The algae (singular is alga) comprise several different groups of living things that produce energy through photosynthesis. They are generally regarded as simple plants, and some are related to the higher plants. Others represent independent lines of evolutionary development, although they all appear to have acquired photosynthesis from cyanobacteria. 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. Thus, the algae are no longer considered a natural grouping, but the term is still used for convenience. The study of algae is called phycology or algology.

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, rather than in specialized organelles.

Eukaryotic algae

All other algae are eukaryotes and conduct photosynthesis within membrane-bound structures (organelles) called chloroplasts. 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:

• Green algae, together with higher plants
• Red algae
• Glaucophytes

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, 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 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.

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 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.

Algal ecology

Algae are a significant 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 might be visible as a discoloration of the water. Some algae, especially the seaweeds, are used as human food, or harvested to make various products.

External links

Australian freshwater algae - Sydney Botanic Gardens

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

• Important publications in phycology