About Algae - Natural History Museum, London · 2019-11-28 · About Algae 'Algae' is a term of convenience used for many oxygen-producing organisms that belong to different evolutionary

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-brown algae

Phylum Ochrophyta, class Chrysophyceae and class Synurophyceae

Xanthophyta Yellow-green algae Phylum Ochrophyta, class Xanthophyceae

Chlorophyta Green algae (includes stoneworts)

Phylum Chlorophyta, phylum Charophyta,

Eustigmatophyta Eustigmatophytes Phylum Ochrophyta, class Eustigmatophyceae

Phaeophyta Brown algae Phylum Ochrophyta, class Phaeophyceae

Prasinophyta 'Primitive' green algae Phylum Chlorophyta

Bacillariophyta (not included in AlgaeVision)

Diatoms Phylum Ochrophyta, class Bacillariophyceae

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