Algae

Scientists' concepts of which organisms should be termed algae (alga, singular; algae, plural; algal, adjective) have changed radically over the past two centuries. The term algae originally referred to almost all aquatic, photosynthetic organisms. But, as more has been learned about the evolutionary

EUKARYOTIC ALGAE
Division	Common Name	Pigments	Habitats	General Morphology
Glaucophyta		Chlorophyll a	Freshwater	Unicellular flagellates
		Phycocyanin
Rhodophyta	Red algae	Chlorophyll a	Mostly marine	Unicells, filaments, thalli; no flagellated stages; some calcified, some mucilaginous
		Phycoerythrin
		Phycocyanin
Cryptophyta	Cryptomonads	Chlorophyll a	Marine and freshwater	Mostly unicells
		Chlorophyll c
		Phycocyanin
		Phycoerythrin
Heterokontophyta
(Ochrophyta)
Chrysophyceae	Golden brown algae	Chlorophyll a	Freshwater	Mostly unicells or colonies; biflagellate
		Chlorophyll c
		Fucoxanthin
Xanthophyceae Chlorophyll		Chlorophyll a	Mostly freshwater and terrestrial; some marine	Coccoid, flagellate, or amoeboid unicells; colonies, uni- and multinucleate filaments; biflagellate
(Tribophyceae)		Chlorophyll c

Eustigmatophyceae		Chlorophyll a	Freshwater and marine	Unicells and coccoid; uni- or biflagellate
		Violaxanthin
Bacillariophyceae	Diatoms	Chlorophyll a	Freshwater and marine	Unicells and colonial coccoids; no flagella
		Chlorophyll c
		Fucoxanthin
Raphidophyceae		Chlorophyll a	Freshwater and marine	Unicellular biflagellates
		Chlorophyll c
		Fucoxanthin	(Marine species only)
		Diadinoxanthin
		Vaucheriaxanthin
		Heteroxanthin
Dictyochophyceae	Silicoflagellates	Chlorophyll a	Marine	Unicellular uniflagellates
		Chlorophyll c
		Fucoxanthin
Phaeophyceae	Brown algae	Chlorophyll a	Marine	Multicellular; reproductive cells biflagellate
		Chlorophyll c
		Fucoxanthin
Dinophyta (Pyrrhophyta)	Dinoflagellates	Chlorophyll a	Mostly marine	Mostly unicells, some coccoids and filaments; biflagellate
		Chlorophyll c
Haptophyta		Chlorophyll a	Mostly marine	Unicellular biflagellates
		Chlorophyll c
Euglenophyta	Euglenoids	Chlorophyll a	Mostly freshwater	Unicellular uniflagellates
		Chlorophyll b
Chlorophyta	Green algae	Chlorophyll a
		Chlorophyll b
Prasinophyceae			Marine and freshwater	Unicells; 1-8 flagella
Chlorophyceae			Mostly freshwater; some terrestrial and marine	Unicellular, coccoid, or colonial flagellates; multicellular or multinucleate filaments; bi- or tetraflagellate


Ulvophyceae			Marine or subaerial	Uni- or multicellular or multinucleate filaments; reproductive cells bi-or tetraflagellate

Pleurastrophyceae			Subaerial	Coccoid or filament; reproductive cells biflagellate
Charophyceae	Stoneworts or brittleworts; desmids		Fresh or brackish water or subaerial	Coccoid or filament, reproductive cells biflagellate or with no flagella; or multinucleate cells, complex thalli with biflagellate male gametes

history of algae, which spans at least five hundred million years, the definition has narrowed considerably. For instance, the assemblage of organisms traditionally called the blue-green algae will not be discussed here. These organisms are now known as cyanobacteria, a name that more accurately reflects their nature as prokaryotes. The algae are now generally considered to include only eukaryotic organisms.

Even after narrowing the group by excluding cyanobacteria, a succinct, precise definition of algae is not really possible. It would be accurate to say that algae are eukaryotic, photosynthetic autotrophs (and their colorless relatives), and that most are aquatic (there are some terrestrial species). The algae include organisms ranging in size from the microscopic to those reaching lengths as tall as a six-story building (e.g., the giant kelp, Macrocystis, which exists off the California coast), but no species of alga achieves the morphological complexity of true land plants; furthermore, sexual reproduction in algae is completely different than that of true land plants. Although not as beautiful to most people as roses and redwood trees, the algae are arguably the most important photosynthesizing eukaryotes on Earth.

Data analysis based on morphological, biochemical, and molecular research has led many systematists (scientists who study relationships among organisms) to conclude that traditional classification schemes for algae, and plants in general, do not reflect natural groupings and so should be abolished. It is useful, nevertheless, to have a classification system that provides a structure for comparing and discussing the various groups in terms that phycologists (scientists who study algae) and other scientists who work with algae (such as ecologists and biochemists) and students can understand. The accompanying table compares different groups of algae at the taxonomic level of division using a scheme that is generally accepted by many phycologists.

Of the eight divisions of algae in the table, only the group called the Chlorophyta is considered to be closely related to green plants. The organisms in the other groups are considered to be more closely related to protists than to green plants.

An ancient unicellular green alga gave rise to all algae in the Chlorophyta lineage. Green algae within the Chlorophyta are further split into two groups, one that contains the charophycean algae and another that consists of all other green algae. It is generally accepted among botanists (scientists who study plants) that a charophycean alga is the closest ancestor to the higher green plants.

Without the ancestral green algae, there would be no land plants, and without the algae and land plants, life as we know it would not be possible. Algae are primary producers in any aquatic environment. They are the basis of the food web, forming the very bottom of the food chain, meaning that they provide, as a byproduct of photosynthesis, a majority of the oxygen humans and animals breathe.

