Микростериас: Жасыл балдырлар туысы

Genus of algae

Micrasterias is a unicellular green alga of the order Desmidiales. Its species vary in size reaching up to hundreds of microns. Micrasterias displays a bilateral symmetry, with two mirror image semi cells joined by a narrow isthmus containing the nucleus of the organism. This dual semi cell structure is unique to the group of green algae to which Micrasterias belongs. Each semi cell contains a single large chloroplast, the site of photosynthesis for Micrasterias. Chloroplasts within Micrasterias contain chlorophyll a and chlorophyll b and the enzymes required for photosynthesis. The sugar created is used to provide energy for the organism or, if not used, taken up by many small round pyrenoids which are embedded in the chloroplast. They convert the sugar to a starch for storage. Micrasterias can produce both asexually and sexually. Asexual reproduction occurs via mitosis. When this occurs the genetic material of Micrasterias is duplicated and two small semi cells grow between the original semi cells, gradually increasing in size. Sexual reproduction occurs through a process called conjugation whereby two organisms come together and fuse their haploid cells to form a diploid zygote. This zygote typically forms a thick protective wall which can allow the organism to remain dormant for many months to survive cold winters and long droughts. When adequate conditions resume, the zygospore will germinate, undergo meiosis, and produce new haploid algal cells.

Micrasterias species are symmetrical and generally consist of two flattened, identical portions called semicells that are almost entirely filled with chloroplasts, with a nucleus that lies at the center where the two semicells are joined together. The gaps between the two semicells are joined by an isthmus. Each semicell is further divided into a polar lobe and two lateral lobes. These lobes can be further subdivided up to the fourth order. Some species, such as Micrasterias laticeps, have a very different morphology, with unbranched lobes. Except for a single filament forming species, Micrasterias foliacea, it is found as single cells. As is common in the green algae, the chloroplasts of Micrasterias contain pyrenoids. The genus Prescottiella contains a single species, Prescottiella sudanensis, formerly known as Micrasterias sudanensis. It is distinguished from Micrasterias in that its semicells are not identical, making the cell asymmetrical along one axis; the spines of one semicell curve towards the isthmus, while the spines of the other semicell curve away.

As with other desmids, Micrasterias grows in freshwater habitats. It prefers oligotrophic to mesotrophic lakes and bogs, often associated with aquatic plants. Freshwater microalgae, along with other microscopic organisms, are often presumed to be cosmopolitan in distribution (see the Baas Becking hypothesis). However, many desmids are an exception to this, likely because of their high morphological complexity allowing for easier identification, and the fact that they mostly do not form resting spores that would allow for wider dispersal. In particular, a number of Micrasterias species are restricted to certain biogeographical realms or continents. For example, Micrasterias muricata appears to be endemic to North America, while Micrasterias ceratofera is restricted to Southeast Asia and northern Australia.