In this article we will discuss about:- 1. Characters of Slime Moulds 2. Types of Slime Moulds 3. Economic Importance.
Characters of Slime Moulds:
Slime moulds were formerly included amongst mycetozoa or fungus-animals. However, they are included in the division of gymnomycota by mycologists. Because of their protistan nature, these microbes are also called protistan fungi.
The slime moulds have the following characters:
(i) They lack chlorophyll.
(ii) They are surrounded by the plasma membrane only; vegetative phase lacks cell-wall. However, the spores have the cellulose cell walls.
(iii) At one stage of the life cycle they have amoeboid structure (i.e., vegetative body plasmodial).
(iv) The slime moulds live usually amongst decaying vegetation. They commonly occur on lawns and moist fields.
(v) They exhibit wide range of colouration.
(vi) They are saprophytes and have phagotrophic mode of nutrition. Parasitic forms are not known (Bold et al., 1987).
(vii) Both asexual and sexual modes of reproduction are present. They produce spores within sporangia. A spore possesses a cell wall of cellulose.
(viii) The slime moulds resemble both protozoa and the true fungi. They are like protozoa in their amoeboid plasmodial stage and similar to true fungi in abundant spore formation.
Types of Slime Moulds:
Slime moulds are of two types – acellular and cellular.
1. Acellular Slime Moulds (= Plasmodial Slime Moulds):
Habitat:
The acellular slime 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.
Structure and Life Cycle (Fig. 10.1):
Somatic diploid phase is wall-less multinucleate protoplasm called Plasmodium. It may be coloured variously. Plasmodium creeps over the surface of substratum with the help of pseudopodia.
The chief mode of nutrition of Plasmodium is saprotrophic, absorbing the organic food from the decaying organic matter (substratum of Plasmodium). Plasmodium also feeds on bacteria, protozoa, spores of fungi and other microorganisms through ingestion and engulfing (i.e., phagotrophic or holozoic nutrition).
Under unfavourable conditions such as drought or too much cold, the Plasmodium divides to form many multinucleate cysts. Sometimes even whole plasmodium forms a hard dormant structure called sclerotium. On the return of favourable conditions, the cyst or the sclerotium liberates the multinucleate diploid plasmodium. Thus, these structures serve for perennation.
(i) Formation of Sporangia:
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.
(ii) Formation of Spores:
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 cell wall to become spore. The sporangium also develops a system of threads called capillitium.
(iii) Liberation of Spores from the Sporangia:
When fully mature, the wall of the sporangium bursts to release the spores. The spores are dispersed by air. A large number of spores are often present in the air. Spores of some species such as Fuligo septica cause allergic reactions.
(iv) 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 sivarm cell or a non-flagellate myxamoeba. The myxamoebae feed on bacteria and yeasts and multiply in number. The swarm cell swim about actively and finally fuse in pairs at the posterior non-flagellate ends to form zygotes.
(v) Formation of Plasmodium:
The zygote creeps over the substratum and feeds on bacteria, yeasts and the other organic matter. It grows in size. The diploid nucleus of 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:
Physarum, Physarella, Fuligo, Dictydium, Lycogala, Tubifera.
2. Cellular Slime Moulds:
Habitat:
The cellular slime moulds occur in all humus-containing upper layers of damp soil.
Life cycle of Cellular Slime Moulds:
Life cycle (Fig. 10.2) of a typical cellular slime mould includes the following:
(i) 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 unfavourable conditions, a myxamoeba secretes a rigid cellulose wall to form the microcysts.
Microcyst formation is a means of perennation. The microcysts can be dispersed. On the return of favourable conditions, the microcyst wall ruptures to release a myxamoeba. The latter resumes its function of feeding, growth and multiplication forming amoeboid cells.
(ii) Formation of Pseudoplasmodium:
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 3’, 5’ adenosine monophosphate (cyclic AMP) from the amoeboid cells. This aggregated mass of cells is called pseudoplasmodium. 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.
(iii) Formation of Sporangium:
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.
(iv) Formation of Spores:
The cells present inside the sporangium become rounded and become surrounded by the cellulose wall to form the spores. Each spore is a void, haploid, uninucleate mass of protoplast covered by a cellulose cell wall. The spore germinates to produce a single naked amoeba-like cell called myxamoeba.
Sexual Reproduction:
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 macrocyst. Karyogamy occurs inside the macrocyst which is followed by meiotic and several mitotic divisions. Ultimately the macrocyst wall ruptures to release a number of haploid myxamoebae.
Examples of Cellular Slime Moulds are:
Dictyostelium, Polysphondylium.
Economic Importance 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.