Kingdom Protista

Kingdom Protista is often described as a 'link' or 'bridge' kingdom, because it includes single-celled eukaryotic organisms that show characteristics transitional between the simplest prokaryotes and the more complex multicellular fungi, plants and animals. All protists are primarily aquatic, and although most are unicellular, some -- such as certain algae -- may form loose colonies.

Protists possess a true, membrane-bound nucleus along with other membrane-bound organelles, and they reproduce both asexually and by a sexual process involving cell fusion and zygote formation. Within Kingdom Protista, five major groups are recognised for the purposes of this syllabus: chrysophytes, dinoflagellates, euglenoids, slime moulds and protozoans.

Chrysophytes. This group includes the diatoms and the golden algae (desmids). Diatoms have a unique cell wall that forms two thin, overlapping shells that fit together as neatly as the two halves of a soap box; the walls are embedded with silica, making them virtually indestructible. Because of this, diatom cell walls accumulate over long periods of time on ocean and lake floors, forming a fine, gritty sediment known as 'diatomaceous earth.' Being both gritty and chemically inert, diatomaceous earth is put to a wide range of industrial uses, including polishing, filtration and in the manufacture of insulating materials. As chief 'producers' in oceans, diatoms are also of major ecological importance.

Dinoflagellates. These are mostly marine and photosynthetic organisms, whose cell wall is covered by stiff cellulose plates and which bear two flagella -- one running longitudinally and one lying in a transverse groove between the plates, giving the cell a spinning motion as it swims. Their colour depends on the main pigment present, so a dinoflagellate population may appear yellow-green, brown, blue, red or golden. A rapid multiplication of certain red-pigmented dinoflagellates such as Gonyaulax can turn the sea visibly red, an event known as a 'red tide.' Toxins released in such large numbers can even kill other marine animals, including fish, in the surrounding water.

Euglenoids. Most members of this small group live in freshwater, especially in stagnant water. Instead of a rigid cell wall, euglenoids have a protein-rich, flexible outer layer called a pellicle, which allows the cell to change shape. They are typically bi-flagellated, with one flagellum being short and the other long. Interestingly, euglenoids display characteristics of both plants and animals: in sunlight they behave photosynthetically, since chlorophyll is present in their cells, but in the absence of sunlight they behave as heterotrophs, engulfing smaller organisms as prey. Euglena, the best-known example, also possesses a light-sensitive eyespot near its flagellar base.

Slime moulds. These are saprophytic protists that produce spores and show a fascinating life cycle. Under favourable conditions, they exist as an aggregated mass of protoplasm called a plasmodium, which can grow to a size of several feet and moves along decaying twigs and leaves, engulfing organic matter as it goes -- much like a giant, mobile, multinucleate amoeba. When food or moisture becomes scarce, the plasmodium differentiates and forms fruiting bodies bearing spores at its tip. These spores possess extremely resistant walls and can survive for many years, even under adverse conditions such as intense heat, cold, and unfavourable pH -- a striking parallel to how bacteria form resistant spores in tough times. …