cyanobacterial cell structure, colour, unlabeled
cyanobacterium structure b&w unlabeled
Cyanobacteria
See also: algal bodies.

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.
oscillatoria, a Pov-ray model
Anabaena
structure of a cyanobacterium, labeled
Above: 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:
Phycobilisome
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.
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.
Above: a filament of a nitrogen-fixing cyanobacterium such as Ananbaena or Nostoc. The large cell on the left is
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

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.

Anoxygenic Photosynthesis

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.
Cyanobacterial cell envelope
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.
junctional pores and slime jets
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.
More 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.

Multicellularity

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.
cyanobacteria under the microscope
Green 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.
Left: a cyanobacterial filament found growing on
tree bark. Can you spot a heterocyst?
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).