What is Slime Mould?
Slime moulds or simply slime molds are grouped under fungi earlier, but later, they are kept in the Protista kingdom with other small multicellular and unicellular eukaryotic organisms.
They are the saprophytic type and feed on dead and decaying organic matter.
The name 'slime' is taken from the gelatinous appearance of macroscopic slime moulds. They form aggregates under any unfavourable conditions, and this is common in acellular or plasmodial slime moulds. Their size varies from some centimetres to various square metres.
When there is abundant food, they can live as a single-celled organism, primarily cellular slime molds.
Slime Moulds Classification
Slime moulds are classified under the Protista kingdom. They resemble fungi and protozoa as well. In modern taxonomy, the true slime moulds fall under Mycetozoa. They are further classified into various classes. The main classes of slime moulds are given below.
Dictyostelium - These are cellular slime moulds. They don't form huge coenocytes
Myxomycetes - True or acellular slime moulds. They are characterised by plasmodial stage, syncytial (multinucleated).
Acrasia- Cellular slime moulds are similar to dictyostelids but have eruptive pseudopodia.
Protostele A - Minute, simple, and amoeboid slime moulds.
Labyrinthulae - These are the net slime moulds. They form a network of tubes in which an amoeba without pseudopodia can swim freely.
Plasmodiophomycetes - These are the parasitic slime molds. These are found as an internal parasite in cabbage roots and cause various diseases in some plants like clubroot disease of cabbage.
Fonticula- These are the form volcano-shaped fruiting bodies
Slime moulds are mainly classified as two types. They are Acellular, Cellular slime moulds, where acellular slime moulds are also called Plasmodial slime molds.
Acellular Slime Moulds (Also called Plasmodial)
The feeding stage is a multinucleate protoplasm's mass, which is, plasmodium
They reproduce asexually when the food is scarce, and moisture is less
They can be found creeping as a slimy mass over leaf litter, decaying, and moist logs. It feeds on the dead and decaying microorganisms and organic matter
They can grow up to 1 foot in diameter
Acellular slime molds examples are, Cribaria, Physarum, Lycogala, Tubifera, Fuligo
The Life Cycle of Acellular Slime Moulds
Drying plasmodium forms the stalked fruiting bodies, those bear sporangia
Haploid spores are formed in sporangia by meiosis. They also have a thick cell wall and are resistant to adverse conditions extremely
These spores germinate to form haploid cells under favourable conditions
These haploid cells act as a gamete. Different cell kinds are formed depending on the availability of moisture
Biflagellated swarm cells are produced when the conditions are wet. Otherwise, amoeboid myxamoebae are formed. Such cells act as gametes
Such haploid gametes fuse to form a diploid zygote
Also, the diploid zygote undergoes multiple mitotic division producing multinucleated plasmodium because it doesn't undergo cytoplasmic division (cytokinesis)
Cellular Slime Moulds
The feeding stage in a cellular slime moulds is a single-celled amoeboid, which lives as a solitary organism
Individual cells feed on the microorganisms and other food matter while creeping on freely swimming or decaying log in freshwater
They have a closer resemblance with amoebas
Just similar to an amoeba, each cell has a haploid nucleus, and it can divide mitotically
When there is a portion of less food, they form aggregate, but retain their individuality because of the thin plasma membrane presence and reproduces asexually by spore formation
The chemical then diffuses out and binds to the receptors that present on the surface of the nearby cells by resulting in the movement of cells towards the cAMP (Cyclic Adenosine Monophosphate)
When the moisture or food is depleted, they send out a cAMP-mediated chemical signals
Cellular slime molds examples are Acytostelium, Dictyostelium, Polysphondylium
Given below is the discussion on two major groups of cellular slime moulds.
Dictyostelids
The cellular slime molds in this group of cellular slime exist as individual amoeboid cells that aggregate on a periodic basis. One particular cellular slime mold of this group, scientifically known as Dictyostelium discoideum, has been an vital part of scientific research, acting as a study organism for understanding cell differentiation, because it has both single-celled and multicellular life stages, during which the cells show some degree of differentiation in the multicellular form.
The organisms of this group lead a complex life cycle during which they undergo unicellular, multicellular, spore producing, and amoeboid stages. In this group, it is observed that hundreds of thousands of individual amoebae closely group together into a slimy mass and each of these cells can retain its identity (this is unlike the plasmodial slime molds). The aggregating cells are attracted and group with each other by the activity of cyclic AMP (cAMP) that they release when conditions become stressful, such as a scarcity of food. Individual amoebae respond to this chemical signal and move to areas of higher cAMP concentration (a process known as chemotaxis), eventually aggregating into a single slug. The slug that is formed is highly receptive to moisture and light gradients, and can easily locate itself to a good spot for spore production. Some cells in the sluggish mass contribute to a 2–3-millimeter stalk, where they may dry up and eventually perish in the process. Cells that are at the top of the stalk form an asexual fruiting body that contains many haploid spores. The spores are dispersed and can now germinate if they successfully land in an environment with good moisture conditions.
