About Algae

'Algae' is a term of convenience used for many oxygen-producing organisms that belong to different evolutionary lineages but have similar ecological requirements. All contain chlorophyll a, although the green colour of the pigment is often masked by accessory pigments.

These organisms are extraordinarily diverse and range from solitary cells to complex multicellular forms reaching several metres in length. Those possessing internal membranes and therefore organelles (such as chloroplasts and nuclei) are the eukaryotic algae and are usually placed in four of five supergroups or kingdoms, including the Plantae. The evolutionary history of the plastids of these eukaryotic algae is exceedingly complex and involves several endosymbiotic events.

Another important group is the 'blue-green algae'. These are prokaryotic organisms because they lack membrane-bound organelles. The group is more commonly called the cyanobacteria because of their close relationship to bacteria, although they contain chlorophyll a, like eukaryotic algae and vascular plants.

Some of the most important identifying features of algae are frequently lost on preservation. Even microalgae mounted on glass slides may deteriorate in time and rarely possess any useful diagnostic features. The diatoms are one of the most notable exceptions since their silica walls normally provide all the features necessary for identification. Many permanently preserved samples of freshwater algae therefore provide little useful information. For this reason, the type of microscopic algae is frequently not a specimen but a line-drawing or photomicrograph ('iconotype') and any preserved voucher material has limited use for cross-checking identifications.

Classification

The classification follows the 2011 edition of The Freshwater Algal Flora of the British Isles and therefore recognises 15 phyla (see below). It differs in part from AlgaeBase, which to a large extent follows the consensus classification presented in Ruggiero et al. (2015).

Phylum in AlgaeVision Common name AlgaeBase classification Cyanophyta Blue-green algae Phylum Cyanobacteria Rhodophyta Red algae Phylum Rhodophyta Euglenophyta Euglenoids Phylum Euglenophyta/Euglenozoa Cryptophyta Cryptomonads Phylum Cryptophyta/Cryptista Dinophyta Dinoflagellates Phylum Miozoa, class Dinophyceae Raphidophyta Raphidophytes Phylum Ochrophyta, class Raphidophyceae Haptophyta Haptophytes Phylum Haptophyta Chrysophyta Golden or golden- Phylum Ochrophyta, class Chrysophyceae brown algae and class Synurophyceae Xanthophyta Yellow-green algae Phylum Ochrophyta, class Xanthophyceae Chlorophyta Green algae Phylum Chlorophyta, phylum Charophyta, (includes stoneworts) Eustigmatophyta Eustigmatophytes Phylum Ochrophyta, class Eustigmatophyceae Phaeophyta Brown algae Phylum Ochrophyta, class Phaeophyceae Prasinophyta 'Primitive' green algae Phylum Chlorophyta Bacillariophyta Diatoms Phylum Ochrophyta, class Bacillariophyceae (not included in AlgaeVision) Glaucophyta Glaucophytes Phylum Glaucophyta

Phylum Cyanophyta (blue-green algae/cyanobacteria)

 blue-green, grey-green, violet, brown, purplish or red dependent on relative proportions of chlorophyll, phycocyanin and phycoerythrin  sometimes a brown sheath pigment (scytonemin) present  unicellular, colonial or filamentous (simple or branched)  internal membranes absent and so no organelles

Phylum Rhodophyta (red algae)

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

Phylum Euglenophyta (euglenoids)

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

Phylum Cryptophyta (cryptomonads)

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

Phylum Dinophyta (dinoflagellates)

 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 directed  food storage materials - starch and oil

Phylum Raphidophyta

 yellow-green due to predominance of accessory pigment diatoxanthin in two 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 other backwards  food storage material - oil

Phylum Haptophyta

 cells are golden or yellow-brown due to presence of accessory pigments (principally fucoxanthin)  unicellular  flagellates have amoeboid, coccoid, palmelloid or filamentous stages  walls often possess calcified scales  two flagella, and between them an appendage known as a haptonema  food storage material - principally chrysolaminarin

Phylum Chrysophyta (golden-brown algae)

 cells are 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 (lorica)  silica scales sometimes present  food storage material - oil or leucosin

Phylum Xanthophyta (yellow-green algae)

 cells are typically yellow-green due to present of the accessory pigment diatoxanthin in two or more chloroplasts  unicellular, filamentous, colonial or coenocytic  motile forms have two subapical flagella  walls frequently of overlapping parts  food storage material - oil, fat or leucosin

Phylum Chlorophyta (green algae)

 cells with one to several green chloroplasts  unicellular, colonial, filamentous, coenocytic or macrophytes with robust axes bearing whorls of branches and branchlets  motile or non-motile - if motile then normally have one, two or four usually apical flagella  food storage material - principally starch surrounding in one to several pyrenoids  sexual reproduction oogamous in some orders

Phylum Eustigmatophyta

 cells are yellow-green, with main accessory pigment usually violaxanthin in one or more chloroplasts  unicellular and coccoidal  motile forms have one flagellum or two unequal flagella inserted near apex  eyespot unique, independent of chloroplast  pyrenoid unique  food storage material unknown

Phylum Phaeophyta (brown algae)

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

Phylum Prasinophyta (primitive green algae)

 cells have green, rarely yellow-green, chloroplasts  unicellular flagellates, rarely non-motile, with one to eight lateral or apical flagella, usually arising at base of a depression  walls and flagella mostly covered with organic scales  food storage material - starch or mannitol

Phylum Glaucophyta

 cells are bright blue-green due to presence of phycocyanin and other pigments in cyanelles (not equivalent to chloroplasts)  unicellular or colonial  food storage material – starch, produced outside the cyanelles