Reproduction and life histories
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Algae regenerate by sexual reproduction, involving male and female gametes (sex cells), by asexual reproduction, or by both ways.
Asexual reproduction is the production of progeny without the union of cells or nuclear material. Many small algae reproduce asexually by ordinary cell division or by fragmentation, whereas larger algae reproduce by spores. Some red algae produce monospores (walled, nonflagellate, spherical cells) that are carried by water currents and upon germination produce a new organism. Some green algae produce nonmotile spores called aplanospores, while others produce zoospores, which lack true cell walls and bear one or more flagella. These flagella allow zoospores to swim to a favourable environment, whereas monospores and aplanospores have to rely on passive transport by water currents.
Sexual reproduction is characterized by the process of meiosis, in which progeny cells receive half of their genetic information from each parent cell. Sexual reproduction is usually regulated by environmental events. In many species, when temperature, salinity, inorganic nutrients (e.g., phosphorus, nitrogen, and magnesium), or day length become unfavourable, sexual reproduction is induced. A sexually reproducing organism typically has two phases in its life cycle. In the first stage, each cell has a single set of chromosomes and is called haploid, whereas in the second stage each cell has two sets of chromosomes and is called diploid. When one haploid gamete fuses with another haploid gamete during fertilization, the resulting combination, with two sets of chromosomes, is called a zygote. Either immediately or at some later time, a diploid cell directly or indirectly undergoes a special reductive cell-division process (meiosis). Diploid cells in this stage are called sporophytes because they produce spores. During meiosis the chromosome number of a diploid sporophyte is halved, and the resulting daughter cells are haploid. At some time, immediately or later, haploid cells act directly as gametes. In algae, as in plants, haploid cells in this stage are called gametophytes because they produce gametes.
The life cycles of sexually reproducing algae vary; in some, the dominant stage is the sporophyte, in others it is the gametophyte. For example, Sargassum (class Phaeophyceae) has a diploid (sporophyte) body, and the haploid phase is represented by gametes. Ectocarpus (class Phaeophyceae) has alternating diploid and haploid vegetative stages, whereas Spirogyra (class Charophyceae) has a haploid vegetative stage, and the zygote is the only diploid cell.
In freshwater species especially, the fertilized egg, or zygote, often passes into a dormant state called a zygospore. Zygospores generally have a large store of food reserves and a thick, resistant cell wall. Following an appropriate environmental stimulus, such as a change in light, temperature, or nutrients, the zygospores are induced to germinate and start another period of growth.
Most algae can live for days, weeks, or months. Small algae are sometimes found in abundance during a short period of the year and remain dormant during the rest of the year. In some species, the dormant form is a resistant cyst, whereas other species remain in the vegetative state but at very low population numbers. Some large, attached species are true perennials. They may lose the main body at the end of the growing season, but the attachment part, the holdfast, produces new growth only at the beginning of the next growing season.
The red algae, as exemplified by Polysiphonia, have some of the most complex life cycles known for living organisms. Following meiosis, four haploid tetraspores are produced, which germinate to produce either a male or a female gametophyte. When mature, the male gametophyte produces special spermatangial branches that bear structures, called spermatangia, which contain spermatia, the male gametes. The female gametophyte produces special carpogonial branches that bear carpogonia, the female gametes. Fertilization occurs when a male spermatium, carried by water currents, collides with the extended portion of a female carpogonium and the two gametes fuse. The fertilized carpogonium (the zygote) and the female gametophyte tissue around it develop into a basketlike or pustulelike structure called a carposporophyte. The carposporophyte eventually produces and releases diploid carpospores that develop into tetrasporophytes. Certain cells of the tetrasporophyte undergo meiosis to produce tetraspores, and the cycle is repeated. In the life cycle of Polysiphonia, and many other red algae, there are separate male and female gametophytes, carposporophytes that develop on the female gametophytes, and separate tetrasporophytes.
The life cycles of diatoms, which are diploid, are also unique. Diatom walls, or frustules, are composed of two overlapping parts (the valves). During cell division, two new valves form in the middle of the cell and partition the protoplasm into two parts. Consequently, the new valves are generally somewhat smaller than the originals, so after many successive generations, most of the cells in the growing population are smaller than their parents. When such diatoms reach a critically small size, sexual reproduction may be stimulated. The small diploid cells undergo meiosis, and among pennate (thin, elliptical) diatoms the resulting haploid gametes fuse into a zygote, which grows quite large and forms a special kind of cell called an auxospore. The auxospore divides, forming two large, vegetative cells, and in this manner the larger size is renewed. In centric diatoms there is marked differentiation between nonmotile female gametes, which act as egg cells, and motile (typically uniflagellate) male gametes.
Evolution and paleontology of algae
Modern ultrastructural and molecular studies have provided important information that has led to a reassessment of the evolution of algae. In addition, the fossil record for some groups of algae has hindered evolutionary studies, and the realization that some algae are more closely related to protozoa or fungi than they are to other algae came late, producing confusion in evolutionary thought and delays in understanding the evolution of the algae.
The Euglenophyceae are believed to be an ancient lineage of algae that includes some zooflagellate protozoa, which is supported by ultrastructural and molecular data, though the group is taxonomically contentious. Some scientists consider the colourless euglenophytes to be an older group and believe that the chloroplasts were incorporated by symbiogenesis more recently. The order of algae with the best fossil record are the Dasycladales, which are calcified unicellar forms of elegant construction dating back at least to the Triassic Period (about 252 million to 201 million years ago).
