Way back in the day, back when blogging was actually a thing that people paid a modicum of attention to (as opposed to its current status as a way for old fogies to scream into the void), I used to have a link to this blog at some indexing/promotional site that advertised its coverage as including, among other things, "multicellular bacteria". Now, when one is considering micro-organisms, the line between 'multicellular' and 'colonial' is a vague one. Nevertheless, there are certain lineages of colonial bacteria in which individual cells within the colony may become differentiated in a way that renders them incapable of surviving on their own. A definite argument could therefore be made that such colonies have crossed the boundary into true multicellularity.
Light microscopy image of Anabaena circinalis at 400–600×, copyright Imre Oldal. The lighter coloured cells are heterocysts.
A particularly diverse such bacterial lineage is the heterocyst-forming members of the Cyanobacteria, the blue-green algae, of which the genus
Anabaena is a widespread representative.
Anabaena species grow as long strings of cells referred to as trichomes. These trichomes are often embedded within a layer of dense mucilage though
Anabaena species lack the hard external sheath produced by some other cyanobacterial genera. The cells within a trichome are more or less spherical, cylindrical or barrel-shaped and are not differentiated from each other in such a way that a trichome could be said to have a 'base' or 'apex'. Trichomes may be planktonic or benthic, depending on the species. Benthic species are capable of slow movement and the cells at each end of a trichome are conical in shape. Planktonic species are immobile; the cells contain gas vesicles to provide buoyancy and those at the ends of the trichomes are not differentiated from the remainder (Boone
et al. 2001).
The aforementioned heterocysts are specialised cells within the trichome of
Anabaena species and related Cyanobacteria that are capable of fixing molecular nitrogen from the surrounding environment (trichomes growing in a medium providing a surfeit of previously fixed nitrogen will not produce heterocysts). The enzymes responsible for nitrogen fixation require the absence of oxygen to function and so heterocysts devlop a thick, multi-layered envelope outside the original cell wall. They also lose the capacity to conduct their own photosynthesis. As a result, the heterocyst becomes completely dependent on the surrounding cells in the trichome for the production of carbohydrates, supplying them in turn with nitrogen incorporated into amino acids (Golden & Yoon 2003).
Anabaena species will generally have individual heterocysts separated by about ten to twenty photosynthetic cells; the heterocysts are most commonly at internal positions within the trichome though they may occasionally occupy a terminal position. One species usually included in
Anabaena,
A. azollae, lives in close association with the small, floating aquatic ferns of the genus
Azolla.
Anabaena azollae trichomes are contained within cavities on the underside of the leaves. Heterocyst formation is much more extensive than in free-living
Anabaena with fully 20–30% of the cells being heterocysts. Developing sporocarps on the
Azolla also become infested with
A. azollae akinetes (thick-walled cells that act as resting spores) that are picked up by emerging embryos so the symbiont is transmitted down through the generations (Peters 1989). Because of this association,
Azolla growth is often encouraged as a source of nitrogen for crops grown in water such as rice. Other
Anabaena species, conversely, are less welcomed by humans due to their production of harmful toxins.
The advent of molecular studies of bacterial phylogeny has confirmed the integrity of the heterocyst-formers as a monophyletic lineage within the Cyanobacteria. However, the internal classification of this clade is far more uncertain. Though well recognised from a morphological standpoint, molecular studies have questioned whether the genus should continue to be recognised in its current form. A study by Gugger
et al. (2002) comparing planktonic strains of
Anabaena with another genus
Aphanizomenon, distinguished by differences in cell and trichome shape, found that the two were well and truly intermingled genetically. Some of the features hitherto used in cyanobacterial classification may be affected by the environment. For instance,
Anabaena azollae will, under certain conditions, produce hormogonia, small, motile chunks of trichome that function as disseminules. Hormogonia production is supposed to be a feature of another cyanobacterial genus,
Nostoc, rather than
Anabaena (and it is worth noting that other Cyanobacteria involved in symbioses with plants have been assigned to
Nostoc) (Peters 1989). There is a need out there for an extensive investigation into the relationships of these genera, and maybe a thorough re-analysis of their definitions.
REFERENCES
Boone, D. R., R. W. Castenholz & G. M. Garrity (eds) 2001.
Bergey’s Manual of Systematic Bacteriology 2
nd ed. vol. 1.
The Archaea and the Deeply Branching and Phototrophic Bacteria. Springer.
Golden, J. W., & H.-S. Yoon. 2003. Heterocyst development in
Anabaena.
Current Opinion in Microbiology 6: 557–563.
Gugger, M., C. Lyra, P. Henriksen, A. Couté, J.-F. Humbert & K. Sivonen. 2002. Phylogenetic comparison of the cyanobacterial genera
Anabaena and
Aphanizomenon.
International Journal of Systematic and Evolutionary Microbiology 52: 1867–1880.
Peters, G. A., & J. C. Meeks. 1989. The
Azolla-Anabaena symbiosis: basic biology.
Annual Review of Plant Physiology and Plant Molecular Biology 40: 193–210.
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