Algae comprise a diverse group of (with few exceptions) photosynthetic oxygen-producing organisms, ranging in size from microscopic single cells to gigantic seaweeds.
The study of algae is known as phycology (in Greek, phycos means "algae"). Currently, most authors place eukaryotic algae in the kingdom Protista (domain Eukarya) and prokaryotic algae in the domain Bacteria.
In the past algae were considered to be lower plants because some forms look like plants. As in plants, the primary photosynthetic pigment in algae is chlorophyll a, and oxygen is produced during photosynthesis.
What Are Algae?
Algae can be found nearly everywhere on earth: oceans, rivers, lakes, in the snow of mountaintops, on forest and desert soils, on rocks, on plants and animals (such as within the hollow hair of the polar bear), or even on other algae. They are involved in diverse interactions with other organisms, including symbiosis, parasitism, and epiphytism.
Lichens are symbiotic associations between algae (blue-green algae, or cyanobacteria) and fungi. Atmospheric nitrogen-fixing cyanobacteria occur in symbiotic associations with plants such as bryophytes, water ferns, gymnosperms (such as cycads), and the angiosperms.
The aquatic fern Azolla, commonly used as a biofertilizer in rice fields in Asian countries, harbors the symbiotic cyanobacterium Anabaena azollae. Gunnera, the only flowering plant to house symbiotic cyanobacterium Nostoc, is widely distributed in the tropics.
Symbiotic dinoflagellates known as zooxanthellae livewithin the tissues of corals. Corals get their colors and obtain energy from their photosynthetic symbionts. About 15 percent of red algae occur as parasites of other red algae. Parasitic algae may even transfer nuclei into host cells and transform them.
After transformation, the reproductive cells of the host algae carry the parasite’s genes. Various algae live on the surfaces of plants and other algae as epiphytes. Sometimes algae can be found in strange places—the pink color of flamingos originates, for example, comes from a pigment in the algae consumed by these birds.
Algal Structure and Properties
Algal cells are bounded by a cell wall. Algal cells are either prokaryotic or eukaryotic. All prokaryotic algae belong to Cyanophyta (cyanobacteria) and lack both a nucleus and complex membrane-bound organelles, such as chloroplasts and mitochondria.
Photosynthesis occurs in cyanobacteria in thylakoid membranes similar to those of plants. However, there is no double membrane surrounding the thylakoids of cyanobacteria.
All other algal groups are eukaryotic. Eukaryotic algae differ from cyanobacteria in that they possess chloroplasts and flagella with associated structures and in their cellwall composition. According to the endosymbiont hypothesis, some eukaryotic algae (red and green algae) obtained their chloroplasts by acquiring symbiotic prokaryotic cyanobacteria. This is known as primary endosymbiosis.
Other eukaryotic algae probably obtained their chloroplasts by taking up eukaryotic endosymbiotic algae, a process known as secondary endosymbiosis. The existence of secondary endosymbiosis is indicated by the occurrence of more than two membranes around the chloroplasts of some algae, such as haptophytes, euglenophytes, dinoflagellates, and cryptomonads.
Pigments found in algae include chlorophylls, phycobilins, and carotenoids. All algae contain chlorophyll a. Accessory pigments vary among different algal groups.
Photoautotrophy is the principal mode of nutrition in algae; in other words, they are "self-feeders", using light energy and a photosynthetic apparatus to produce their own food (organic carbon) from carbon dioxide and water. The majority of algal groups contain heterotrophic species, which obtain their organic food molecules by consuming other organisms.
Numerous algae are mixotrophs; that is, they use different modes of nutrition (such as autotrophy and heterotrophy), depending on the availability of resources. The molecules used as food reserves differ among and are characteristic for algal groups. Food reserve molecules are polymers of glucose with different links between monomers.
Many algae are capable of movement. Movement is accomplished by means of flagellar action and by extrusion of mucilage. There are also peristaltic and amoeba-like algal movement. Within algal cells, movement of the cytoplasm, plastids, and nucleus also occurs.