Some scientists consider the red algae, which bear little resemblance to any other group of organisms, to be very primitive eukaryotes that evolved from the prokaryotic blue-green algae (cyanobacteria). Evidence in support of this view includes the nearly identical photosynthetic pigments and the very similar starches among the red algae and the blue-green algae. Many scientists, however, attribute the similarity to an endosymbiotic origin of the red algal chloroplast from a blue-green algal symbiont. Other scientists suggest that the red algae evolved from the Cryptophyceae, with the loss of flagella, or from fungi by obtaining a chloroplast. In support of this view are similarities in mitosis and in cell wall plugs, special structures inserted into holes in the cell walls that interconnect cells. Some evidence suggests that such plugs regulate the intercellular movement of solutes. Ribosomal gene sequence data from studies in molecular biology suggest that the red algae arose along with animal, fungal, and green plant lineages.
The green algal classes are evolutionarily related, but their origins are unclear. Most consider the class Micromonadophyceae to be the most ancient group, and some fossil data support this view. The class Ulvophyceae is also ancient, whereas the classes Charophyceae and Chlorophyceae are more recent.
The class Dinophyceae is of uncertain origin and is taxonomically contentious. During the 1960s and ’70s the unusual structure and chemical composition of the nuclear DNA of the Dinophyceae were interpreted as somewhat primitive features. Some scientists even considered the Dinophyceae to be mesokaryotes (intermediate between the prokaryotes and the eukaryotes); however, this view is no longer accepted. Their peculiar structure is considered as a result of evolutionary divergence, perhaps about 300 or 400 million years ago. The Dinophyceae may be distantly related to the chromophyte algae, but ribosomal gene sequence data suggest that their closest living relatives are the ciliated protozoa. It is likely that the Dinophyceae arose from nonphotosynthetic ancestors and that later some species of Dinophyceae adopted chloroplasts by symbiogenesis and thereby became capable of photosynthesis, although many of these organisms still retain the ability to ingest solid food, similar to protozoa.
The origin of the chromophyte algae also remains unknown. Ultrastructural and molecular data suggest that they are in a protistan lineage that diverged from the protozoa and aquatic fungi about 300 to 400 million years ago. At that time, chloroplasts were incorporated, originally as endosymbionts, and since then the many chromophyte groups have been evolving. Fossil, ultrastructural, and ribosomal gene sequence data support this hypothesis.
The Cryptophyceae are an evolutionary enigma. They have no fossil record, and phylogenetic data are conflicting. Although some researchers align them near the red algae, because both groups possess phycobiliproteins in their chloroplasts, most scientists suggest that independent symbiotic origins for the red or blue colour of their chloroplasts could explain the similarity. Cryptophytes have flagellar hairs and other flagellar features that resemble those of the chromophyte algae; however, the mitochondrial structure and other ultrastructural features are distinct and argue against such a relationship.
The fossil record for the algae is not nearly as complete as it is for land plants and animals. Red algal fossils are the oldest known algal fossils. Microscopic spherical algae (Eosphaera and Huroniospora) that resemble the living genus Porphyridium are known from the Gunflint Iron Formation of North America (formed about 1.9 billion years ago). Fossils that resemble modern tetraspores are known from the Amelia Dolomites of Australia (formed some 1.5 billion years ago). The best characterized fossils are the coralline red algae represented in fossil beds since the Precambrian time (4.6 billion to 541 million years ago).
Some of the green algal classes are also very old. Organic cysts resembling modern Micromonadophyceae cysts date from about 1.2 billion years ago. Tasmanites formed the Permian “white coal,” or tasmanite, deposits of Tasmania and accumulated to a depth of several feet in deposits that extend for miles. Similar deposits in Alaska yield up to 568 litres (150 gallons) of oil per ton of sediment. Certain Ulvophyceae fossils that date from about one billion years ago are abundant in Paleozoic rocks. Some green algae deposit calcium carbonate on their cell walls, and these algae produced extensive limestone formations. The Charophyceae, as represented by the large stoneworts (order Charales), date from about 400 million years ago. The oospore, the fertilized female egg, has spirals on its surface that were imprinted by the spiraling protective cells that surrounded the oospore. Oospores from before about 225 million years ago had right-handed spirals, whereas those formed since that time have had left-handed spirals. The reason for the switch remains a mystery.
Fossil Dinophyceae date from the Silurian Period (443.4 million to 419.2 million years ago). Some scientists consider at least a portion of the acritarchs, a group of cystlike fossils of unknown affinity, to be Dinophyceae. The acritarchs occurred as early as 700 million years ago.
The Chromophyta have the shortest fossil history among the major algal groups. Some scientists believe that the group is ancient, whereas others point out that there is a lack of data to support this view and suggest that the group evolved recently, as indicated by fossil and molecular data. The oldest chromophyte fossils, putative brown algae, are approximately 400 million years old. Coccolithophores, coccolith-bearing members of the Prymnesiophyceae, date from the Late Triassic (227 million to 201.3 million years ago), with one reported from approximately 280 million years ago. Coccolithophores were extremely abundant during the Mesozoic Era (252.2 million to 66 million years ago), contributing to deep deposits such as those that constitute the white cliffs of southeast England. Most species became extinct at the end of the Cretaceous Period (145 million to 66 million years ago), along with the dinosaurs, and indeed there are more extinct species of coccolithophores than there are living species. The Chrysophyceae, Bacillariophyceae, and Dictyochophyceae date from about 100 million years ago, and despite the mass extinctions 66 million years ago, many species still flourish. In Lompoc, California, U.S., their siliceous remains have formed deposits of diatomite almost 0.5 km (0.3 mile) in depth, while at Mývatn in Iceland the lake bottom bears significant amounts of diatomite in the form of diatomaceous ooze, many metres in depth.
The Xanthophyceae may be even more recent, with fossils dating from about 20 million years ago, while fossil records of the remaining groups of algae, notably the Euglenophyceae and the Cryptophyceae, which lack mineralized walls, are negligible.
