Traditionally the cyanobacteria have been included here, referred to as the blue-green algae, though recent literature often specifically excludes them. Cyanobacteria are one of the first groups to appear in the fossil record, dating back to ~3800 million years ago, and probably played a large role in creating the Earth's oxygen atmosphere. They have a prokaryotic cell structure, and conduct photosynthesis directly in the cytoplasm.
All other algae are eukaryotes, and conduct photosynthesis within membrane-bound chloroplasts. These contain DNA and are very similar in structure to the cyanobacteria, and presumably represent reduced cyanobacterial endosymbionts. The exact nature of the chloroplasts is different among the different lines of algae, reflecting different endosymbiotic events. There are three groups which have primary chloroplasts:
In these 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 plants have chloroplasts with chlorophyll b, which is found in some cyanobacteria but not most. It is still not entirely clear whether these groups acquired chloroplasts independently, or diverged from a single ancestral form.
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 of these is not known from any prokaryotes or primary chloroplasts, but genetic similarities suggest they are derived from red algae. 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.
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. Large,
Algae are an extremely important part of the ecology. Larger algae, called seaweeds, grow in large underwater "forests" that provide distinctive habitats. Microscopic forms, called phytoplankton, provide most of the energy for marine ecosystems. Sometimes these are present in exceptionally large quantities, called algal blooms, which are typically visible as a weaker or stronger discoloration of the water. A few algae are eaten, or used to make various products. A great deal more could be said here...