Algue

L' algue, dont l'étude est appellée phycologie or algologie, est une plante qui produit de l'énergie par la photosynthèse mais qui n'a ni feuille, ni racine, ni fleur ou autres structure tissulaire trouvé dans les autres plantes plus évoluées.

Relations parmi les groupes d'algues

Algues prokaryotes

Traditionellement les cyanobactérie ont été inclues parmi les algues, référencés comme cyanophytes ou algues bleu-vert, bien que certains traités les ont spécifiquement exclues. Ce sont parmi les groupes de matière vivante apparaissant dans les fossiles de 3,8 milliards d'années pendant le (Precambrian) quand elles auraient jouée un rôle majeur en créant l'oxygène de l'atmosphère. Elles ont une structure de la cellule prokaryotic typique des bactéries et elles conduisent la photosynthèse directement dans le cytoplasme.

Algues eukaryotes

Toutes les autres algues sont eukaryotes et conduisent la photosynthèse dans la structure entourée d'une membrane appellé chloroplastes qui contient l'ADN et sont similaire dans leur structure aux cyanobactéries pratiquant l'hypothèse de l'endosymbiose. Il y a trois groupes qui ont des chloroplastes primaire:

• Algue verte et plantes supérieures,
• Algue rouge
• Glaucophytes

Dans ces groupes le chloroplaste est entouré par deux membranes,

-- à traduire

both now thought to come from the chloroplast.  The chloroplasts of red algae have a more or less typical cyanobacterial pigmentation, while the green algae and higher plants have chloroplasts with chlorophyll a and b, the latter found in some cyanobacteria but not most.  There is reasonably solid evidence that these three groups originated from a common colored ancestor, i.e. chloroplasts developed in a single endosymbiotic event.

Two other groups have green chloroplasts containing chlorophyll b, the euglenids and chlorarachniophytes. These are surrounded by three and four membranes, respectively, and were probably retained from an ingested green alga. Those of the chlorarchniophytes contain a small nucleomorph, which is the remnant of the alga's nucleus. It has been suggested that the euglenid chloroplasts only have three membranes because they were acquired through myzocytosis rather than phagocytosis.

The remaining algae all have chloroplasts containing chlorophylls a and c. The latter chlorophyll type is not known from any prokaryotes or primary chloroplasts, but genetic similarities with the red algae suggest a relationship there. These groups include:

• Heterokonts (e.g., golden algae, diatoms, brown algae)
• Haptophytes (e.g., coccolithophores)
• Cryptomonads
• Dinoflagellates

In the first three of these groups (Chromista) the chloroplast has four membranes, retaining a nucleomorph in cryptomonads, and it now appears that they share a common colored ancestor. The typical dinoflagellate chloroplast has three membranes, but there is considerable diversity in chloroplasts among the group, some members presumably having acquired theirs from other sources. The Apicomplexa, a group of closely related parasites, also have plastids though not actual chloroplasts, which appear to have a common origin with those of the dinoflagellates.

Forms of Algae

Most of the simpler algae are unicellular flagellates or amoeboids, but colonial and non-motile forms have developed independently among several of the groups. Some of the more common organizational levels, more than one of which may occur in the life cycle of a species, are:

• Colonial - small, regular groups of motile cells
• Capsoid - individual non-motile cells embedded in mucilage
• Coccoid - individual non-motile cells with cell walls
• Palmelloid - non-motile cells embedded in mucilage
• Filamentous - a string of non-motile cells connected together, sometimes branching
• Parenchymatous - cells forming a thallus with partial differentiation of tissues

In three lines even higher levels of organization have been reached, leading to organisms with full tissue differentiation. These are the brown algae—some of which may reached 70 m in length (kelps)—the red algae, and the green algae. The most complex forms are found among the green algae (see Charales), in a lineage that eventually led to the higher land plants. The point where these non-algal plants begin and algae stop is usually taken to be the presence of reproductive organs with protective cell layers, a characteristic not found in the other algal groups.

Algal Ecology

Algae are an important part of aquatic ecology. Larger algae, called seaweeds, grow mostly in shallow water and provide distinctive habitats. Microscopic forms, called phytoplankton, provide the food base for marine food chains. Phytoplankton can be present in high densities, called algal blooms, which are visible as discoloration of the water. Some algae are used as human food, or harvested to make various products.