About algae

Algae is a general term of convenience for phylogenetically unrelated organisms that undertake photosynthesis and/or possess plastids. They are extraordinarily diverse and range from solitary cells through to complex multicellular forms several metres in length. The absence of a sterile layer of protecting cells surrounding the reproductive organs was the main reason why those algal groups having organelles (chloroplasts, nuclei) - the eukaryotic algae - were commonly placed in the kingdoms Protista and Plantae. It is now apparent that the evolutionary history of the plastids of these algae is exceedingly complex and has involved several endosymbiotic events that have led to their transmission from one group to another. There is beginning to emerge the broad outlines of a global phylogenetic tree with the eukaryotic algae placed in four of five supergroups or 'kingdoms', including the Plantae. The 'blue-green algae' (Cyanophyta) lack membrane-bound organelles and are therefore prokaryotic organisms. They are more closely related to the bacteria than to other algae and often referred to as 'Cyanobacteria'.

Some of the most crucially important diagnostic characters seen with a good quality light microscope are often lost or no longer visible when an alga is preserved. Even when microalgae are mounted on glass slides they can deteriorate in time and rarely retain useful diagnostic features; one of the notable exceptions are diatoms whose silica walls normally provide all the characters required for identification. As a result, many 'permanently preserved' samples of freshwater algae provide little useful information. For this reason, the type of many microscopic algae is frequently not a specimen but an illustration, photograph or figure ('iconotype') and preserved voucher material is of limited use for cross-checking an identification.


The algae are important as primary producers of organic matter and in many aquatic environments lie at the base of the food chain. Besides being present in all marine and freshwaters (including thermal springs), they are also commonly associated with damp soil, soil-free surfaces (tree trunks, rocks, snow, ice) and are a component (phycobiont) of lichens. In the fossil record there are recognisable algae in the Precambian dating back to at least 3 billion years.


The number of species of algae remains uncertain since there is no authoritative inventory of names in current use. Various figures have been given and these generally range from about 27,000-36,000 with less than one third of the species occuring in marine or brackish water. In the United Kingdom the number of known freshwater and terrestrial algal species is less than 5,000 of which diatoms (Bacillariophyta) and green algae (Chlorophyta) are by far the largest groups.

The phyla of algae are still distinguished on a combination of characteristics, including chorophyll pigments, accessory pigments, food reserve products, cell covering, reproductive features and various aspects of cellular organisation. Modern molecular phylogenetic studies are revealing relationships within the 'algae' to be much more complicated than originally believed. These studies are leading to a radical reorganisation of the traditionally recognised algal groups although a consensus has still to emerge.

The phyla

There are fifteen recognised phyla:

  • Cyanophyta (Cyanobacterial, bacteria / blue-green algae)
  • Rhodophyta (red algae)
  • Euglenophyta (euglenoids)
  • Cryptophyta (cryptomonads)
  • Pyrrophyta (dinoflagellates)
  • Raphidophyta 
  • Haptophyta (=Prymnesiophyta)
  • Chrysophyta (golden/golden brown algae)
  • Xanthophyta (=Tribophyta; yellow-green algae)
  • Chlorophyta (green algae, including stoneworts)
  • Eustigmatophyta
  • Phaeophyta (=Fucophyta, brown algae)
  • Prasinopyta
  • Bacillariophyta (diatoms)
  • Glaucophyta


Nostoc commune
PHYLUM CYANOPHYTA (Blue-Green Algae/Cyanobacteria)

Blue-green, grey-green, violet, brown, purplish or red dependent on relative proportions of chlorophyll, phycocyanin, phycoerythrin and sometimes brown sheath pigments; unicellular, colonial or filamentous; internal membranes absent and so no organelles.

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Hildenbrandia rivularis



Commonly red due to predominance of phycocyanin and phycoerythrin in chloroplasts; unicellular, filamentous or pseudoparenchymatous (flagellated stages absent); storage material various including floridean starch; unique features associated with reproduction.

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Phacus acuminatus



Commonly unicellular, green, often exhibit squirming movements, sometimes surrounded by an envelope or lorica; chloroplasts variously shaped; one or 2 flagella arising in a flask-shaped invagination; eyespot red, usually evident; walls with longitudinal or spiral striations; storage material variously shaped paramylon.

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Cryptomonas platyuris



Cells brown, blue, blue-green, red, red-brown, olive green, or yellow-brown due to accessory pigments in often 1 or 2 chloroplasts; unicellular (rarely colonial), often bean-shaped, frequently dorsiventrally flattened; two or more unequal subapical flagella arising in an anterior invagination; storage material starch or starch-like.

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Ceratium carolinianum


PHYLUM PYRROPHYTA (Dinoflagellates)

Cells usually brown due to presence of accessory pigments; unicellular, rarely coccoid or filamentous; walls firm or of regularly arranged polygonal plates; biflagellate, one flagellum transverse and encircling the cell, other directed posteriorly; storage materials starch and oil.

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Gonyostomum semen



Cells yellow-green due to predominance of accessory pigment diatoxanthin in 2 or more chloroplasts; unicellular, dorsiventrally organised, with no outer wall (naked); two flagella arising in an apical, funnel-shaped invagination, one flagellum directed forwards and the other backwards; storage material oil.

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Hymenomanas roseola



Cells golden or yellow-brown due to presence of accessory pigments (principally fucoxanthin); unicellular flagellates having also amoeboid, coccoid, palmelloid or filamentous stages; walls often possessing calcified scales; two flagella and between them an appendage known as a haptonema; storage material principally chrysolaminarin.

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Dinobryon cylindricum var. alpinum



Cells golden to yellow-brown due to presence of accessory pigments; single coccoidal cells or palmelloid, filamentous or parenchymatous; mostly uniflagellate or with two flagella, one long and the other short; outer wall absent or cell(s) within an often urn-shaped envelope or lorica; silica scales sometimes present; storage material oil or leucosin.

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Vaucheria geminata




Cells typically yellow-green due to present of the accessory pigment diatoxanthin in 2 or more chloroplasts; unicellular, filamentous, colonial or coenocytic; motile forms with 2 subapical flagella; walls frequently of overlapping parts; storage material oil, fat or leucosin.

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Micrasterias crux-meltensis



Cells with 1 to several green chloroplasts, colonial, filamentous, coenocytic or macrophytes with robust axes bearing whorls of branches and branchlets; motile or non-motile, if motile then normally having (1-)2 or 4 usually apical flagella (except orders Charales and Oedogoniales); storage material principally starch surrounding 1 to several pyrenoids; sexual reproduction oogamous in some orders.

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Chlorobotrys regularis



Cells yellow-green, with main accessry pigment usually violaxanthin in 1 or more chloroplasts; unicellular and coccoidal; motile forms having a single flagellum or with 2 unequal flagella inserted near apex; eyespot unique, independent of chloroplast; pyrenoid unique; food storage material unknown.

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Cells brownish due to presence of carotenoids pigments (principally fucoxanthin) in 1 to several chloroplasts; freshwater species of microscopic branched filaments (often closely packed); motile stages pear-shaped with 2 laterally inserted flagella; walls frequently contain alginic acid and fucinic acid; food storage materials laminarin and mannitol.


Cells with green, rarely yellow-green, chloroplasts; unicellular flagellates, rarely non-motile, with 1-8 lateral or apical flagella, usually arising at base of a depression; walls and flagella mostly covered with organic scales; food storage materials starch or mannitol.


Chlorobotrys regularis

Cells bright blue-green due to presence of phycocyanin and other pigments in cyanelles (not equivalent to chloroplasts); unicellular or colonial; storage material starch produced outside the cyanelles.

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