Belongs within: Cyanobacteriia.
Phormidium is a genus of filamentous green algae with cells typically longer than wide, and with the end few cells of the filament tapering.
Mats of obscurity
Published 19 January 2026
The blue-green algae, Cyanobacteria, were one of the first groups of bacteria to receive extensive systematic attention, even before the concept of bacteria was much of a thing. Obviously, these early classifications of cyanobacteria were based primarily on such things as cell and colony structure. And while some cyanobacteria may produce surprisingly complex structures, others are frustrating in their simplicity. The genus Phormidium tends more towards the latter than the former.
Phormidium has historically been recognised as a genus of the Oscillatoriales, the order of cyanobacteria in which cells form simple, more or less unbranching filaments without strongly differentiated cells. Individual filaments are often surrounded by a mucilaginous sheath. The structure and development of the overall filament and its sheath has historically been more important in distinguishing oscillatorialean genera than features of the largely undifferentiated cells. Phormidium has therefore been recognised for species with thin, hyaline, mucous sheaths that are often partially to completely degraded. A more recent attempt to recognise Phormidium on more cell-specific features defined the genus by the presence of radially oriented thylakoids in transverse cell sections, but it remains uncertain whether this character applies to many of the 150+ species historically included in Phormidium (Marquadt & Palinska 2007).
The majority of species assigned to Phormidium grow in fresh or brackish water, though some have been recognised from the marine littoral zone. As a result of their poor sheath development, Phormidium filaments growing adjacent to one another have a tendency to stick together and form dense mats. These mats may come to dominate their substrate under certain conditions. A few species deposit calcium carbonate in their sheaths, leading to the development of nodular accretions (Pentecost 2003). Other species are of concern because of their production of dangerous toxins that have been known to cause rapid fatalities in animals such as dogs when fragments of mat are inadvertently swallowed (McAllister et al. 2016). Not all Phormidium produce such toxins, however, and both toxic and non-toxic strains may be found growing in close proximity.
Because the features on which they are defined are ultimately simple (and not always easy to define), it should hardly come as a surprise that more recent molecular studies have not supported the historical concepts of either Oscillatoriales or Phormidium as monophyletic (Marquadt & Palinska 2007). Whether finer details of cellular structure or chemistry can be applied to more reliable classifications remains a subject of ongoing study. When dealing with organisms that appear so simple on the surface, a closer look is required to determine their true nature.
Systematics of Phormidium
Phormidium Kützing 1843C01
|--P. ambiguumL02
|--P. calcicocolaSG05
|--P. coriumW93
|--P. geysericola Copeland 1936 (n. d.)GH01
|--P. hendersoniiAG03
|--P. laminosumGP03
|--P. laysanenseAG03
|--P. mucicolaL02
|--P. retziiSA07
|--P. submembranaceumRA05
`--P. tenueSG05
*Type species of generic name indicated
References
[AG03] Abed, R. M. M., S. Golubic, F. Garcia-Pichel, G. F. Camoin & S. Sprachta. 2003. Characterization of microbialite-forming cyanobacteria in a tropical lagoon: Tikehau Atoll, Tuamotu, French Polynesia. Journal of Phycology 39: 862–873.
[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.
[GH01] Garrity, G. M., & J. G. Holt. 2001. Phylum BVI. Chloroflexi phy. nov. 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. 427–446. Springer.
[GP03] Gupta, R. S., M. Pereira, C. Chandrasekara & V. Johari. 2003. Molecular signatures in protein sequences that are characteristic of cyanobacteria and plastid homologues. International Journal of Systematic and Evolutionary Microbiology 53: 1833–1842.
[L02] Litvaitis, M. K. 2002. A molecular test of cyanobacterial phylogeny: inferences from constraint analyses. Hydrobiologia 468: 135–145.
Marquardt, J., & K. A. Palinska. 2007. Genotypic and phenotypic diversity of cyanobacteria assigned to the genus Phormidium (Oscillatoriales) from different habitats and geographical sites. Archives of Microbiology 187: 397–413.
McAllister, T. G., S. A. Wood & I. Hawes. 2016. The rise of toxic benthic Phormidium proliferations: a review of their taxonomy, distribution, toxin content and factors regulating prevalence and increased severity. Harmful Algae 55: 282–294.
Pentecost, A. 2003. Taxonomic identity, ecology and distribution of the calcite-depositing cyanobacterium Phormidium incrustatum (Oscillatoriaceae). Cryptogamie Algologie 24 (4): 307–321.
[RA05] Rath, J., & S. P. Adhikary. 2005. A check list of algae from Chilika Lake, Orissa. Bulletin of the Botanical Survey of India 47: 101–114.
[SA07] Samad, L. K., & S. P. Adhikary. 2007. Cyanobacteria from two prehistoric caves of India. Bulletin of the Botanical Survey of India 49: 191–194.
[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.
[W93] Westphalen, D. 1993. Stromatolitoid microbial nodules from Bermuda—a special micro habitat for meiofauna. Marine Biology 117: 145–157.