Cyanobacterial morphology

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

In eukaryotes, these manifold tasks are fulfilled by the cytoskeleton: proteinaceous polymers that assemble into stable or dynamic filaments or tubules in vivo and in vitro.

However, unlike other Gram-negative bacteria, cyanobacteria contain an unusually thick peptidoglycan (PG) layer between the inner and outer membrane, thus containing features of both Gram phenotypes.

Cyanobacteria show a high degree of morphological diversity and can undergo a variety of cellular differentiation processes in order to adapt to certain environmental conditions.

[30][8] Morphological plasticity, or the ability of one cell to alternate between different shapes, is a common strategy of many bacteria in response to environmental changes or as part of their normal life cycle.

[35] The precise molecular circuits that govern those morphological changes are yet to be identified, however, a so-far constant factor is that the cell shape is determined by the rigid PG sacculus which consists of glycan strands crosslinked by peptides.

[23] Changes in cellular or even trichome morphologies are tasks that would require active cell wall remodelling and thus far no genes attributed to the different morphotypes have been identified in cyanobacteria.

[8] Cyanobacteria present remarkable variability in terms of morphology: from unicellular and colonial to multicellular filamentous forms.

In aquatic habitats, unicellular cyanobacteria are considered as an important group regarding abundance, diversity, and ecological character.

Heterocysts are specialized nitrogen-fixing cells formed during nitrogen starvation by some filamentous cyanobacteria, such as Nostoc punctiforme, Cylindrospermum stagnale, and Anabaena sphaerica.

[59] Cyanobacteria are ubiquitous, finding habitats in most water bodies and in extreme environments such as the polar regions, deserts, brine lakes and hot springs.

For example, filamentous cyanobacteria live in long chains of cells that bundle together into larger structures including biofilms, biomats and stromatolites.

In addition, cyanobacteria-based biofilms can be used as bioreactors to produce a wide range of chemicals, including biofuels like biodiesel and ethanol.

[65] However, despite their importance to the history of life on Earth, and their commercial and environmental potentials, there remain basic questions of how filamentous cyanobacteria move, respond to their environment and self-organize into collective patterns and structures.

[52] All known cyanobacteria lack flagella;[66] however, many filamentous species move on surfaces by gliding, a form of locomotion where no physical appendages are seen to aid movement.

[68][69] One theory suggests that gliding motion in cyanobacteria is mediated by the continuous secretion of polysaccharides through pores on individual cells.

Other scholars have suggested surface waves generated by the contraction of a fibril layer as the mechanism behind gliding motion in Oscillatoria.

[80][52] Through collective interaction, filamentous cyanobacteria self-organize into colonies or biofilms, symbiotic communities found in a wide variety of ecological niches.

Their larger-scale collective structures are characterized by diverse shapes including bundles, vortices and reticulate patterns.

[85] Further, biofilms and biomats show some remarkably conserved macro-mechanical properties, typically behaving as viscoelastic materials with a relaxation time of about 20 min.

UV radiation is especially deadly for cyanobacteria, with normal solar levels being significantly detrimental for these microorganisms in some cases.

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.

An alternative hypothesis is that the cells use contractive elements that produce undulations running over the surface inside the slime tube like an earthworm.

Different forms of cyanobacteria [ 1 ]
(A) spherical and ovoid unicellular, (B) colonial, (C) filamentous, (D) spiral, (E) unsheathed trichome, (F) sheathed trichome, (G) false branching, (H) true branching, (I) different cell types in filamentous cyanobacteria.
Cyanobacterial cell division and cell growth mutant phenotypes in Synechocystis , Synechococcus , and Anabaena . Stars indicate gene essentiality in the respective organism. While one gene can be essential in one cyanobacterial organism/ morphotype , it does not necessarily mean it is essential in all other cyanobacteria. N/A indicates that no mutant phenotypes have been described. WT: wild type . [ 8 ]
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
Modeling filamentous cyanobacteria [ 3 ]
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.
(a) Under ideal conditions active gliding specimens of Oscillatoria lutea appear as long thin curved filaments. (b) When rendered inactive, for example by being briefly cooled, the same filaments adopt a more random shape. (c) Under higher magnification O. lutea is seen to be composed of one-cell-wide strands of connected cells. [ 52 ]
Oscillatoria are capable of a waving motion
Häder's cyanograph experiment [ 3 ]
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.