a computer model of the filamentous cyanobacterium Oscillatoria. Three filaments are
shown, each filament being a chain of cells (the cells being
disc-like in Oscillatoria). Cyanobacteria, also called
blue-green algae, are a major group of photosynthetic bacteria.
Their photosynthetic pigments give them their blue-green
colouration (though sometimes other pigments mask this as some
forms are orange or red in colour).
The humble cyanobacteria dominated life on Earth for an immense period of time, from about 3 billion years ago to 500 million years ago - what could be called the Age of the Cyanobacteria. This age was crucial for the development of animal and plant life on Earth, since during this time the cyanobacteria created the first oxygen atmosphere. They are still widespread, though often overlooked. Their varied forms, vibrant colours and, in some cases, their graceful gliding movements make them beautiful subjects for the microscope. On close examination, they prove to be much more sophisticated and complex than first impressions might suggest.
The internal structure of a typical cyanobacterium is shown below:
As prokaryotes, the blue-greens have no nuclear envelope and no
true nucleus, instead the DNA is circular and free in the cytosol (cytosol = liquid component of the cytoplasm), though intricately folded and attached to special scaffolding proteins. Typical of bacteria, the DNA is often confined to a central region of the cell, the nucleoplasm, forming a nucleoid. Surrounding this is cytoplasm rich in prokaryotic ribosomes - the riboplasm. The peripheral cytoplasm contains the photosynthetic apparatus. Flattened membrane vesicles called thylakoids house the pigments and proteins that make up the photosynthetic machinery. Each outer cytosolic surface of each thylakoid (the surface facing into the cytosol) is studded with particles called phycobilisomes, which
consist of chlorophyll type a and accessory pigments, called phycobiliproteins, such as phycoerythrin (red) and phycocyanin (cyan). The accessory pigments both screen and protect the chlorophyl from damaging UV light and also trap photons and funnel
them to the chlorophyl, acting as antennae, that increase the wavelengths of light that can be used for photosynthesis.
The photosynthetic membranes, or thylakoids, are not confined inside chloroplasts as in plants and eukaryotic algae (see photosynthesis in plants) but are free in the cytoplasm. They are also single and not stacked as in chloroplasts.
Gas vacuoles (clusters of protein gas-filled rods called gas vesicles) are often present in aquatic forms. These act to regulate buoyancy, helping the cells to float at the right depth in the water column where light levels are optimal for photosynthesis. Cyanophycin granules are large, up to 500 nanometres in diameter, and typically located near the cell periphery. Cyanophycin is a polypeptide (polymer or chain of amino acids) produced by a ribosome-independent mechanism and involved in nitrogen metabolism. Lipid droplets, storing lipids for later use, may also be found in the periphery of the cell. Large polyphosphate bodies are granules that store phosphate and tiny glycogen granules or rods (30 to 65 nanometres in diameter) are situated between the thylakoids and act as a store of glucose (for carbon and energy - a fuel store). Carboxysomes (polyhedral bodies) are 200 to 300 nanometres in diameter and consist of the main enzyme involved in photosynthesis, rubisco (ribulose -1,5-bisphosphate carboxylase).
Cyanobacteria are Gram negative bacteria, meaning that they do not stain purple with Gram's stain. The Gram stain dyes peptidoglycan purple. Peptidoglycan is the polymer that makes up the tough cell-wall layer in the cell envelopes of most bacteria. However, in Gram negative bacteria the peptidoglycan wall is covered on the outside by an additional membrane, the outer membrane, which prevents the gram stain from reaching the peptidoglycan and so this type of bacteria do not stain purple. (In Gram positive bacteria, the peptidoglycan sits on top of the cell membrane, which is also called the inner membrane in Gram negative bacteria, and so is reached by the stain, staining the cells deep purple.) Bacteria have other optional layers in the cell envelope. Cyanobacteria typically have an S-layer, a layer of proteins fitted together like a mosaic, which covers the outer membrane and on top of this is another layer of protein fibrils, called oscillin fibrils, wound around the cell in a helix.
