Identify that alga:
Web-based key to genera - 5

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Colonies and coenobia
with sheath
without sheath  

flat plate

Colonies and coenobia : Cyanobacteria
with sheath

(under development)
 + chloroplast(s)

(under development)

non-flagellate cells
without case
(under development)
 in silica case
= diatoms
(under development)
flagellate cells

(under development)

no chloroplast:


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Unbranched filaments:
No Chloroplast ( = unbranched cyanobacteria)
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Cells spherical or bead-like, linked together like beads on a string, with occasional larger or different cells present (heterocysts) 

Heterocysts are associated with nitrogen fixation in Cyanobacteria, which may also produce Akinetes, or resting spores- the sausage-shaped cells seen below (Anabaena)

Filaments planktonic, solitary, though may form blooms - dense aggregations of filaments which float at the surface due to the presence of gas vacuoles




Filaments clustered together in a tough mucilaginous sheath, usually attached to surfaces




Filaments very narrow, consisting of cylindrical or barrel-shaped cells which appear slightly separated


cells short-cylindric, forming smooth filaments (trichomes)


(a) filaments with heterocysts (and akinetes); short planktonic filaments which may be aggregated in clumps


(b) trichomes without heterocysts:


(i) unsheathed, even diameter throughout

  Oscillatoria: usually seen to be motile, gliding as well as showing bending and flexing movements. May form dense aggregations of interwoven filaments.



(ii) trichomes sheathed

(In sheathed genera, the string of cells is referred to as the trichome, while filament refers to the trichome plus the sheath)

trichomes tapering





trichomes sheathed, even diameter throughout:


Sheath thin, filament long



often forms dense mats of filaments, blue-green or grey in colour. (May occasionally show motility)

(Sheath thin, 'filament' usually just one elongated cell, attached to other algae at one end; sometimes with spherical exospores at the other end)

    Chamaesiphon: epiphytic

Sheath varying from thick to thin depending on species. Not easily separated from Phormidium

Lyngbya: Trichomes may break up into hormogonia (short lengths comprising a number of cells eg. upwards of 10 or so), which may be confused with Oscillatoria. These may be released from the sheath and go on to develop into a new filament. (May occasionally show motility)


Several trichomes within one sheath


Trichomes tapering from base to tip, sheath thick, each trichome with a basal heterocyst:


filaments forming a clump, radiating out from the centre

  Gloeotrichia: may form spherical planktonic colonies

filaments single or in a small group: false branching is infrequent

  Calothrix: sheath is firm, covering the thicker, basal part of the trichome, but does not usually cover the heterocyst

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John Kinross