Slime moulds or consumer-decomposer protists

They were included in class myxomycetes of fungi in two-kingdom classification. They were called mycetozoa by DeBary as they are closely related to animals. Mycologists include them in gymnomycota. Because o their nature they are called protistan fungi.

General characteristics of the slime moulds are

1. They are usually free-living, creeping over debris like fallen leaves and rotting logs of wood.
2. They have naked protoplast, not covered by any cell wall in vegetative stage.
3. They lack chlorophyll and have saprobic or phagotrophic mode of nutrition.
4. The body moves along decaying twigs and leaves engulfing organic material. Under favourable conditions, they form an aggregation called plasmodium which may groW and spread over several feet. During untavourable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips.
5. During life cycle they are amoeboid and non-cellulosic, but spores have cellulosic wall so that their vegetative phase resembles with animals while reproductive phase resembles with plants.
6. Amoeboid plasmodial stage resembles protozoa and spore forming nature is like fungi.
7. Spores are extremely resistant and survive for many years, even under adverse conditions. The spores are dispersed by air currents.
8. Reproduction is both asexual and sexual. This group is represented by two separate types of organisms i.e. acellular and cellular.

A. Acellular or Plasmodial Slime Moulds

General Characters

1. Slimy masses found on decaying leaves and lumber.
2. Somatic body is free living, multinucleate, naked, diploid mass called Plasmodium. Movement occurs by means of pseudopodia.
3. During unfavourable conditions, entire plasmodium forms many fructifications/fruting bodies (polycentric). The fruiting body is called sporocarp which contains a stalk having a sporangium at its tip. The wall of sporangium is called peridium.
4. Sporangium has an intricate network of cytoplasmic threads called capillitium.
5. Diploid protoplast forms haploid spores by meiosis.
6. Spore wall is double, outer wall is spiny and sculptured.
7. On germination, spores produce biflagellate swarm cells or non-motile myxamoebae which act as gametes.
8. Sexual reproduction is isogamous.
9. Diploid zygote directly forms the plasmodium which becomes multinucleate by repeated mitotic divisions of the diploid nucleus..
10. Chief mode of nutrition is saprotrophic.
11. Vegetative reproduction is by fission. e.g., Physarum, Physarella, Fligo.

B. Cellular slime moulds or communal slime moulds

General characters

1. Wall less, uninucleate myxamoebae present. Complete absence of flagellated cells during life cycle.
2. Sporangia are naked.
3. Spores have cellulosic wall.
4. Sexual reproduction is anisogamouus.

Common cellular slime mould, Dictyostelum, is a colonial form in which hundreds of uninucleate, haploid amoeboid cells are aggregated without any fusion to form a colony. The colony gives the appearance of single multinucleate mass of protoplasm and thus, called pseudoplasmodium.

Under exhausted food supply and stimulation by cAMP and chemical acrasin, many cells come close together by chemotactic movement during the formation of pseudoplasmodium. Pseudoplasmodium exhibits primitive form of multicellularity and division of labour. So these are also called as communal slime moulds. On these basis cellular slime moulds are regarded as advanced protists and primitive fungi. During unfavourable conditions, the myxamoebae may form a cyst called microcyst for perennation and dispersal.

Under dry conditions, the pseudoplasmodium produces stalked sporocarp, which may be branched or unbranched, each branch bearing single sporangium terminally (monocentric). Sporangium is wall less. Within the sporangium, amoeboid celis become rounded to secrete a spore wall around. On the approach of favourable conditions, spores are liberated. Each spore germinates by rupturing cellulosic wall to form myxamoeba and the myxamoebae may live independently, multiply by repeated mitotic divisions or get aggregated to form pseudoplasmodium.

Sexual reproduction is anisogamous type. During sexual reproduction, a number of myxamoebae form a clump. One of the myxamoeba becomes larger and engulfs the surrounding smaller myxamoebae. The plasmogamy occurs and the fused protoplast secretes a thick wall around to form macrocyst. In the macrocyst, karyogamy ocCurs and it thus, becomes zygote. It is followed by meiosis and several mitotic divisions to form a large number of haploid myxamoebae, which are released by rupture of macrocyst wall. e.g., Dictyostelium, Polysphondylium.

By taking example of slime mould, now we can justify that protista forms a connecting link with plants, animals and fungi.

• Fungi like feature: Formation of fruiting bodies.
• Plant like feature: Cell wall around spores.
• Animal like feature: Plasmodium is without cell wall.