Anabaena flos-aquae, copyright Proyecto Agua.

Belongs within: Hormogoneae.
Contains: Tolypothrichaceae, Nostoc.

The Nostocaceae are filamentous blue-green algae in which trichomes do not exhibit basal-apical polarity (Castenholz 2001).

The Nostocaceae: tangled filaments
Published 13 March 2012
Macroscopic growth of the cyanobacterium Nostoc commune, from here.

The cyanobacteria, commonly referred to as the ‘blue-green algae’, were one of the first groups of bacteria to be recognised as distinct. As our knowledge of bacteria has improved over the years, the distinctiveness of cyanobacteria continues to be supported: they are the only bacteria to contain chlorophyll, capturing energy from the light through the oxidation of water. Unfortunately, this confidence in their separation has not necessarily been carried over at lower levels. Many cyanobacterial ‘families’ and ‘genera’ have not been supported by more recent molecular analyses. Nevertheless, one clade that has been well supported is the nitrogen-fixing cyanobacteria, Hormogoneae.

Morphology-based classifications of cyanobacteria started, firstly, with the question of whether a species existed as independent cells, or whether they formed thread-like chains. The Hormogoneae are all chain-forming species, with the chain contained within a but they are also distinguished by the formation of heterocysts, large morphologically distinct cells within the chain that specialise in nitrogen fixation. Heterocysts are so specialised that they are unable to photosynthesise for themselves and are dependent on their neighbour cells for nutrition. So, with the presence of differentiated, interdependent cells, Hormogoneae can be regarded not simply as colonies of cells but as true, multicellular bacteria. A number of species of Hormogoneae (particularly in the genus Nostoc) form symbiotic associations with plants that take advantage of their nitrogen-fixing properties. One of the best-known examples is the association of the aquatic floating fern Azolla with the cyanobacterium ‘Anabaena’ azollae, and Azolla is used as a source of nitrogen in rice paddies. In another post, I have described the association between a Nostoc species and the plant genus Gunnera.

Trichomes of Anabaena, from here. The yellowish cells are heterocysts.

Within the Hormogoneae, classifications have traditionally distinguished between the orders Nostocales and Stigonematales. Stigonematales have branching trichomes, while those of Nostocales are unbranched. The Nostocales have been divided between the Nostocaceae, Rivulariaceae and Scytonemataceae: Rivulariaceae have trichomes that show a distinct base-to-apex polarity, while those of Scytonemataceae show a feature called ‘false branching’*. Nostocaceae are defined by the lack of these features: however, it should not be surprising that molecular studies have not supported a group defined solely by the absence of characters, and both Rivulariaceae and Scytonemataceae (and possibly Stigonematales as well) are probably derived (possibly polyphyletically) from ‘nostocacean’ ancestors. However, actual relationships within the Hormogoneae remain poorly resolved, and no formal reclassification has been proposed (one of the authoritative texts on bacterial classification, Bergey’s Manual of Systematic Bacteriology, replaces the cyanobacterial ‘orders’ with numbered subsections [Nostocales, for instance, is treated as Cyanobacteria subsection IV]—Castenholz 2001).

*True branching as in Stigonematales occurs when cells within a trichome divide at right angles to the direction of the trichome. In ‘false branching’, the trichome breaks within the containing sheath and then grows out of the sheath, but the division of the individual cells remains linear.

Trichomes of Cylindrospermum licheniforme, from André Advocat. The heterocysts are the round terminal cells, while the large elongate cells behind them are akinetes.

Phylogenetic analyses have also failed to confirm many of the genera recognised within the Nostocaceae (treated by Bergey’s Manual as ‘form-genera’ only), distinguished by features such as whether trichomes are generally straight or coiled, and the positions within the trichome of heterocysts and other specialised cells called akinetes, thick-walled cells that function as resistent spores. The traditional genus Nostoc also differs from other Nostocaceae by the formation of hormogonia, motile trichomes with smaller cells and without differentiated heterocysts. Hormogonia form the dispersal stage of the Nostoc life cycle; it is as hormogonia, for instance, that symbiotic Nostoc are transmitted to new hosts. Mature Nostoc trichomes are embedded in a gelatinous matrix, and in some species this matrix may form a globular ball containing large numbers of radially arranged trichomes. Though usually microscopic, these globular clusters can get very large, sometimes more than twenty centimetres in diameter. Species attributed to other genera of Nostocaceae do not generally produce differentiated hormogonia (though some do, such as the aforementioned Anabaena azollae). Trichomes of these species may remain motile throughout the life cycle, or they may be permanently immotile (the latter state is characteristic of planktonic species).

