Slime Molds

Slime Molds, multicellular organisms that form tiny, spore-producing fruiting bodies of various shapes. The two subgroups are the true slime molds, myxomycetes, and the cellular slime molds, acrasiomycetes.

Myxomycetes are most often found in moist, decaying leaf litter and logs in the forest. About 500 species can be distinguished by the size and shape of their fruiting bodies; some are delicately stalked, some branched, some show large masses of spores surrounded by a hard wall. Their life cycle is invariably sexual; two single cells of opposite sex fuse. The cells may be flagellated or amoeboid (see Amoeba; Flagellates), depending on whether the soil is moist or dry. After fertilization the nucleus divides, but no internal cell walls are formed, resulting in a multinucleate mass of protoplasm called a plasmodium. The plasmodium becomes a large, often orange, glistening mass of mucuslike material that slowly moves over the rotten wood or humus. This mass is the feeding structure, which engulfs and ingests food (usually bacteria). When food becomes scarce and external temperature and humidity are right, the plasmodium transforms in a matter of hours into a series of bumps that become the fruiting bodies or sporangium. The spores are produced in the final stages of this differentiation. When the dispersed spores germinate, each one liberates a gamete.

Acrasiomycetes are smaller than most myxomycetes and have fewer species (about 50). The cells are always amoeboid and uninucleate; they do not form plasmodia. Cellular slime molds constitute a large portion of the soil amoebae population, feeding separately on bacteria. When they have cleared an area of food, they form fruiting bodies, but in a radically different manner from the myxomycetes. Starved amoebae stream together into central collection points, attracted by a chemical called acrasin that they themselves give off. The resulting multicellular organism seems remarkably well organized: It has anterior and posterior ends, moves toward light, and orients toward or away from heat, depending on the temperature. When it comes to rest, the amoebae at the anterior end start producing a delicate stalk, while those at the posterior end turn into spores that form a terminal ball at the tip of the new stalk. This cycle is asexual, although the cellular slime molds also have a sexual cycle. The fusion of the sex cells (individual amoebas) produces a large resistant body called a macrocyst, which upon germination, produces new cells capable of continuous asexual reproduction.

Scientific classification: There is debate as to where the slime molds should be classified since they share some characteristics with amoebas and others with fungi. Most often they are classified in the Kingdom Protista with the amoebas, where they form two phyla, the Myxomycota and the Acrasiomycota.