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Algae

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A seaweed on a California beach.
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A seaweed on a California beach.

The algae (singular alga) comprise several different groups of living organisms usually found in wet places or water bodies and that capture light energy through photosynthesis, converting inorganic substances into simple sugars with the captured energy. Algae were traditionally regarded as simple plants, and some are closely related to the higher plants. Others appear to represent different protist groups, alongside other organisms that are traditionally considered more animal-like (protozoa). Thus algae do not represent a single evolutionary group, but a level of organization that may have developed several times in the early evolutionary history of life on earth.

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.

Contents

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:

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:

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.
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