The algae (singular is alga) comprise several different groups of living things that are simple plants. However, most of these groups are not classified in the Kingdom Plantae. All algae lack true leaves, roots, flowers, and other tissue structures found in higher plants. They are distinguished from bacteria and protozoa in that they are photoautotrophic, obtaining energy through photosynthesis. The algae are no longer considered a natural grouping, but the term is still used for convenience. The botanical study of algae is called Phycology.

Table of contents
1 Relationships Among Algal Groups
2 Forms of Algae
3 Algal Ecology

Relationships Among Algal Groups

Prokaryotic Algae

Traditionally the cyanobacteria have been included among the algae, referred to as the cyanophytes or Blue-green Algae, though some recent treatises on algae specifically exclude them. Cyanobacteria is one of the first groups of living things to appear in the fossil record, dating back some 3800 million years ago (Precambrian) when they may have played a major role in creating Earth's oxygen atmosphere. They have a prokaryotic cell structure typical of bacteria and conduct photosynthesis directly within the cytoplasm.

Eukaryotic Algae

All other algae are eukaryotes and conduct photosynthesis within membrane-bound structures called chloroplasts. These chloroplasts contain DNA and are similar in structure to cyanobacteria, presumably representing reduced cyanobacterial endosymbionts. The exact nature of the chloroplasts is different among the different lines of algae, possibly reflecting different endosymbiotic events. There are three groups that have primary chloroplasts:

In these groups the chloroplast is surrounded by two membranes, of which the outer comes from the "host" and the inner 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. It is still not entirely clear whether these groups acquired chloroplasts independently (i.e., different endosymbiotic events), or diverged later from a single ancestral form (i.e., 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:

In the first three of these groups the chloroplast has four membranes, retaining a nucleomorph in cryptomonads. It has been suggested that these groups, sometimes referred to as the Chromista, also share a common origin but this is far from certain. 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 may 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:

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, the red algae, and the green algae. The last have developed even more complex forms, giving rise to the land plants. The exact point where these begin, and the algae stop, is usually taken to be the presence of reproductive organs with productive cell layers, which are not found in the other lines.

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.