Protostelids
Protostelid is another group among the cellular slimes, although they seem to have received little popularity than either the cousin Dictyostelids or plasmodial slime molds, as each of the latter groups consists of a model organism that has been used to study a specific system by biologists.
Protostelids, like other cellular molds, make simple fruiting bodies (just like in case of the Dictyostelids discussed previously) with a stalk and spores at the apex. One interesting member of this group, the slime mold Ceratiomyxa looks very similar to a plasmodial slime mold, and only under close inspection the minute spores of the body are observable, which otherwise remain hidden, as the stalked fruiting bodies of the mold stealthily cover the external surface of the tentacle-like structures. Some argue that Ceratiomyxa may not actually be a member of the protostelid group, but the small, stalked fruiting bodies found externally on their surface resemble what has been found in members of a true protostelid.
The Lifecycle of Cellular Slime Moulds
Thousands of cells aggregate, producing a multicellular slug, that creeps for a short distance and settles.
Each cell in the slug retains its individuality and its plasma membrane.
After settling down, the fruiting bodies are formed in the slug that contains the spores.
Spores are then released and germinate under the favourable conditions into a single haploid amoeboid cell, as the vegetative stage.
Spores formation is an asexual method of reproduction.
They lack sexual reproduction mostly and also the flagellated gamete stage.
General Slime Molds Characteristics
A few of the general slime moulds characteristics are listed below.
The slime moulds are creeping on debris, decaying on twigs or leaves, in soil, on tree canopies and moist found on the tree, on the forest floor, and in cold and dark conditions.
In the vegetative phase, the protoplast is not surrounded by a cell wall .
They lack chlorophyll and saprophytic too. They feed on microorganisms like fungi, bacteria, and yeasts and decompose the dead organic matter.
A few of the slime moulds are parasitic and are found in the roots of cabbage and other plants that belong to Brassicaceae family.
The plasmodial stage resembles fruiting, and protozoa bodies form spores resembling fungi.
Spores have a cell wall, which is made up of cellulose and are resistant to adverse conditions. They can also survive for many years.
FAQs on Slime Moulds
1. Are Slime Molds Organisms in their Own Right?
There are many slime mold species that are cellular, maintaining their cell integrity throughout development, and others are multinuclear - developing into large single cells with many nuclei.
The cellular slime mould, Dictyostelium discoideum, became an essential model organism studied in great detail by the developmental biologists because it was the fascinating case where an organism starts out as a spore developing into a single amoeboid cell which grows and divides; and then when they are running out of food in the soil, they aggregate into a multicellular slug which differentiates into various cell types and sporulates spores eventually that will spread to other locations. And again, the developmental cycle starts all over. Every slime mould species has its own developmental process in its own right.
2. In What Form is the Food Stored in Slime Molds?
Slime molds are basically clusters of amoeba-like cells and therefore consume small microbes by the process called phagocytosis (pseudopodia-like appendages engulf food particles/microbes and are digested in vacuole-like structures in the cell). The digested food may then be stored in the form of scratch molecules for a definite amount of time and utilized in an event of food shortage.
3. What is the difference between the two groups of slime molds discussed here (the cellular and acellular types)?
The two types of slime molds found in nature differ in their cellular construction and life cycle stages. Cellular slime molds spend the majority of their life cycle (sometimes referred as the feeding stage) as a unicellular member of the species. Even if they aggregate (for reproductive purposes), they do not lose their unique identity and as a cell remain distinct to other aggregated members of the cell mass (known as “pseudoplasmodium”). Their cytoplasmic structures never fuse. Acellular slime molds, on the other hand, are found to exist as a single cytoplasmic structure, the plasmodium, with multiple nuclei during their entire feeding stage.
4. Explain in brief the crucial steps in the life cycle of slime molds.
Following are the main steps in the life cycle of slime molds:
The Unicellular growth stage (during which the spore develops into small amoebae and vegetative growth takes place)
The Multicellular growth stage (during which the amoebae aggregate and form slime mass and later the fruiting body to produce and release spores).
5. How do slime molds reproduce?
The reproductive stage is a multistep process in the life cycle of slime molds. When met with favorable conditions, the mature amoebae aggregate (via chemotaxis) to form a mass of cells (plasmodium in case of acellular molds and “pseudoplasmodium” in case of cellular molds). The slime mass or slug then bears a fruiting body attached to a stalk-like structure, which produces spores and desiccates under favorable conditions.