|MadSci Network: General Biology|
Aimee, Let's start at the top and work our way down to more and more refined classification. First the plankton are a group of organisms subject to the movement of the water in which they live. While many are capable of small-scale movement, their large scale movements and distribution are determined by currents, thus they are the "drifters" in the sea (and lakes, too). At this level, "plankton" includes ZOOPLANKTON, or animal drifters - non-photosynthetic, heterotrophic organisms (such as copepods, and many larval forms of sessile animals, and jellyfish), BACTERIOPLANKTON, bacteria (including both photosynthetic cyanobacteria and non-photosynthetic, or heterotrophic, bacteria), and the PHYTOPLANKTON, which refers primarily to microalgae and cyanobacteria (also called blue-green algae) but this is a slightly misleading label. "Phytos" means plant in latin, but most phytoplankton are not true plants. The presence of true plants in the plankton is limited. The plankton also includes members of the fungi and viruses. Separating zooplankton and phytoplankton seems to be the fairly obvious functional difference between "animals" and "plants", but read on. The subset of plankton known as phytoplankton includes diatoms, dinoflagellates, coccolithophorids, and cyanobacteria. At least these are the major components of the phytoplankton of the sea. The cyanobacteria are true prokaryotic bacteria, sometimes included in bacterioplankton, sometimes in phytoplankton because they photosynthesize. The dinoflagellates, coccolithophorids and diatoms are considered Protists, not true plants despite similarities in how they perform photosynthesis. The Protista, in the 5 Kingdom classification scheme, are not true plants (Kingdom Planta) due to their simple life histories and lack of specialized body tissues. True plants include some single celled organisms such as Chlorella, but for the most part include those organisms we generally think of as plants from seaweeds to trees. Their representation in the plankton is limited to planktonic sexual stages and a small number of special cases such as the floating seaweed, Sargassum. However, things can get complicated here. Some species of dinoflagellates, for example, are not photosynthetic. They do not produce their food from inorganic substrates but rather eat (usually bacteria and other phytoplankton) and some do both depending on what is available. Here the distinction between 'phyto'plankton and 'zoo'plankton can get blurred. So dinoflagellates complicate the distinction between phyto- and zoo-plankton. Remember, though, that the distinction is a functional one. Phytoplankton photosynthesize. So some dinoflagellates are zoo- and some are phytoplankton. The 'switch-hitters' are both. This confusion is not the case for diatoms. Diatoms (Bacillariophyceae) are obligate autotrophs. They require sunlight for growth. There are some that can absorb organic compounds to supplement their nutrition, but they cannot live only that way. Diatoms are a diverse group of photosynthetic unicellular (although some form chains) organisms characterized by their siliceous (glass) "shells". These shells, called frustules, are the main determinant of classification. The diatoms are divided into two subgroups, based on the symmetry of their frustules. If the frustule is radially symmetrical, they are part of the "Centric" group (Centrales) while those that are bilaterally symmetrical belong in the "Pennate" group (Pennales). Frustules come in a variety of shapes and can be highly ornamented. Since they persist after death, they are also the subject of paleoclimate research as frustules from sediments can be identified. Knowing who lived in the waters above the sediment can give clues to past environmental conditions based on the tolerance range of the species that are found. They are classified in the classical Linnean method of morphological characters (size, shape, etc.). But more and more research is being done using molecular biology to examine the genetic relationships. Sometimes two speices that look quite alike are not closely related genetically. This has caused a minor revolution within the science of classification of organisms (taxonomy) I am including Dr. R. Sweets' Diatom HomePage address where you can find many links, images and other information about this interesting group of organisms. http://www.indiana.edu/~diatom/diatom.html Good Luck!
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