In this article we will discuss about the Significance and Types of Slime Moulds.
Significance of Slime Moulds:
(i) The slime moulds cause the decay and decomposition of the organic matter in the soil.
(ii) They creep over the ornamental plants and make them look ugly.
(iii) Their attractive colours are of artistic value.
(iv) The plasmodia of slime moulds are an excellent material for the study of structure and physiology of protoplasm.
Types of Slime Moulds:
1. Acellular Slime Moulds (= Plasmodial Slime Moulds):
These moulds are commonly found on dead and decaying leaves, twigs, logs of wood and the other decaying vegetable matter. They prefer to grow in damp places rich in decaying vegetable matter in the forests a little after and during the rainy seasons.
II. Somatic Phase:
It is diploid multinucleate plasmodium.
A free living thalloid body of the acellular slime moulds is called plasmodium. The plasmodium is wall-less mass of multinucleate protoplasm covered by slime. All the nuclei in the plasmodium divide simultaneously. The plasmodia (pi. of plasmodium) are often coloured. They may be even colourless.
However, chlorophyll is always absent. The plasmodium often possesses a number of branched “veins”. The protoplasm present in the veins shows reversible streaming movement. The “veins” disappear and reappear as the plasmodium moves about. The plasmodium creeps over the surface of the substratum with the help of pseudopodia.
Internally the plasmodium has contractile librils and streaming cytoplasm along with eukaryotic organelles except plastids. The chief mode of nutrition of plasmodium is saprotrophic, absorbing the organic food from the decaying organic matter. Plasmodium also feeds on bacteria, protozoa, spores of fungi and other microorganisms through ingestion and engulfing (i.e., phagotrophic or holozoic nutrition).
During injury, the plasmodium (multinucleate) may undergo division to form two or more plasmodia.
Perennation means surviving from year to year by vegetative means.
Under un-favourable conditions such as drought or too much cold, the plasmodium divides to form two types of perennating structures:
(i) Cyst. Plasmodium divides into small multinucleate fragments. Each fragment secretes a thick covering to form a cyst.
(ii) Sclerotium (Gk. Skeleros — hard). Sometimes the whole plasmodium secretes a thick covering around itself called sclerotium. Cyst and Sclerotium can remain dormant for a few months to many years. On return of favourable conditions, the Cyst or the sclerotium releases the multinucleate plasmodium. Thus these structures serve for perennation.
VI. Life Cycle:
When the plasmodium reaches a certain stage of maturity or the food supply is nearly exhausted, the contents of plasmodium concentrate at one or more places forming papilla like mounds that grow into sessile or stalked sporophores. Each sporophore bears one or more sporangia (= fruiting bodies). Each sporangium is surrounded by a hard and brittle wall-like layer, the peridium.
The numerous diploid nuclei in the sporangium undergo meiotic division. The multinucleate protoplasm of the sporangium undergoes cleavage to form uninucleate tiny segments. Each uninucleate tiny segment becomes rounded and secretes a ceil wall to become spore.
The sporangium also develops a system of threads called capillitium. When fully mature, the wall of the sporangium bursts to release the spores. The spores are dispersed by air. Spores of some species such as Fuligo septica, cause allergic reactions.
(iii) Germination and Sexual Reproduction:
After falling on a suitable substratum, each spore germinates only when water is available. On germination, a spore generally releases one biflagellate, spindle-shaped swarm cell or a non-flagellate myxamoeba.
The myxamoeba feeds on bacteria and yeasts and multiplies in number. Ultimately myxamoebae fuse in pairs to form zygote. The swarm cells swim about actively and finally fuse in pairs at the posterior non-flagellate ends to form zygote.
(iv) Formation of Plasmodium:
The zygote creeps over the substratum and feeds on the bacteria, yeasts and the other organic matter. It grows in size and diploid nucleus of the zygote undergoes repeated mitotic divisions. As a result, the zygote gradually changes into a multinucleate amoeboid structure the plasmodium. The plasmodium repeats the life cycle.
Examples of Acellular Slime Moulds:
Physarum, Physarella, Fuligo, Dictydium, Lyсоgala, Tubifera.
Differences between Plasmodium and Plasmodium
2. Cellular Slime Moulds (Acrasiomycetes):
The cellular slime moulds occur in all humus-containing upper layers of damp soil.
II. Somatic Phase:
It is represented by haploid and uninucleate cells called myxamoebae.
These are uninucleate, haploid and amoeba-like cells. Myxamoebae are without cell wall. They are covered by plasma membrane. They move by amoeboid movements. Myxamoebae feed on bacteria and other microorganisms through ingestion (phagotrophic or holotrophic nutrition). They grow and divide to form a large population of individuals.
Under un-favourable conditions, a myxamoeba secretes a rigid cellulose wall to form the micro cyst. Micro cyst formation is a means of perennation. The micro cysts can be dispersed. On the return of favourable conditions, the micro cyst wall ruptures to release a myxamoeba. The latter resumes its function of feeding, growth and multiplication forming amoeboid cells.
IV. Life Cycle:
(i) Pseudo plasmodium:
When the food supply is exhausted, the amoeboid cells get aggregated without any fusion. The stimulus for the aggregation process is due to release of cyclic adenosine monophosphate (cyclic AMP) from the amoeboid cells. This aggregated mass of cells is called pseudo plasmodium. It is a sort of community association. Because of this reason, cellular slime moulds are called the communal slime moulds.
Significance of Pseudo plasmodium:
The pseudo plasmodium exhibits a primitive form of multi cellularity, where cells maintain their identity but can live together. It also shows division of labour as some cells form fruiting body (sporangium) while others form spores. For this reason the cellular slime moulds are regarded as advanced protists or primitive fungi.
The aggregated cells of pseudo-plasmodium differentiate and migrate to form a stalked sporocarp. The sporocarp bears a sporangium at its terminal end. The sporangium of cellular slime moulds is naked. The stalk may remain upright or become slightly bent.
The cells present inside the sporangium become rounded and are surrounded by the cellulose wall to form the spores. Each spore is an ovoid, haploid, uninucleate mass of protoplast covered by a cellulose cell wall. The spore germinates to produce a single naked amoeba like cell called myxamoeba.
(iv) Sexual Reproduction in Cellular Slime Moulds:
Sexual reproduction in cellular slime moulds is controversial. In this process, the myxamoebae form clusters. The central myxamoeba of the cluster engulfs a surrounding myxamoeba to become larger structure which forms a thick wall to form the zygote.
This zygote is called macro cyst. Kayos-gamy occurs inside the macro cyst which is followed by meiotic and several mitotic divisions, ultimately the macro cyst wall ruptures to release a number of haploid myxamoebae.
Examples of Cellular Slime Moulds:
The cellular slime moulds have the characters of both plants and animals. The reproductive phase is plant-like as the spores have a cell wall composed of cellulose. However, vegetative phase is animal like having no cell wall and feeding like amoeba.