On Anabaena
Published 13 July 2020

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.

Systematics of Nostocaceae

Characters (from Castenholz 2001, as Cyanobacteria subsection IV.I): Trichomes never exhibiting basal-apical polarity, all vegetative cells of uniform size. Cell divisions occuring at similar rates throughout trichome. Distinct development cycle occasionally present, involving hormogonia production. False branching extremely rare. Heterocysts differentiated under nitrogen-limiting conditions in terminal and/or intercalary positions. Akinetes (if present) may be formed singly or in short chains of two or three, adjacent to or very close to heterocysts, or may be differentiated equidistant between two heterocysts, with successive differentiation on either side of first akinete often leading to very long chains of akinetes that may comprise the majority of the cell population.

<==Nostocaceae [Nostochineae]
    |--+--‘Anabaena’ cylindrica Lemmerman 1896L02, C01
    |  `--Cylindrospermopsis Seenayya & Subba-Raju 1972L02, C01
    |       `--*C. raciborskii (Woloszynska) Seenayya & Subba-Raju 1972 [=Anabaenopsis raciborskii Woloszynska 1912]C01
       |  |  `--GodleyaceaeHB14
       |  |       |--Godleya Novis & Visnovsky 2011HB14
       |  |       |    `--*G. alpinaHB14
       |  |       `--Toxopsis Lamprinou, Skaraki et al. 2012HB14
       |  |            `--*T. calypsusHB14
       |  `--+--+--Sphaerospermopsis torques-reginaeHB14
       |     |  `--Raphidiopsis curvataHB14, SG05
       |     `--+--*Cuspidothrix issatchenkoiHB14
       |        `--+--Dolichospermum flos-aquaeHB14
       |           `--Aphanizomenon Morren 1838HB14, C01
       |                |--*A. flos-aquae (Linnaeus) Ralfs ex Bornet & Flahault 1888 [=Byssus flos-aquae Linnaeus 1753]C01
       |                |--A. gracileHB14
       |                |--A. incurvumG64
       |                `--A. ovalisporumJK06
       `--+--*Calochaete cimrmaniiHB14
             |  |  `--‘Microchaete’ diplosiphon Gomont ex Bornet & Flahault 1886HB14 [=Fremyella diplosiphon (nom. illeg.)C01]
             |  `--+--*Aulosira laxaHB14
             |     `--‘Nostoc’ carneumHB14
                |  `--+--Cylindrospermum Kützing 1843HB14, C01
                |     |    |--*C. stagnale Kützing ex Bornet & Flahault 1888HB14, C01
                |     |    |--C. alatosporumHB14
                |     |    |--C. doryphorumSG05
                |     |    `--C. majus Kützing ex Bornet & Flahault 1888C01
                |     `--Anabaena Bory de St Vincent 1822L02, C01
                |          |--A. ambiguaSG05
                |          |--A. azollaeC01
                |          |--A. bergiiJK06
                |          |    |--A. b. var. bergiiJK06
                |          |    `--A. b. var. limneticaJK06
                |          |--A. circinalisC01
                |          |--A. constrictaG64
                |          |--A. flos-aquae (Lyngbye) de Brebisson 1835M70
                |          |--A. impalpebralisG64
                |          |--A. licheniformisG64
                |          |--A. oscillarioidesJK06
                |          |--A. sphericaSG05
                |          |--A. spiroides Klebahn 1895M70
                |          |--A. torulosa (Carmichael) Lagerheim 1883L27
                |          `--A. variabilis Kützing 1843 [=Trichormus variabilis]C01
                `--+--+--‘Aulosira’ bohemensis Lukešová et al. 2009HB14
                   |  `--‘Microchaete’ tenera Thuret 1875HB14, C01
                   `--Fortiea De Toni 1936HB14
                        |  i. s.: F. monilisporaHB14
                        |         F. rugulosaHB14
                        |         F. spirulinaHB14
                        |         F. striatulaHB14
                        |--F. coimbrae Hauer, Bohunická & Mareš in Hauer, Bohunická et al. 2014HB14
                        `--+--F. contorta Hauer, Bohunická & Mareš in Hauer, Bohunická et al. 2014HB14
                           `--F. laiensis Vaccarino & Johansen in Hauer, Bohunická et al. 2014HB14
Nostocaceae incertae sedis:
  0--Nodularia Mertens 1822C01
  |    |--N. harveyana (Thwaites) Thuret 1875C01
  |    `--N. spumigena Mertens ex Bornet & Flahault 1886C01
  |         |--N. s. var. spumigenaC01
  |         `--N. s. var. vacuolataC01
  `--+--Cyanospira Florenzano, Sili et al. 1985 non Chodat 1920 (ICZN)C01
     |    |--*C. rippkae Florenzano et al. 1985C01
     |    `--C. capsulata Florenzano et al. 1985C01
     `--Anabaenopsis (Woloszynska) Miller 1923C01
          |--A. arnoldiiJK06
          |--A. circularisC01
          `--A. elenkiniiC01
  Anabaenidium Schopf 1968E93, G79
    `--*A. johnsonii Schopf 1968G79
  Veteronostocale Schopf & Blacic 1971E93
  Monormia intricataG64
    |--S. berkeleyanaG64
    |--S. broomeiG64
    |--S. carmichaeliiG64
    |--S. jacobiG64
    `--S. thwaitesiiG64
    |--S. harveyanaG64
    `--S. litoreaG64
  Trichormus incurvusG64

