Algae

As considered here, all of the algae are eukaryotic organisms, meaning their cells have nuclear material of deoxyribonucleic acid (DNA) organized within a discrete, membrane-bounded organelle, known as the nucleus. In view of this definition, the so-called blue-green algae are not discussed in this article, because those organisms are prokaryotic (that is, without an organized nucleus) and are more appropriately referred to as blue-green bacteria, or as cyanobacteria. The cyanobacteria are also different from the true algae in that they do not contain the photosynthetic pigment known as chlorophyll a, they do not have cell walls made of cellulose, and they do not store energy as starch or related polysaccharides.

Virtually all species of algae are photosynthetic. They have a relatively simple anatomy, which can range in complexity from single-celled organisms to colonial filaments, plates, or spheres, to the large, multicellular structures of the brown algae, known as thalli. Algal cell walls are generally made of cellulose, but can contain pectin, a class of hemicellulose polysaccharides that give the algae a slimy feel. The larger, multicellular algae have relatively complex tissues, which can be organized into organ-like structures that serve particular functions.