Cyanobacterial morphology

Microphotographs of bundle-forming filamentous cyanobacteria
A–C: Microcoleus steenstrupii D–E: Tolypothrix desertorum F: Scytonema cf. calcicola G: S. cf. calcicola H: S. cf. c alcicola
Scale bar =10 µm

Cyanobacteria are a large and diverse phylum of bacteria defined by their unique combination of pigments and their ability to perform oxygenic photosynthesis.^[1][2]

Cyanobacteria often live in colonial aggregates that can take a multitude of forms.^[2] Of particular interest among the many species of cyanobacteria are those that live colonially in long trichomes of hundreds to thousands of cells.^[2] These 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 polar regions,^[3] as well as being widely distributed in more mundane environments.^[2]

Many filamentous species are also motile, gliding along their long axis, and displaying photomovement by which a trichome modulates its gliding according to the incident light. The latter has been found to play an important role in guiding the trichomes to optimal lighting conditions, which can either inhibit the cells if the incident light is too weak, or damage the cells if too strong.^[2]

Overview

Cyanobacteria present remarkable variability in terms of morphology: from unicellular and colonial to filamentous forms. Filamentous forms exhibit functional cell differentiation such as heterocysts (for nitrogen fixation), akinetes (resting stage cells), and hormogonia (reproductive, motile filaments). These, together with the intercellular connections they possess, are considered the first signs of multicellularity.^[4][5][6][7]

Many cyanobacteria form motile filaments of cells, called hormogonia, that travel away from the main biomass to bud and form new colonies elsewhere.^[8][9] 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.

Single cell species

Each individual cell (each single cyanobacterium) typically has a thick, gelatinous cell wall.^[10] They lack flagella, but hormogonia of some species can move about by gliding along surfaces.^[11]

Colonial species

• 
  Fischerella
• 
  True branching phenotype of a Fischerella thermalis colony
• 
  Lyngbya species form long, unbranching filaments inside rigid mucilaginous sheaths which can form tangles or mats, intermixed with other phytoplankton species
• 
  Cyanobacterial colony of Lyngbya majuscula

• 
  Ball-shaped colony of Gloeotrichia echinulata stained with SYTOX
• 
  Colonies of Nostoc pruniforme
• 
  Colonial cyanobacteria Stratonostoc

Filamentous species

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^[12][13][14] in an anaerobic environment due to its sensitivity to oxygen.^[14]

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.^[15] These vesicles are not organelles as such. They are not bounded by lipid membranes but by a protein sheath.

• 
  Anabaena sperica
• 
  Anabaena is used as a model organism to study simple vision^[16]
• 
  Helical filaments of cyanobacteria
• 
  Helical filament from Dolichospermum
• Nassula consuming oscillatoria through its cytopharyngeal basket

Diversity

Diversity in cyanobacteria morphology

Unicellular and colonial cyanobacteria
scale bars about 10 µm

Simple cyanobacterial filaments Nostocales, Oscillatoriales and Spirulinales

Branched forms
Tolypothrix, Scytonema, Stigonema and Fischerella

Cyanobacteria associated with tufa
Schizothrix calcicola, Gloeocapsa, Coccochloris, Microcoleus vaginatus and Rivularia

Drawings by Allan Pentecost

Heterocysts

Heterocysts are specialized nitrogen-fixing cells formed during nitrogen starvation by some filamentous cyanobacteria, such as Nostoc punctiforme, Cylindrospermum stagnale, and Anabaena sphaerica.^[17] They fix nitrogen from atmospheric N₂ using the enzyme nitrogenase, in order to provide the cells in the filament with nitrogen for biosynthesis.^[18]

Movement

Modeling filamentous cyanobacteria ^[2]

Model components: (A) Trichomes are modeled as thin flexible rods that are discretized into sequences of 50 µm edges. Each edge is loaded with a linear spring. (B) The local bending moment is a function of the radius of curvature. (C) Trichomes can glide along their long axis and reverse their direction of movement photophobically. (D) Trichome collisions are defined between edge-vertex pairs. A vertex that penetrates an edge's volume is repulsed by equal and opposite forces between the pair.