Some algae form symbiotic relationships with other organisms. Specific algae, in association with various types of fungi, form lichens of many different species, one of which is a major food source for reindeer in arctic regions. Algae can also form symbiotic relationships with animals, as evidenced by the very successful association of some reef-forming corals and the dinoflagellate algae of the species Symbiodinium.

Many algae are of economic importance. The fossilized remains of diatoms, known as diatomaceous earth, are used in cleaning products and as filtering and inert processing agents. Algal polysaccharides provide agar, used to prepare media for culturing bacteria, fungi, and plant tissues and in the purification and separation of nucleic acids and proteins. In Asia, certain algae are a major source of food. A tour through an Asian food store will turn up innumerable products made with algae, including the red alga, Porphyra (also known as nori or laver), which is used as a wrapper for sushi; prepared packets of dried soups featuring green algae; and several species of red and brown algae that are packaged, dried, salted, refrigerated, pickled, or frozen. The red alga Chondrus crispus provides carrageenan, used in the food industry as a thickener and emulsifier in many brands of ice cream, pudding, baby food, and chocolate milk. Brown algae provide alginates, also used as thickeners and stabilizers in numerous industries including food, paints, and cosmetics. Algal seaweeds are also collected and used as fodder for livestock in many parts of the world.

Some algae are of concern to humans because of the problems they cause. Some algae grow on the sides of buildings and on statues or other structures, forming unsightly discoloration. Rarely, and generally only in immuno-compromised individuals, certain species of green algae invade human tissues, initially gaining entry through a cut or abrasion on the skin and then proliferating. The green alga Cephaleuros virescens can become parasitic on the leaves of economically important plants such as coffee and tea. But, by far, the most destructive algal incidents are harmful algal blooms (HABs), the consequences of which can cost millions of dollars and cause serious health problems to livestock, fish, and even humans. HABs can occur in freshwater, contaminating watering sources for livestock and killing fish, or in marine environments. The marine HAB known as red tide is caused by certain toxin -producing dinoflagellates. The toxin can poison fish and shell-fish, and shellfish contaminated by the toxin can cause mild to severe illness, even death, in humans who consume them. The alga Pfiesteria has caused toxic reactions in fish and humans in estuaries in the southeastern United States. Many HABs can be attributed to pollution, especially runoff into waterways that causes a nutrient-rich environment conducive to the rapid growth of algae.

Divisions of Algae

The type of chlorophyll and other pigments is characteristic of certain groups of algae. For instance, the Chlorophyta (and the pigmented members of the euglenoids) have both chlorophylls a and b. These pigments are contained in chloroplasts that are the result of endosymbiotic events; that is, during the evolutionary history of the algae, photosynthetic, prokaryotic organisms survived being ingested by their algal hosts and became an integral part of them. The main features distinguishing the algal divisions are listed in the accompanying table. Here are a few more details:

Glaucophyta.

The glaucophytes are unusual unicells in which the plastids are recent endosymbionts.

Cryptophyta.

The cryptomonads are unicells with phycobiliprotein pigments like the red algae, but the pigments are located in a different position within the chloroplast.

Haptophyta.

The haptophytes are distinguished by the haptonema, an anterior filament that sometimes serves to attach the unicells to a substrate or to catch prey. The haptophytes include the coccolithophorids, the scales of which formed the white cliffs of Dover on the coast of England.

Dinophyta (or Pyrrhophyta).

The dinoflagellates provide a good example of the problems in classifying algae, as many species do not have chloroplasts and, thus, live heterotrophically. As discussed above, some species are notorious for causing HABs, including red tides.

Euglenophyta.

The euglenoids are motile unicells often found in organically enriched waters; like the dinoflagellates, some species of euglenoids are colorless heterotrophs.

Heterokontophyta (or Ochrophyta).

This large division includes the brown algae (class Phaeophyceae) and the diatoms (Bacillariophyceae). Brown algae are mostly seaweeds, very diverse in form and habitat. They range in size from microscopic filaments to kelps 50 or 60 meters in length. Sargassum floats freely in the Sargasso Sea, but some kelp, such as Laminaria, have a holdfast that attaches to a substrate, leaving the stem and leafy blade to undulate in the current.

Diatoms are noted for their siliceous walls, which can form many intricate and beautiful shapes. Diatoms are very abundant in both freshwater and marine environments and are important primary producers.

Rhodophyta.

The red algae are mostly seaweeds, and they form some of the most beautiful, exotic shapes of all algae. Some species are calcified and resemble corals.

Chlorophyta.

The very diverse green algae form two major lineages. The charophycean algae have complex morphologies and ultrastructural and genetic features that indicate they are ancestral to land plants. The other lineage comprises all other green algae, which range from unicells to large multinucleate filaments.

Russell L. Chapman

Debra A. Waters

Bibliography

Bold, Harold C., and Michael J. Wynne. Introduction to the Algae, 2nd ed. Upper Saddle River, NJ: Prentice-Hall, 1985.

Graham, Linda E., and Lee W. Wilcox. Algae. Upper Saddle River, NJ: Prentice-Hall, 2000.

Lembi, Carole A., and J. Robert Waaland, eds. Algae and Human Affairs. Cambridge: Cambridge University Press, 1988.

Sze, Philip. A Biology of the Algae, 3rd ed. Boston: WCB McGraw-Hill, 1998.

van den Hoek, C., D. G. Mann, and H. M. Jahns. Algae: An Introduction to Phycology.

Cambridge: Cambridge University Press, 1995.