Order Centrales is a traditional, paraphyletic subdivision of the heterokont algae known as diatoms. The order is named for the shape of the cell walls (or valves or frustules) of centric diatoms, which are circular or ellipsoid in valve view. The valves often bear radially symmetrical ornamental patterns that can appear as dots when viewed with an optical microscope. Some also bear spines on their valves, which may either increase cell surface area and reduce sinking, or act as a deterrant to zooplankton grazers. Unlike pennate diatoms, centric diatoms never have a raphe.
In terms of cell cycle, vegetative cells are diploid and undergo mitosis during normal cell division. In sexual species, oogamous meiosis produces haploid gametes, either ova or sperm cells. These fuse to produce a zygote which expands in size to develop into an auxospore from which full-size vegetative cells are produced.
In some taxonomy schemes, the centric diatoms are known instead as Order Coscinodiscophyceae. And in some schemes as order Biddulphiales . However, diatom taxonomy is changing due to the development of new molecular and genetical analysis tools.
Querido DiálogoEcológico: ¿Es verdad que sólo un puñado de centrales eléctricas caducas a carbón generan una cantidad desproporcionada de la contaminación de mercurio engendrada en Estados Unidos? Si ese es el caso, ¿se está haciendo algo para limpiar estos sitios o cerrarlos?-- Frank Pearson, Wichita, KS
Jun 12, 2011; Diálogo Ecológico De los Redactores de E/La Revista Ecológica El Fondo de Defensa Ambiental informa que los 25 emisores más...