Cyanobacteria have strict light requirements. Too little light can result in insufficient energy production, and in some species may cause the cells to resort to heterotrophic respiration.^[3] Too much light can inhibit the cells, decrease photosynthesis efficiency and cause damage by bleaching. UV radiation is especially deadly for cyanobacteria, with normal solar levels being significantly detrimental for these microorganisms in some cases.^[19][20][2]

Filamentous cyanobacteria that live in microbial mats often migrate vertically and horizontally within the mat in order to find an optimal niche that balances their light requirements for photosynthesis against their sensitivity to photodamage. For example, the filamentous cyanobacteria Oscillatoria sp. and Spirulina subsalsa found in the hypersaline benthic mats of Guerrero Negro, Mexico migrate downwards into the lower layers during the day in order to escape the intense sunlight and then rise to the surface at dusk.^[21] In contrast, the population of Microcoleus chthonoplastes found in hypersaline mats at Salins-de-Giraud, Camargue, France migrate to the upper layer of the mat during the day and are spread homogenously through the mat at night.^[22] An in vitro experiment using P. uncinatum also demonstrated this species' tendency to migrate in order to avoid damaging radiation.^[19][20] These migrations are usually the result of some sort of photomovement, although other forms of taxis can also play a role.^[23][2]

Many species of cyanobacteria are capable of gliding. Gliding is a form of cell movement that differs from crawling or swimming in that it does not rely on any obvious external organ or change in cell shape and it occurs only in the presence of a substrate.^[24][25] Gliding in filamentous cyanobacteria appears to be powered by a "slime jet" mechanism, in which the cells extrude a gel that expands quickly as it hydrates providing a propulsion force,^[26][27] although some unicellular cyanobacteria use type IV pili for gliding.^[28] Individual cells in a trichome have two sets of pores for extruding slime. Each set is organized in a ring at the cell septae and extrudes slime at an acute angle.^[29] The sets extrude slime in opposite directions and so only one set is likely to be activated during gliding. An alternative hypothesis is that the cells use contractive elements that produce undulations running over the surface inside the slime tube like an earthworm.^[30] The trichomes rotate in a spiral fashion, the angle of which corresponds with the pitch angle of Castenholz's contractile trichomes.^[2]

The cells appear to coordinate their gliding direction by an electrical potential that establishes polarity in the trichomes, and thus establishes a "head" and the "tail".^[31] Trichomes usually reverse their polarity randomly with an average period on the order of minutes to hours.^[32][33] Many species also form a semi-rigid sheath that is left behind as a hollow tube as the trichome moves forward. When the trichome reverses direction, it can move back into the sheath or break out.^[34][2]

Oscillatoria filaments

Oscillatoria is a genus of filamentous cyanobacterium named after the oscillation in its movement. Filaments in colonies slide back and forth against each other until the whole mass is reoriented to its light source. Oscillatoria is mainly blue-green or brown-green and is commonly found in watering-troughs. It reproduces by fragmentation forming long filaments of cells which can break into fragments called hormogonia. The hormogonia can then grow into new, longer filaments.

Häder's cyanograph experiment

Häder's cyanograph experiment ^[2]

Photographic negative projected onto a Petri dish containing a culture of photophobic filamentous cyanobacteria (Phormidium uncinatum). The trichomes cover the lighter areas of the projection while uncovering the darker areas producing a photographic positive.

In 1987, Häder demonstrated that trichomes can position themselves quite precisely within their environment through photomovement. In Häder's cyanograph experiment a photographic negative is projected onto a Petri dish containing a culture of Phormidium uncinatum.^[35][36] After a few hours, the trichomes move away from the darker areas onto the lighter areas, forming a photographic positive on the culture. The experiment demonstrates that photomovement is effective not just for discrete light traps, but for minutely patterned, continuously differentiated light fields as well.^[2]

See also

• Bacterial cellular morphologies
• Bacterial morphological plasticity
• Colonial morphology

References