Slime Moulds

‘Slime mould’ is the informal name given to different kinds of unrelated saprophytic protists. Once considered to be fungi, they are now recognised as members of the Phylum Mycetozoa or Myxomycota of the Kingdom Protista. Slime moulds vary greatly in their colour, shape and size.

Nine hundred species of slime mould are known worldwide, and there are likely to be hundreds more unknown species, typically occurring in cool, moist, shady places. Slime moulds play important roles in ecosystems, in decomposing organic matter and recycling nutrients.

Slime moulds can live freely as single cells, but under suitable conditions, they can aggregate to form plasmodia, which may grow and spread over several feet. At some point, plasmodia differentiate into spore-bearing structures; the spore-producing fruiting bodies are often very fragile, dispersing the spores when touched.

Versatility of the Slime Moulds

Classification of Slime Moulds

Kingdom: Protista.

Phylum: Mycetozoa (Myxomycota).

Classes under the phylum associated with slime moulds are-

Protosteliomycetes are primitive slime moulds that form small, simple fruiting bodies.
Dictyosteliomycetes are cellular slime moulds that form multicellular structures during their life cycle.
Acrasiomycetes are cellular slime moulds with eruptive pseudopodia that have distinct differences in structures throughout their life cycles.
Myxomycetes are known as true slime moulds that form complex, multinucleate plasmodia.
Plasmodiophoromycetes are the parasitic slime moulds.
Fonticula are the slime moulds that produce volcano-shaped fruiting bodies.

Each of the classes consists of various genera and species, each with unique characteristics.

Characteristics of Slime Moulds

Found in cold, dark and moist conditions- creeping and decaying on debris, twigs, or leaves, in soil, on tree canopies, on the forest floor, crevices of logs, etc.
In the vegetative phase, a cell wall surrounding the protoplast is absent.
They lack chlorophyll and are heterotrophic (specifically saprophytic); thus, they mostly feed on decomposing dead organic matter and microorganisms (like bacteria, fungi, and yeasts).
A few slime moulds are parasitic and are found in the roots of the plants of the Brassicaceae family.
The cellulosic cell wall of the spores is thick-layered and is resistant to adverse conditions, and can survive for a long period.
Formation of plasmodia is similar to protozoa, and the presence of fruiting bodies (bearing spores) is similar to fungi.

Types of Slime Moulds

Slime moulds are mainly of two types-

Acellular or Plasmodial Slime Moulds

Plasmodium is a multinucleate mass of protoplasm, which is the feeding stage.
Creeps as a slimy mass over leaf litter, moist and decaying logs, as it feeds on dead and decaying organic matter and microorganisms.
Reproduce asexually when there is less moisture or scarcity of food.

Examples: Physarum, Physarella, Cribaria, Lycogala, Fuligo, Tubifera, Dictydium, etc.

Life cycle

When food supplies begin to wane, drying plasmodia migrate to the surface to form stalked fruiting bodies.
The fruiting bodies bear sporangia.
Through meiosis, haploid spores are produced within the sporangia, which germinate to form haploid gametes.
The spores can survive in a dried form, and if conditions become too wet, the single-celled units grow flagella to switch to a free-swimming form, which is considered a gamete.
These haploid gametes fuse and produce a diploid zygote.
A diploid zygote undergoes multiple mitotic divisions without cytokinesis, forming a multinucleated plasmodium.

Cellular Slime Moulds

The single-celled, solitary amoeboid structure is the feeding stage.
Feed on microorganisms and dead food matter, while creeping on decaying litter and logs, or freely swimming in freshwater.
Each cell consists of a haploid nucleus that divides mitotically
During scarcity of food, they form aggregates but retain their individuality and reproduce asexually by spore formation.
During moisture depletion, they send out a cAMP-mediated chemical signal, which binds to the receptors present on the surface of nearby cells.

Examples: Dictyostelium, Acytostelium, Polysphondylium.

Lifecycle

Numerous cells aggregate, forming a multicellular slug, which creeps for a short distance.
Each cell retains its individuality in its plasma membrane.
Fruiting bodies within the slug contain spores.
Under favourable conditions, spores are released to germinate into a single haploid amoeboid cell.
Mostly lack the flagellated gamete stage, and thus sexual reproduction.

Economic Importance of Slime Moulds

Decompose the organic matter, which increases the nutrient levels in the soil.
Some slime moulds (e.g., Physarum) have been studied for their pigmentation, though not widely used commercially.
Plasmodium obtained from the organism helps to study the structure and physiology of protoplasm.