Advantages conferred by mobility include achieving optimal light conditions for photosynthesis, avoiding damage caused by excess light, and obtaining inorganic nutrients.
Algal Reproduction and Life Cycles
Algae may reproduce either asexually or sexually. Asexual reproduction among algae includes production of unicellular spores that germinate without fusing with other cells, fragmentation of filamentous forms, and cell division by splitting.
In sexual reproduction, parent cells release gametes, which then fuse to form a zygote. Zygotes may either develop into new filaments or produce haploid spores by meiotic division.
Algae exhibit different types of life cycles. Some algal life cycles are characterized by an alteration of generations similar to that of plants. Two phases occur: sporophyte (usually diploid) and gametophyte (usually haploid).
The sporophyte produces haploid spores through meiosis, and the haploid gametophyte produces male or female gametes by mitosis. Gametophyte and sporophyte may be structurally identical or dissimilar, depending on the algal group.
Roles of Algae
Algae have played significant roles in the earth’s ecosystems since the origin of cyanobacteria (also known as blue-green algae)more than three billion years ago. Early cyanobacteria were responsible for the development of significant amounts of free oxygen in the atmosphere, which then made aerobic respiration possible.
More than 70 percent of all photosynthetic activity on earth is carried out by phytoplankton—floating microscopic algae—rather than plants. Phytoplankton recharge the atmosphere with oxygen and simultaneously absorb carbon dioxide, helping to support the complex web of aquatic biota.
Algae are also very important in the global cycling of other elements, such as carbon, nitrogen, phosphorus, and silicon. Several algal groups—such as cyanobacteria, green algae, red algae, and the haptophyte algae—are able to generate calcium carbonate.
Sedimented algae are the major contributors to deep-sea carbonate deposits (sand), which cover about half of the world’s ocean floor. Calcified coralline red algae contribute to coral reefs in tropical waters. Silica sediments in oceans (sand) are based on abundant growth of another algal group, the diatoms, which contain silica in their cell walls.
Some algae (cyanobacteria) are able to fix atmospheric nitrogen and convert it to ammonia. Ammonia, in turn, can be a nitrogen source for plants and animals. On the other hand, high levels of nitrogen and phosphorus in rivers and lakes owing to pollution can cause the rapid and uncontrollable growth of algae, known as algal blooms.
A bloom of algae is a threat to human and marine health, both directly and indirectly. It clogs fishes’ gills, interferes with water filters, and ruins recreation sites. More than 50 percent of algal blooms produce toxins.
Cases of human respiratory, skin, and gastrointestinal disorders associated with algal toxins have been reported. Certain blooms of algae are called red tides. The water appears to be red or brown because of the color of algal bodies, mainly dinoflagellates that contain the pigment xanthophyll.
Algae have been used as food, medicine, and fertilizer for centuries. The earliest known reference to the use of algae as food occurs in Chinese poetic literature dated about 600 b.c.e. More recently, algae have begun to play important roles in certain biotechnological processes.
Several algae, including reds, browns, greens, and cyanobacteria, are used for food in Pacific and Asian countries, especially Japan. The annual harvest of the red alga Porphyra worldwide is worth several billion dollars. Porphyra (Japanese nori, Chinese zicai) is used as a wrapper for sushi or may be eaten alone. Another edible alga with a high iodine content is the brown alga Laminaria (Japanese kombu). The cyanobacterium Spirulina, with a protein level of 50 to 70 percent, was cultivated for centuries by indigenous Central Americans at Lake Texcoco near modern-day Mexico City for use as human food.
Several gelling agents are produced from red and brown algae. Agar from red algae is used as a medium for culturing microorganisms including algae, as a food gel, and in pharmaceutical capsules. Red algal carrageenan is used in toothpaste, cosmetics, and food such as ice creamand chocolate milk.