In addition, a thin slime sheath, external to the oscillin layer, encloses the cell or filament. Cyanobacteria may be immotile, or they move by gliding motility. A good example of this is seen in the Oscillatoria filament. These filaments glide along a solid surface, leaving the exuded slime sheet behind them as a collapsed tube as they go. The slime sheath is continuously secreted during these movements. Filaments will also glide vertically within their slime sheaths, an important adaptation in some forms for altering their height above the surface. One of the chief reasons why some bacteria have evolved into filaments of cells is to increase their length, allowing them to reach above the stagnant boundary layer. Gliding also helps cyanobacteria to reach optimal lighting levels for photosynthesis.
Gliding Cyanobacterial filaments appear to be driven by jet propulsion! In oscillatoria, for example, there are rows of pores on either side of the annular groove which demarcates one cell from a neighbouring cell. Slime jets from these pores in a controlled manner, propelling the filaments in one direction or the other. The oscillin channels the slime jets in a helix around the cell, causing the filament to rotate on its axis as it glides. The slime eventually forms a collapsed tube trailing behind the filament, as secretion of new slime continues. The slime is not too expensive to produce, as it consists of polysaccharides (chains of sugar molecules) that expand massively upon mixing with water. earlier authors claimed that slime production was a consequence of and not the course of motility, and controversy still surrounds the mechanism.
details on bacterial gliding motility.
It is possible that some cyanobacteria contain contractile protein fibres, and these have been implicated in jerking and clumping movements seen in Spirulina, whose filaments are helical.
In Oscillatoria the cross-walls, which divide one cell from the next in the filament, consist of peptidoglycan lined by the inner membrane of each cell on each side. The outer membrane, S-layer and oscillin layer do not extend into the cross-walls, but instead form continuous layers along the whole filament. This means that the periplasm (the 'space' between the inner and outer membranes is continuous along the filament and may act as a channel for communication from one cell to the next. However, in at least some cases tiny pores, called
microplasmodesmata, have been seen to span the dividing cross-walls, connecting the cytoplasms of
neighbouring cells together. These junctions may function as electrical contacts, allowing electrochemical signals (e.g. hydrogen or calcium ions) to diffuse from one cell to the next, enabling the cells to communicate via electrical signals and so synchronsie their activity. This is important if the filament is to move in a given direction - each cell needs to 'know' which direction to move in.
Cyanobacteria may be single-celled, or they form simple multicellular structures, such as the filaments of cells,
also called trichomes, that we have seen in Oscillatoria. In some forms, e.g. Nostoc, the trichomes may be
coiled up inside shared green, black or red mucilagenous capsules called nodules, which may be a centimetre or two in diameter and look like grapes or plums. Sometimes these nodules or colonies are leaf-like. Presumably, this both protects the cells inside and enables them to better optimise the atmosphere that surrounds them. Many types, including Oscillatoria, do not form colonies or nodules. As we have seen, the cells within a trichome may communicate via pore-junctions, a feature of true multicellularity. Filaments are branched in some species and may even consist of more than one row of cells. Filaments are produced when cells divide in one plane only. Some forms divide in two planes, producing square sheets of cells, others divide in three planes to produce cubes or balls of cells.
Cyanobacteria may form larger structures. Stromatolites (stromatoliths) are large columnar deposits of calcium carbonate built-up over immense periods of time by cyanobacteria. The oldest fossil stromatolites are 2.7 billion years old. These columns allow cyanobacteria to maintain a good position close to the source of light, as generation upon generation adds to the stromatolite. They were once common along the coasts of ancient oceans on Earth, when cyanobacteria dominated the biosphere, and are still found in the Dead sea, where the very high salt concentrations prevent growth of cyanobacterial rivals.
a filament of a nitrogen-fixing cyanobacterium such as Ananbaena or Nostoc. The large cell on the
a heterocyst, the large granular cell on the right is an akinete (spore). The arrangements of heterocysts and akinetes along the chain are characteristic of the species or strain.
