Algae range from single-celled organisms to multi-cellular organisms, some with fairly complex differentiated form and called seaweeds. All lack leaves, roots, flowers, and other organ structures that characterize higher plants. They are distinguished from other Protozoa in that they are photoautotrophic, although this is not a hard and fast distinction as some groups may contain members that are Mixotrophic, deriving energy both from photosynthesis as well as through the uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Some unicellular algae rely entirely on external energy sources and have reduced or lost their photosynthetic apparatus.
All algae have photosynthetic machinery ultimately derived from the Cyanobacteria, and so produce Oxygen as a by-product of photosynthesis, unlike other, non-cyanobacterial photosynthetic bacteria.
Algae are common in terrestrial as well as aquatic environments, but usually inconspicuous on the land and more common in moist, tropical climates (see, however Lichens). The various sorts of algae play significant roles in aquatic ecology. Microscopic forms that live suspended in the water column, called Phytoplankton, provide the food base for most marine food chains. In very high densities (so-called algal blooms) they may discolor the water and outcompete or poison other life forms. The seaweeds grow mostly in shallow marine waters; some are used as human food or are harvested for useful substances such as Agar or fertilizer. The study of algae is called Phycology or algology.
In these groups the chloroplast is surrounded by two membranes, 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 pigmented ancestor; i.e., chloroplasts developed in a single endosymbiotic event.
Two other groups have green chloroplasts containing chlorophyll b:
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 (Chromista), the chloroplast has four membranes, retaining a nucleomorph in cryptomonads, and it now appears that they share a common pigmented 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.
Note many of these groups contain some members that are no longer photosynthetic. Some retain plastids, but not chloroplasts, while others have lost them entirely.
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 reach 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.
Learn about Algae & Algal Blooms - Rural Chemical Industries (Aust.) Pty Ltd.
Harmful Algal Blooms - "Red tide" - National Office for Marine Biotoxins and Harmful Algal Blooms, USA.
Algae Section, National Museum of Natural History - Smithsonian Institution