'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.