of wildlife and humans by the production of powerful toxins such as microcystins, saxitoxin, and cylindrospermopsin. Nowadays, cyanobacterial blooms pose a serious threat to aquatic environments and public health, and are increasing in frequency and magnitude globally.
Cyanobacteria are ubiquitous in marine environments and play important roles as primary producers. They are part of the marine phytoplankton, which currently contributes almost half of the Earth's total primary production. About 25% of the global marine primary production is contributed by cyanobacteria.
Within the cyanobacteria, only a few lineages colonized the open ocean: Crocosphaera and relatives, cyanobacterium UCYN-A, Trichodesmium, as well as Prochlorococcus and Synechococcus. From these lineages, nitrogen-fixing cyanobacteria are particularly important because they exert a control on primary productivity and the export of organic carbon to the deep ocean, by converting nitrogen gas into ammonium, which is later used to make amino acids and proteins. Marine picocyanobacteria numerically dominate most phytoplankton assemblages in modern oceans, contributing importantly to primary productivity. While some planktonic cyanobacteria are unicellular and free living cells ; others have established symbiotic relationships with haptophyte algae, such as coccolithophores. Amongst the filamentous forms, Trichodesmium are free-living and form aggregates. However, filamentous heterocyst-forming cyanobacteria are found in association with diatoms such as Hemiaulus, Rhizosolenia and Chaetoceros.
Marine cyanobacteria include the smallest known photosynthetic organisms. The smallest of all, Prochlorococcus, is just 0.5 to 0.8 micrometres across. In terms of numbers of individuals, Prochlorococcus is possibly the most plentiful genus on Earth: a single millilitre of surface seawater can contain 100,000 cells of this genus or more. Worldwide there are estimated to be several octillion individuals. Prochlorococcus is ubiquitous between latitudes 40°N and 40°S, and dominates in the oligotrophic regions of the oceans. The bacterium accounts for about 20% of the oxygen in the Earth's atmosphere.

Morphology

Cyanobacteria are variable in morphology, ranging from unicellular and filamentous to colonial forms. Filamentous forms exhibit functional cell differentiation such as heterocysts, akinetes, and hormogonia. These, together with the intercellular connections they possess, are considered the first signs of multicellularity.
Many cyanobacteria form motile filaments of cells, called hormogonia, that travel away from the main biomass to bud and form new colonies elsewhere. The cells in a hormogonium are often thinner than in the vegetative state, and the cells on either end of the motile chain may be tapered. To break away from the parent colony, a hormogonium often must tear apart a weaker cell in a filament, called a necridium.
Some filamentous species can differentiate into several different cell types:

Vegetative cells – the normal, photosynthetic cells that are formed under favorable growing conditions
Akinetes – climate-resistant spores that may form when environmental conditions become harsh
Thick-walled heterocysts – which contain the enzyme nitrogenase vital for nitrogen fixation in an anaerobic environment due to its sensitivity to oxygen.

Each individual cell typically has a thick, gelatinous cell wall. They lack flagella, but hormogonia of some species can move about by gliding along surfaces. Many of the multicellular filamentous forms of Oscillatoria are capable of a waving motion; the filament oscillates back and forth. In water columns, some cyanobacteria float by forming gas vesicles, as in archaea. These vesicles are not organelles as such. They are not bounded by lipid membranes, but by a protein sheath.

Nitrogen fixation

Some cyanobacteria can fix atmospheric nitrogen in anaerobic conditions by means of specialized cells called heterocysts. Heterocysts may also form under the appropriate environmental conditions when fixed nitrogen is scarce. Heterocyst-forming species are specialized for nitrogen fixation and are able to fix nitrogen gas into ammonia, nitrites or nitrates, which can be absorbed by plants and converted to protein and nucleic acids. Nitrogen fixation commonly occurs on a cycle of nitrogen fixation during the night because photosynthesis can inhibit nitrogen fixation.
Free-living cyanobacteria are present in the water of rice paddies, and cyanobacteria can be found growing as epiphytes on the surfaces of the green alga, Chara, where they may fix nitrogen. Cyanobacteria such as Anabaena can provide rice plantations with biofertilizer.

Photosynthesis

Carbon fixation

The thylakoids of cyanobacteria use the energy of sunlight to drive photosynthesis, a process where the energy of light is used to synthesize organic compounds from carbon dioxide. Because they are aquatic organisms, they typically employ several strategies which are collectively known as a " concentrating mechanism" to aid in the acquisition of inorganic carbon. Among the more specific strategies is the widespread prevalence of the bacterial microcompartments known as carboxysomes, which co-operate with active transporters of CO₂ and bicarbonate, in order to accumulate bicarbonate into the cytoplasm of the cell. Carboxysomes are icosahedral structures composed of hexameric shell proteins that assemble into cage-like structures that can be several hundreds of nanometres in diameter. It is believed that these structures tether the -fixing enzyme, RuBisCO, to the interior of the shell, as well as the enzyme carbonic anhydrase, using metabolic channeling to enhance the local concentrations and thus increase the efficiency of the RuBisCO enzyme.