Nitrogen fixation is the biochemical process of capturing and converting atmospheric nitrogen gas into usable organic nitrates. (Fixation is an old alchemical term meaning to make solid). Plants are generally incapable of utilising nitrogen gas, though some plants, such as legumes, harbour nitrogen-fixing bacteria in root nodules, and in exchange for nutrients provided by the plant the bacteria supply the plant with nitrates which the plant can use. In general, plants require nitrogen in the form of soil nitrates which their roots can absorb.
Cyanobacteria are the only organisms able to perform both oxygenic (oxygen-generating) photosynthesis and nitrogen fixation. They achieve both functions by a division of labour - vegetative cells carry-out photosynthesis, whilst specialised cells called heterocysts carry out nitrogen-fixation. Low concentrations of oxygen rapidly and irreversibly inactivate the nitrogenase enzymes responsible for fixing nitrogen, so photosynthesis, which generates oxygen, must be kept separate from nitrogen-fixation. Most of these nitrogen-fixing cyanobacteria are filamentaous and produce specialised nitrogen-fixing cells, called heterocysts. Examples include Nostoc and Anabaena. Heterocyst and nitrogenase synthesis is repressed when combined/fixed nitrogen is already present. Lack of combined nitrogen stimulates heterocyst and nitrogenase production, but if the required nitrogen gas is also absent, then development arrests at an intermediate stage, called the proheterocyst. About 5-10% of the cells develop into heterocysts in a 30 hour period.
Heterocyst. Heterocysts are formed at regular intervals along the filaments. The heterocysts have thick outer wall layers and the thylakoids become concentrated near the cell poles and special polar connections form where the heterocyst is attached to vegetative cells. Their thick walls are thought to restrict oxygen diffusion into the cell, whilst enzymes neutralise any oxygen that does enter the cell. Chlorophyll a is present, but phycobiliproteins are absent. Some components of the photosynthetic machinery are down-regulated (PS II is inactive and rubisco is also lacking, so they can neither fix carbon dioxide nor produce oxygen in the light). [For a description of PS II and rubisco see photosynthesis.] However, some components of the photosynthetic
machinery (such as PS I) are upregulated and these components are able to harness light energy to
manufacture ATP in a process called photophosphorylation. This provides the heterocysts with the energy
needed to fix nitrogen. Respiration uses hydrogen generated during nitrogen fixation (nitrogenase produces one hydrogen molecule for every nitrogen molecule fixed). The heterocyst depends upon the vegetative cells to supply nitrogen, reductant (electron donors) to reduce the nitrogen to ammonium, sugars for fuel and possibly acting as the required reductants, and glutamate, all via the microplasmodesmata. The heterocysts fix nitrogen into ammonium ions which diffuse to neighbouring vegetative cells via the microplasmodesmata, at least some of this ammonium combines with the glutamate to form the amino acid glutamine which is then exported to the vegetative cells (which process the glutamine by removing the nitrogen and turning it back into glutamate).
Only cyanobacteria and some other forms of bacteria can fix nitrogen. This is one reason why flood plains are so fertile: the flood waters leaves behind masses of bacteria, including cyanobacteria, which fix nitrogen and so increase the fertility of the soil.
Non-heterocystous nitrogen-fixing cyanobacteria are facultative (meaning they can do this as an option) anoxygenic photosynthesisers (photosynthesis that produces no oxygen) and fix nitrogen under anaerobic growth conditions. Oscillatoria limnetica, an inhabitant of hypersaline lakes, does not produce heterocysts, but can photosynthesise using sulphide, rather than water, as a reductant (electron donor) and producing sulphur rather than oxygen. Since they are not producing oxygen they can perform nitrogen fixation in the same cells. In the dark they can still produce ATP by respiring their polyglucose food reserves using the sulphur as a final electron acceptor, rather than oxygen. They can also respire anaerobically by fermentation. There are other mechanisms of protecting nitrogenase from oxygen in cyanobacteria that are poorly understood.
