Algae

A seaweed (Laurencia) up close: the "branches" are multicellular and only about 1 mm thick. Much smaller algae are seen attached to the structure extending upwards in the lower right quarter

Algae (singular alga) are a diverse grouping of photosynthetic, eukaryotic organisms, that are generally classified in the kingdom Protista, and which use chlorphyll in capturing light energy. Traditionally, they have been regarded as simple plants, but lack many characteristic plant structures, such as leaves, roots, flowers, and seeds, and thus are typically not classified in the kingdom Plantae. They are distinguished from the other main protists, the protozoa, in that they are photoautotrophic, although this is not a hard and fast distinction as some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Some scientists include as algae the prokaryotic cyanobacteria, which are aquatic and photosynthetic and are commonly known as "blue-green algae." However, in general, the designation of algae is limited to eukaryotic photosynthetic organisms. The name alga (plural algae) comes from the Latin word for seaweed.

Algae range from single-celled organisms to multi-cellular organisms, some with fairly complex differentiated form and (if marine) called seaweeds. Some of the single-celled organisms may be as small as 1 micrometer, while the giant kelp reaches 60 meters in length. Some algae consist of a row of cells, appearing as a filament, and some may have spiky branches. Seaweeds themselves have many forms, including looking like terrestrial plants, with leaves and stems, such as those that look like moss, mushrooms, a leaf lettuce, or even a palm tree.

Algae are usually found in damp places or bodies of water and thus are common in terrestrial as well as aquatic environments. However, terrestrial algae are usually rather inconspicuous and far more common in moist, tropical regions than dry ones, because algae lack vascular tissues and other adaptions to live on land. Algae can endure dryness and other conditions in symbiosis with a fungus as lichen. 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.

All algae have photosynthetic machinery ultimately derived from the cyanobacteria, and so produce oxygen as a by-product of photosynthesis, unlike the non-cyanobacterial photosynthetic bacteria. It is believed that more than three-quarters of the oxygen in the atmosphere comes from algae and cyanobacteria, rather than from plants. Although all algae utilize chlophyll, at times other pigments mask the green color, resulting in organisms with red and brown colors.

Algae are also key as the base of the aquatic food chain, and as an original source of petroleum.

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 are some of the oldest organisms to appear in the fossil record, dating back about 3.8 billion years (Precambrian). Ancient cyanobacteria likely produced much of the oxygen in the Earth's atmosphere.

Cyanobacteria can be unicellular, colonial, or filamentous. They have a prokaryotic cell structure typical of bacteria and conduct photosynthesis directly within the cytoplasm, rather than in specialized organelles. Some filamentous blue-green algae have specialized cells, termed heterocysts, in which nitrogen fixation occurs.

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. Red and green algae have an alternation of generations life cycle. This is the same life cycle as the mosses, suggesting that the mosses evolved from the green algae.

Two other groups have green chloroplasts containing chlorophyll b:

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.

Algae and symbioses

Algae frequently form part of a symbiosis with other organisms. In these symbioses, the algae photosynthesise and supply photosynthates to their host. The host organism is then capable of deriving some or all of its energy requirements from the alga. Examples include:

lichens - a fungus is the host, usually with a green alga or a cyanobacterium as the symbiont. Both fungi and algae found in lichens are capable of living independantly.
corals - several algae form symbioses (zooxanthellae) with corals. Notable amongst these is the dinoflagellate Symbiodinium, found in many hard corals. The loss of Symbiodinium, or other zooxanthellae, from the host leads to coral bleaching.

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

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