Diatoms —- Living Snowflakes

Scientists estimate that there may be as many as 12,000 species of diatoms, each identifiable by their delicate, ornate shells. Diatoms are a dominant species of phytoplankton, and are known scientifically as the class Bacillariophyceae and may be grouped in one division (Chrysophyta) of the kingdom Protista along with the golden-brown and the yellow-green algae. Diatoms are one of the major producers in the ocean, and serve as a major component of the ocean food web. At home in the sunlit layer, diatoms are autotrophic–they require energy from sunlight, nutrients like nitrates and phosphates, and carbon dioxide for photosynthesis. These tiny, one-celled creatures are abundant in temperate water, and they can grow up to 1 millimeter across, although most are smaller. Diatoms may thrive in both fresh and marine waters systems, and on moist soil. In the water, diatoms may live attached to rocks, sand, or plants, or they may float freely. Diatoms are divided into two basic types: the elongated forms called pennate diatoms which are most often found in shallow areas and fresh water, and the centric diatoms which are circular, triangular, or irregular in shape and are most abundant in the ocean. Diatoms have thin, double shells of silica called frustules that fit together, one of top of the other, like the two parts of a pillbox. The top of the frustule, the epitheca, is perforated with many holes, arranged in a pattern characteristic of the species The holes permit the cell’s plasma membrane to maintain close contact with the environment, and allows for the diffusion of materials into and out of the cell. Numerous pores also reduce the weight of the floating diatom Some diatoms also have longitudinal slots in the wall of the base, or hypotheca, through which the cell can make contact with a solid surface and crawl about. In the center of the frustule is the nucleus which contains the genetic materials and the chloroplasts, or photosynthetic organelles. Diatoms reproduce through cell division–one cell divides into two cells. After a diatom cell divides each new cell retains one part of the parent shell and builds a new part to fit into it. Movement: Diatoms cannot move under their own power, but have developed adaptations that keep them afloat. Some diatoms have pairs of thin spines or setae projecting from the ends of the cells. These setae fuse with other cells to form long chains, thereby increasing the buoyancy of the chained groups or colonies. Surface Blooms: Massive surface blooms of buoyant diatoms have been observed where the cold waters of polar seas converge with warm equatorial currents. The blooms are marked by a narrow line where the water changes from blue to green. These blooms are detectable by radar and even by satellites in orbit around the earth. Diatomaceous Earths: When diatoms die, their hard, silicate shells remain in tact. The shells sink to the bottom of the sea, and over long periods of geologic time, the layer of diatom shells may become very deep. Diatoms began to accumulate 100 million years ago, and reached a peak of abundance in the middle part of the Cenozoic, the Miocene, wherethere are thick deposits composed largely of their skeletons. The beds are called diatomites, or diatomaceous earths, and are of commercial value. This fine, crumbly substance is used in insulating materials, abrasives, ceramics, and in filtering and filling materials. More than 270,000 metric tons of it are extracted annually from a quarry near Lompoc, California. ((FALL 1997))












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