Nomen invalidum: Nodularia sphaerocarpaC01

*Type species of generic name indicated


Boone, D. R., R. W. Castenholz & G. M. Garrity (eds) 2001. Bergey’s Manual of Systematic Bacteriology 2nd ed. vol. 1. The Archaea and the Deeply Branching and Phototrophic Bacteria. Springer.

[C01] Castenholz, R. W. 2001. Phylum BX. Cyanobacteria: oxygenic photosynthetic bacteria. In: Boone, D. R., R. W. Castenholz & G. M. Garrity (eds) Bergey’s Manual of Systematic Bacteriology 2nd ed. vol. 1. The Archaea and the Deeply Branching and Phototrophic Bacteria pp. 473–599. Springer.

[E93] Edwards, D. 1993. Monera (bacteria, blue-green algae). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 3–7. Chapman & Hall: London.

[G79] Glaessner, M. F. 1979. Precambrian. In: Robison, R. A., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt A. Introduction. Fossilisation (Taphonomy), Biogeography and Biostratigraphy pp. A79–A118. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

Golden, J. W., & H.-S. Yoon. 2003. Heterocyst development in Anabaena. Current Opinion in Microbiology 6: 557–563.

[G64] Gray, J. E. 1864. Handbook of British Water-weeds or Algae. R. Hardwicke: London.

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.

[HB14] Hauer, T., M. Bohunická, J. R. Johansen, J. Mareš & E. Berrendero-Gomez. 2014. Reassessment of the cyanobacterial family Microchaetaceae and establishment of new families Tolypothrichaceae and Godleyaceae. Journal of Phycology 50: 1089–1100.

[JK06] John, J., & A. Kemp. 2006. Cyanobacterial blooms in the wetlands of the Perth region, taxonomy and distribution: an overview. Journal of the Royal Society of Western Australia 89 (2): 51–56.

[L27] Laing, R. M. 1927. A reference list of New Zealand marine algae. Transactions and Proceedings of the New Zealand Institute 57: 126–185.

[L02] Litvaitis, M. K. 2002. A molecular test of cyanobacterial phylogeny: inferences from constraint analyses. Hydrobiologia 468: 135–145.

[M70] Meel, L. van. 1970. Etudes limnologiques en Belgique. VI.—Les méandres de la Durme à Hamme (Province de Flandre Orientale). Bulletin de l’Institut Royal des Sciences Naturelles de Belgique 46 (13): 1–56.

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.

[SG05] Sau, A., & R. K. Gupta. 2005. Algal flora of Indian Botanic Garden, Howrah, West Bengal. Bulletin of the Botanical Survey of India 47: 63–86.

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