The Kingdom Protista was established in the 1860s as a place for the slime molds that are plant‐like in forming spores in multinucleate, erect, sporangia and having cellulose in their cell walls, animal‐like in having an amoeboid stage in their life cycle during which they creep about ingesting their food, and fungal‐like in general appearance and habits.
Like fungi these slime molds grow in damp, organic‐rich sites. Rotting logs or decaying plants on the forest floor are favorite habitats. Their amoeboid form is frequently a brightly colored orange or yellow blob of viscous, slippery protoplasm that streams slowly into a network of branching, anastomosing projections that move the whole mass forward. This is the feeding stage and bacteria, yeasts, fungi, or bits of vegetation are incorporated into the mass as it moves. Two principal groups of slime molds are recognized, with a third unrelated group closely associated:
- Dictyosteliomycota form a motile mass of protoplasm—a “slug”—by aggregating individual amoeboid cells that retain their identity in the slug, hence their common name, cellular slime molds.
- Myxomycota, the plasmodial slime molds, lose their cell membranes when they come together and the nuclei float freely in the combined, membrane bound mass of cytoplasm, which is called a plasmodium. (The slug of the Dictyostelio‐mycota is a false plasmodium or a pseudoplasmodium.)
- Labyrinthulomycota, the slime nets, are basically unicellular, but live together in colonies. They secrete a membrane outside of their cells, which forms a network of filaments through which the cells travel. The net, sometimes several centimeters in diameter, resembles the plasmodium of the slime molds, but if examined microscopically the cells in their tracks can be seen as distinctively different.
The three groups are neither related to the fungi nor to each other, but they have in common heterotrophy and a sporangia form in their life cycle.
The myxomycetes are reproductively more advanced than the cellular slime molds and form a true plasmodium that can be several centimeters in diameter and look to the untrained eye like a patch of yellow vomitus spewed over a decaying log on the forest floor. The plasmodium is multinucleate and diploid. When food supplies are limited or the environment dries, the plasmodia cease their streaming and may form thick-walled structures called sclerotia (singular, sclerotium) in which the protoplasm can withstand adverse environmental conditions. Growth is resumed with the return of more favorable circumstances.
Both asexual and sexual reproduction occurs, but the initiation clues to each process remain obscure. Under some conditions, the plasmodium produces erect sporangiophores with sporangia on their tips. Meiosis takes place and haploid spores result. Like the sclerotia, the spores are resistant and able to sustain the slime mold over adverse growth periods. When conditions again are suitable for growth, the spores germinate. Some develop into amoebas that move about, feeding. Others become flagellated gametes. After a period of time, the cytoplasm of a pair of genetically different amoeboid or flagellated cells fuse, but individuality of the nuclei is retained, a process known as plasmogamy. Karyogamy, the fusion of the nuclei, soon follows after both types of unions; the resultant cells are the zygotes. Growth of a zygote and the repeated division of its nuclei by mitosis result in the characteristic multinucleate, diploid plasmodium.
Another distinctive group, the oomycetes, includes the water molds and some other taxa. Some are simple unicellular forms, but there are, as well, branched, coenocytic, filamentous fungi-appearing individuals in the group. They have cellulose in their cell walls, reproduce sexually by oogamy, and asexually with biflagellate zoospores. A large number of the 700 species are aquatic, both marine and freshwater, hence the common name “water molds” for the group. Even the terrestrial members form flagellated zoospores when water is available in their habitat. Among the terrestrial forms are several highly destructive plant pathogens—downy mildew of grapes and late blight of potatoes are caused by oomycetes. Less well known, but as significant economically, are several oomycetes that live in the soil and attack, and ultimately kill, the roots of many commercially grown fruits. Large areas in southern California, for example, are unsuitable for avocado growth because of oomycetes in the soil.
Phylogeny of the fungi-like protists
The slime molds represent three stages in development towards multicellularity: a single, coenocytic mass of protoplasm; a mass of protoplasm in which separate cells float; and a third mass of protoplasm in which the cells are enclosed and move in membranous structures. All of these are able to come together and function in a mass that resembles a multicellular organism.