Many strains are facultatively chemotrophic in the dark, but these maintain constituitve photosynthetic apparatus and can photosynthesise immediately when light is introduced. Many phycoerythrin-producing strains exhibit complementary chromatic adaptation: when grown in green light they have a high phycoerythrin to phycocyanin ratio, but when grown in red light they have very little phycoerythrin. This response appears to be mediated by a phytochrome-like light-sensitive pigment.
Ecology of Cyanobacteria
Cyanobacteria thrive in aquatic environments, and although all require moisture for growth, many are terrestrial. The surface of desert soils may be encrusted with cyanobacteria, as in the deserts of Utah. These cyanobacteria dehydrate and become inactive when dry but rapidly hydrate and resume growth when moisture is present, producing nodules.
snow in springtime glacial regions contains cyanobacteria.
Cyanobacteria also occur inside rocks (endolithic cyanobacteria)
in Arctic and Antarctic deserts and inside limestone and inside
coral rubble and coral sand. Others deposit limestone in reefs
and hot springs.
Cyanobacteria occur in the surface waters of stratified freshwater lakes. See purple bacteria for more about such lakes.
Many cyanobacteria form symbioses with other living organisms, such as with fungi in some lichens, with sea squirts (e.g. the cyanobacterium Prochloron, sometimes classified as separate from other cyanobacteria). The chloroplasts of red algae are very similar to cyanobacteria, possessing single thylakoids (not stacked) and chlorophyll a but not chlorophyll b and in possessing phycobiliproteins. The chloroplasts of plants and green and brown algae are different, but still clearly descended from photosynthetic prokaryotes. The ancestors of these chloroplasts must have been taken up by ancestral cells (or else they invaded the ancestral cells) and became endosymbionts, in a similar manner to the evolution of mitochondria. Today we still see all stages in such evolutionary processes, with the cells of some organisms taking up microbes to perform the same roles, sometimes transiently, whilst others live in necessary symbiosis, unable to survive without their symbiotic partners.
Reproduction of Cyanobacteria
Reproduction is asexual, though being prokaryotes mutation rates are high, so new forms can be constantly produced. Cyanobacteria grow by binary fission, in which a vegetative cell splits or divides into two daughter cells which are genetic clones of the mother cell (apart from mutations). Some reproduce by multiple fission, a cell splitting into many tiny spores called baeocytes. Akinetes, such as those produced by Nostoc, are dormant resilient spores which can germinate in suitable conditions to produce a new filament. Some filamentous forms also produce chains of motile cells, called hormogonia (singular: hormogonium) for dispersion. For example, Nostoc, which is generally immotile, produces special gliding filaments called hormogonia that detach from the ends of the parent filament. Filaments may also break, giving rise to two new chains.
Above: Oscillatoria. Below: Oscillatoria-like cyanobacteria exhibiting gliding movements: the uppermost filament was gliding back and forth in its slime sheath. The presence of a definite and fairly rigid slime sheath suggests that this is probably a species of Lyngbya (although some Oscillatoria have thin sheaths and may secrete a mucilaginous coat when irritated). The bottom image is approximately 40 seconds later than the top image.
tip of gliding filaments often appear to swing or oscillate like
a pendulum, but this is apparently due to the fact that the
filament rotates as it glides, so that any curvature of the
filament appears as a swinging motion.
Note the separation discs in the uppermost filament: these are formed by the death of cells at intervals along a filament, the dead cells filling with mucilage. Filaments will eventually fragment at these separation discs into separate segments called hormogonia or hormogones, a form of asexual reproduction. Although separate filaments may occur, perhaps during dispersal by gliding motility, filaments generally occur as clumps or mats in which the individual filaments may be parallel or interwoven.
Note also that the front-most or apical cell has characteristic morphology, which aids species identification, in this case the apical cell is ovoid.
Above: Oscillatoria from the bottom of a salt marsh. There were considerable differences in size and coloration indicative of different species in the sample. Lyngbya can also escape its sheath and be confused with Oscillatoria. The whole Oscillatoria-Lyngbya grouping is rather artificial and DNA analysis is starting to tease these assemblages into groups with evolutionary connections. Oscillatoria, for example, has been split into several groupings by this kind of analysis.
Article updated: 9 Feb 2021