The life cycles of algae vary widely. Some reproduce only asexually, whereas others have complex sexual life cycles involving alternation of generations. The single-celled alga Chlamydomonas, for example, has a lengthy haploid phase (in which the cell contains only one set of chromosomes) followed by a restricted diploid phase (with two sets) – the opposite of the situation in most plants. Some complex algae, such as the seaweed Fucus, have a haploid phase almost as short as in seed-producing plants.
All algae are autotrophs, using the Sun’s energy to build their own food molecules by photosynthesis. Most possess the green, light-trapping pigment chlorophyll, although in many groups the green color is masked by other photosynthetic pigments, such as carotenoids.
Classification
The algae are an ancient and complex group of organisms that was highly diverse before true plants evolved. Because of this they do not fit into a neat system of classification, and scientists disagree over the best way to group them.
The largest algae are seaweeds, which are usually classified as plants. Single-celled algae are usually placed in the kingdom Protista. Cyanobacteria, which were formerly called blue-green algae, are now considered to be bacteria, not algae. They are placed in the kingdom Monera. Lichens consist of algae living in symbiosis with fungi.
The modern system divides the algae into seven divisions (phyla), largely on the basis of their pigmentation, which is thought to reflect their phylogeny (evolutionary relationship). These are:
- Green algae (Chlorophyta)
- Brown algae (Phaeophyta)
- Red algae (Rhodophyta)
- and yellow-green algae (Chrysophyta)
- Euglenoid algae (Euglenophyta)
- (Bacillariophyta)
- Dinoflagellates (Pyrrhophyta)
Of these, the first three divisions are predominantly multicellular seaweeds, and so are placed in the plant kingdom (although, confusingly, many green algae are single-celled). The remaining four divisions are predominantly single-celled, and so are placed in the kingdom Prostista.
Green Algae
The green algae (division Chlorophyta) include about 7,000 species, ranging from simple, single-celled microorganisms to large seaweeds, such as Ulva (sea lettuce). They are thought to be closely related to true plants because they have cell walls made of cellulose, their chloroplasts contain the pigments chlorophyll a and b, and they store food as starch. The chlorophyll is unmasked by other pigments, giving them a bright green color.Chlorella
Chlorella is an amazingly simple alga, consisting of a single spherical cell containing one large chloroplast. It inhabits damp places and is also found living inside the cells of the green hydra (Chlorohydra viridissima) – an intimate example of symbiosis.
Motile Algae
Some single-celled species of green alga, such as Chlamydomonas and Volvox, are described as motile because they are able to swim around. To do this they use flagella – threadlike projections from one end of the cell.
Spirogyra
Spirogyra is an extremely common type of green alga found in freshwater. It consists of long, unbranched threads of tubular cells joined end to end and tangled together in a slimy mass, which is found either floating in the water or attached to solid objects.
Stoneworts
The most complicated of the green algae are the stoneworts, such as Chara, which resemble miniature trees growing in freshwater ponds.
Brown Algae
The brown algae (division Phaeophyta) include all of the larger seaweeds, as well as many smaller plants. About 1,500 species are known, most of which grow in shallow water. Sargassum is a notable exception: it forms the huge floating masses in the Sargasso Sea.The life cycle of many brown algae involves conspicuous alternation of generations, in which a sexually produced diploid plant in one generation is followed by an asexually produced haploid plant in the next. Alternation of generations occurs in true plants also but is much less obvious, the haploid stage appearing only briefly in the form of an egg cell or germinating pollen grain.
Types of Brown Algae
The largest of the brown algae are the kelps, of which Laminaria is a common example. Macrocystis is the largest kelp – individuals are sometimes claimed to exceed 60 meters in length and to weigh as much as 50 kg.
The brown rock weed, Fucus, is probably the best-known brown alga. This familiar seaweed is often found on beaches, where it clings to rocks with its rootlike holdfast, from which grows a cylindrical stalk called a stipe. The stipe branches into flattened lamellae, which in some species bear air-filled bladders that keep the plant afloat when the tide comes in. Fucus reproduces by the fusion of a motile sperm cell with a nonmotile egg cell. A zygote is formed, from which a new plant develops.
Red Algae
The red algae (division Rhodophyta) include about 3,000 species, most of which are marine. They are all multicellular but seldom exceed several centimeters in length.All red algae have a well-developed haploid generation. In some, the zygote undergoes meiosis (cell division forming two haploid cells from a diploid one) immediately after fusion of the haploid gametes. In others the zygote develops into a multicellular, diploid plant attached to the haploid plant. Diploid spores from this plant develop into diploid offspring plants, which then produce haploid spores from which the new haploid plant grows. These are the only known plants with this type of “alternation” of three generations.
The gelling agent agar is extracted commercially from a type of Japanese red alga (Gelidium).
Golden and Yellow-green Algae
The golden and yellow-green algae (division Chrysophyta) derive their color from photosynthetic pigments called carotenoids. They also possess certain types of chlorophylls, including one not found in true plants or green algae (chlorophyll c). Unlike true plants, which store energy in the form of starch, these algae produce an energy-rich substance called chrysolaminaran. Most chrysophytes are unicellular and live in freshwater.Euglenoid Algae
Euglenoids (division Euglenophyta) are single-celled algae that move actively using a flagellum. Unlike green algae they do not store food as starch, and they lack rigid cellulose cell walls, possessing instead a finely sculptured protein coat called a pellicle. There are some 1,000 species of euglenoid alga, most of which possess chloroplasts containing chlorophylls a and b as well as carotenoid pigments. Although most are autotrophs, many lack chloroplasts and feed heterotrophically. Some scientists classify heterotrophic euglenoids as protozoa because they are more animal-like than plantlike. Euglenoids are very common in freshwater habitats, particularly in water polluted by decaying organic matter.Diatoms
Diatoms (division Bacillariophyta) are microscopic algae that are common in both marine and freshwater habitats. They are the second-most common type of plankton (after cyanobacteria) found in the sea – a single gallon of seawater can contain over a million of them. Together with cyanobacteria they form the base of the marine food chain, on which all marine animals ultimately depend. Their silica-rich cell walls are intricately sculpted and look like jewels when seen through a microscope.Dinoflagellates
The 21,000 or so species of dinoflagellate typically have two flagella for propulsion and an outer protective coat, which varies enormously in shape between species. Dinoflagellates are found in freshwater and among the marine phytoplankton. They are thought to have been the dominant primary producers on Earth during the Paleozoic era (570–250 million years ago).
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