Alginates from brown algae have extensive applications in the cosmetics, soap, and detergent industries. Sources of alginates are Laminaria, some Fucus species, and the giant kelp Macrocystis,which can grow to more than 60 meters long. Algae are also used as feed in the culture of commercially important fish and shrimp.
Mass cultivation of algae (microalgae)—in open ponds and photobioreactors for production of fuels (such as biomass) and biochemicals (such as carotenoids, amino acids, and carbohydrates) and for water purification—is a rapidly developing area based on the use of solar energy as energy source. The green alga Dunaliella is used in the industrial production of carotene. In wastewater treatment plants, algae are used to remove nutrients and heavy metals and to add oxygen to the water.
Algae are used worldwide as indicators (biomonitors) of water quality, helping to detect the presence of toxic compounds in water samples. Several fast-growing algae are used, including the green alga Selenastrum capricornutum.
Many algae are widely employed as research tools because they are easy to culture and manipulate. Danish biologist Joachim Hammerling’s experiments with the green alga Acetabularia identified the nucleus as the likely storage site of hereditary information.
The algae are divided into ninemajor phyla, which differ in their photosynthetic pigments, food reserves, cell structure, and reproduction. These groups include euglenoids, cryptomonads, dinoflagellates, haptophytes, and red algae.
Phylum Euglenophyta contains mostly unicellular formswith one or two flagella. Only one-third of this group possess chlorophyll-containing chloroplasts. Other euglenoids are strictly heterotrophic.
The phylum contains more than nine hundred, mostly freshwater, species. The food reserve is the carbohydrate paramylon, a polymer of glucose. Euglenophytes have chlorophyll a and b as well as carotenoids as their photosynthetic pigments. There is no cell wall.
Cells have several small chloroplasts; each is surrounded by three membranes. A close relative of euglenophytes is the protozoan Trypanosoma, which causes the human disease African sleeping sickness. Reproduction in the euglenophytes occurs by division of cells. Sexual reproduction is unknown.
Phylum Cryptophyta includes unicellular biflagellates. In addition to chlorophyll a, chloroplasts can contain chlorophyll c, carotenoids, and phycobilins. The carotenoid pigment alloxanthin is unique to Cryptophyta. Four membranes surround each chloroplast.
Chloroplast endoplasmic reticulum borders the chloroplasts. The principal food reserve is starch. Instead of a typical cell wall, a periplast composed of protein plates occurs beneath the cell membrane. There are about two hundred freshwater and marine species. Reproduction is primarily asexual.
Members of the phylum Dinophyta, or dinoflagellates, have unicellular forms with two different flagella. There are between two thousand and four thousand marine species and about two hundred freshwater forms. Many have chlorophylls a and c as well as the unique carotenoid peridinin. Some members of Dinophyta have fucoxanthin.
Chloroplasts have three closely associated membranes. The primary food reserve is starch, but lipids are also important storage molecules. A dinoflagellate cell is not surrounded by a cell wall but has a theca (a sort of armor) made of cellulose. Dinoflagellates can reproduce asexually and sexually.
Phylum Haptophyta includes primarily marine unicellular biflagellated algae. A haptophyte cell also has a flagellum-like haptonema, used to capture food. There are about three hundred species. The photosynthetic pigments include chlorophyll a and accessory pigments chlorophyll c and the carotenoid fucoxanthin.
Each chloroplast has four membranes. The food reserve is chrysolaminarin, which is a polymer of glucose. Several layers of scales, or coccoliths, composed primarily of calcium carbonate may cover the haptophyte cell. Asexual and sexual reproduction is widespread.
Phylum Rhodophyta, or the red algae, has between four thousand and six thousand species. Red algae lack any flagellated stages. The photosynthetic pigments include chlorophyll a as well as accessory phycobilins and carotenoids. Two membranes surround each chloroplast.
The food reserve is a floridean starch. A red algal cell is encircled by a wall composed of cellulose. Asexual and sexual re production, as well as alteration of generations, are widespread among Rhodophyta. A triphasic life cycle is unique for this group of algae.