Two or more compatible myxamoebae combine to form the second trophic (feeding) stage called a plasmodium (plural plasmodia). A plasmodium is a mass of protoplasm with numerous nuclei—sometimes numbering in the millions.
Plasmodia have a consistency like egg-white and in natural situations they can flow through labyrinthine microhabitats in search of nourishment. They feed in the soil and decaying organic material on various kinds of bacteria; fungal hyphae, fruiting bodies and spores; algae (which may remain alive and impart a greenish tinge to a plasmodium) and possibly lichens. They are also known to parasitise and predate plasmodia of other myxomycetes. They are generally regarded as scavengers or predators, but some species are known to produce amylase and cellulase that break down components in animal and plants respectively. There are several fungivorous slime moulds containing mycolase that breaks down components in fungi.
If conditions are unfavourable plasmodia can transform to a hard structure called a sclerotium and revert to a plasmodium when favourable conditions return. Sclerotia and microcysts can remain viable for long periods, a strategy that probably ensures their survival in arid and other hostile habitats.
There are at least three types of plasmodia found in the five myxomycete orders: Liceales, Echinosteliales, Trichiales, Physarales and Stemonitales.
The most commonly seen type of plasmodium is the often large and conspicuous phaneroplasmodium (Gk phaneros visible) that can attain sizes of up to one metre. Phaneroplasmodia pulsate over substrates such as logs, tree trunks and leaf litter, sometimes travelling several metres within days. They are usually pigmented and can be white, hyaline (transparent), yellow, orange or bright red and their colour can change depending on what they have been ingesting. They have a fan-shaped feeding front edge ahead of a reticulate network of thick cylindrical veins. The network of veins is usually ‘closed’ i.e. there are usually no free ends at the margin (visible with a x 10 hand lens). Each vein has an outer sponge-like layer where no streaming (flow) occurs and an inner layer of streaming granular protoplasm. Under a low power (x 20 or x 40) microscope the protoplasm can be observed flowing in one direction for about sixty to ninety seconds, the flow will then stop, and then the protoplasm will flow in the reverse direction.
Phaneroplasmodia have a thick slime sheath, a viscous outer layer possibly made of galactose-containing glycoprotein and polysaccharide which is believed to prevent desiccation.
Phaneroplasmodia have a complex ‘invaginated’ (folded) structure that increases their surface area by two to tenfold; food absorption and defecation take place on this surface. Inside the organism are pathways for ingestion, intracellular digestion and defecation vacuoles (i.e. membrane-bound sacs). Plasmodial strands that have a reduced invagination system are probably involved in movement rather than in food absorption. Phaneroplasmodia eventually produce several to several thousand tiny, usually stalked, fruiting bodies often dotted equidistantly along the substrate. They occur in the Physarales, characterised by calcium carbonate (lime) in all or part of their fruiting bodies and some Trichiales, which have white or brightly coloured spores.
The type of plasmodia characteristic of the order Stemonitales are the usually invisible aphanoplasmodia (Gk aphanes invisible). Aphanoplasmodia have an extensive branched reticulate system that is thin, flat, non granular, delicate and without pigment. Some become pigmented when they are about to fruit so developing fruiting bodies can be yellow, pink or white when they first appear.
Aphaneroplasmodia have a ‘spiky’ appearance. They lack the fan-shaped front edge of phaneroplasmodia and their ‘open’ reticulum has numerous free ends around the margins. Their hyphae-like plasmodial strands are 5–10 micrometers in diameter, so they are narrow enough to inhabit the micropores in hard woody substrates.
Aphaneroplasmodia seem to require a wetter environment than phaneroplasmodia and they don’t form hard sclerotia when conditions dry out. Instead the plasmodial strands contract and separate into microscopic droplets which form cysts called aphanosclerotia that are not visible to the naked eye. The rapid development of fruiting structures of Stemonitales on bark in moist chambers and in the field suggests that aphanosclerotia are present and quickly transform into plasmodia upon rehydration.
A type of plasmodium intermediate between the phaneroplasmodium and aphanoplasmodium has been found in the Trichiales. It is likely that other intermediate forms will be found.
The microscopic protoplasmodium is believed to be the most primitive type of plasmodium (Gk protos first). It has the smallest surface-to-volume ratio of all the plasmodia and resembles a juvenile phaneroplasmodium. It is a tiny structure 100 to 300 micrometers in diameter that gives rise to one or several minute fruiting bodies. It is known in all species of Echinosteliales, some species of Licea and Clastoderma debaryanum.
Protoplasmodia are found most frequently on the bark of living trees and vines and less often on the ground. They seem to have evolved to exploit the unpredictability of bark and vine habitats that are subjected to extremely dry, hot or cold conditions interspersed with brief periods of rain. Once wet the spores germinate within hours and grow rapidly into minute fruiting bodies. They have an evanescent peridium through which their spores disperse. During dry conditions the entire protoplasmodium forms a cyst.