Slime Moulds

All the living organisms on earth are grouped together according to their similarities, dissimilarities and phylogenetic order. The scientific method of arranging organisms into different groups and subgroups is known as biological classification. Classification of organisms is important as it helps in identifying new organisms. The study of every organism is not possible so they are divided into separate groups and selective organisms of the group are studied which gives sufficient information about the group. A better understanding of relationships among different groups of organisms. 

Classification

Previously various classification system was used for grouping the organisms. The artificial system of classification also known as Aristotle classification is used for grouping organisms using one or two morphological characteristics, flowering and non-flowering, vertebrate and non-vertebrate. The natural system of classification also known as Bentham and Hooker classification is the comparative study of a number of characteristics of various organisms to review the similarities, dissimilarities, and natural relationships among them. The phylogenetic system of classification also known as Engler and Prantl classification and Hutchinson classification is a system in which organisms are categorized based on their evolution on earth from primitive to highly evolved stage.

Kingdom system of Classification

Based on the system of classification, all organisms are classified into the following kingdom system.

• Two kingdom classifications: In this kingdom system all the living organisms are categorized into kingdom Plantae and kingdom Animalia on the basis of the presence and absence of cell walls. It was proposed by Linnaeus (father of taxonomy). There was a certain limitation with this type of classification as it does not distinguish between prokaryotes and eukaryotes, unicellular and multicellular, and photosynthetic and non-photosynthetic organisms. 
• Three kingdom classifications: In this kingdom system organisms are classified into kingdom Plantae, kingdom Protista, and kingdom Animalia. This system was proposed by Haeckel. He introduced the new kingdom Protista and separated unicellular animals, algae and fungi on the basis of lack of tissue differentiation. 
• Five kingdom classifications: In this kingdom system organisms are categorized into five kingdoms in order to develop phylogenetic classification. R.H.Whittaker divided all the organisms into kingdom Monera, Kingdom Protista, Kingdom Fungi, Kingdom Plantae and Kingdom Animalia. Whittaker classified organisms based on cellular structure, complexity, mode of reproduction, mode of nutrition, phylogenetic relationship and ecological role performed by them. 

Kingdom Protista

Unicellular, colonial eukaryotic organisms are mostly placed under the kingdom Protista. They form plankton and are mostly found in aquatic ecosystems. They contain membrane-bound cell organelle, Nucleus is well defined that is surrounded by a nuclear membrane. They can move using both cilia and flagella made of tubulin. Respiration can be both aerobic and anaerobic, and the mode of reproduction can be both Sexual and Asexual. Mode of nutrition can be photosynthetic, saprophytic, parasitic or holozoic. A photosynthetic protist is a link between plants and animals. Genetic material consists of 2 or more DNA molecules. They are divided into Chrysophytes, Dinoflagellates, Euglenoids, Protozoans and Slime moulds.

Slime Moulds 

Slime moulds were previously classified under fungi, but later they were placed in the kingdom Protista with other small multicellular and unicellular eukaryotic organisms. The name ‘slime’ comes from the gelatinous or jelly-like appearance of macroscopic slime moulds. They are found in different sizes varying from a few centimetres to several square meters. They are saprophytic and feed on dead and decaying organic matter. When there is abundant food, they can live as single-celled organisms, primarily cellular slime moulds. When food is in short supply, these organisms merge together and start moving as a single body. Acellular or plasmodial slime moulds form aggregates under unfavourable conditions. 

Classification

In modern taxonomy, the true slime moulds fall under Mycetozoa. Slime moulds are categorized under the Protista kingdom. They also resemble fungi and protozoa. They are further classified into several different classes.

• Dictyostelium – These are cellular slime moulds mainly related to plasmodial slime moulds. They don’t form huge coenocytes and remain individual.
• Myxomycetes – These are multinucleated plasmodium through multiple nuclear cell divisions without further cell division. They are characterized by the plasmodial stage, and syncytial.
• Acrasia – Cellular slime moulds that have a similar lifestyle to dictyostelids, but their amoebae behave differently, having eruptive pseudopodia. 
• Protostele A – These are intermediate between myxomycetes and dictyostelids. Minute, simple, and amoeboid slime moulds. 
• Labyrinthula – Slime nets that are marine, they form a network of tubes in which an amoeba without pseudopodia can travel freely. 
• Plasmodiophomycetes – These are mainly parasitic slime moulds. It is parasitic protists that can cause cabbage club root disease and powdery scab tuber disease. These are internal parasites of plants. 
• Fonticula – These are cellular slime moulds that form a multicellular fruiting body in a ‘volcano’ shape.

Reproduction

The reproduction in slime moulds is of primitive type. The nuclei present in the plasmodia contain two sets of chromosomes. When this organism is exposed to the sunlight, the plasmodium forms a short stalk with a minute mushroom-like cap. the nuclei in the stalks undergo meiosis and chromosomes are reshuffled and the nucleus divide forming spores that are haploid (i.e. just one set of chromosome). These fruiting bodies are called sporangia. 

Forming these spore capsules is deadly for slime moulds causing the end of the organism but these spores are spread with the help of wind and germinate in damp places. The spores hatch into an amoeba and propagate by engulfing the bacteria around them, they further reproduce by dividing into two mitotically. The amoeba is actually gametes or sex cells. Mostly there are two types of gametes, one is big and immovable and the other is small with flagella for movement. But in this case, slime moulds have similar types of gametes with the same size and shape, this is called isogamy. 

When two amoebae with compatible mating cells meet, their gametes merge and their nuclei fuse. After fusion, the nuclei go from haploid to diploid. After this process, the nucleus stops dividing and the cell expands and forms a new plasmodium. 

Life Cycle 

Sporangia are present on drying plasmodium which forms the stalked fruiting bodies. Through meiosis haploid spores are formed in sporangia by meiosis. A thick cell wall is present and is resistant to adverse conditions extremely. The spores germinate to form haploid cells under favourable conditions. These haploid cells act as a gamete. Depending on the availability of moisture, different cell kinds are developed. Such haploid gametes fuse to form a diploid zygote. As the plasmodium doesn’t undergo cytoplasmic division (cytokinesis), it develops the diploid zygote that undergoes multiple mitotic divisions producing multinucleated plasmodium.

General Characteristics of Slime Moulds 

• The slime moulds are found 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. 
• The protoplast is not surrounded by a cell wall in the vegetative phase. They lack chlorophyll and are saprophytic, they feed on microorganisms such as bacteria, fungi and yeasts, and decompose dead organic matter.
• Few slime moulds are parasitic and they are found in the roots of cabbage and other plants of the Brassicaceae family. 
• The plasmodial stage is similar to protozoa and fruiting bodies form spores similar to fungi. The cell wall of spores is made up of cellulose and is resistant to adverse conditions. They can survive for many years.

Economic Importance of Slime Moulds 

• Slime moulds are responsible for the decay and decomposition of the organic matter in the soil. This increases the nutrient level in the soil. 
• The colour obtained from the slime moulds has artistic value. 
• The plasmodium obtained from slime moulds is an excellent material for the study of the structure and physiology of protoplasm. 

FAQs on Slime Mould

Question 1: State a few characteristics of Slime moulds 

Answer: 

• Deficiency of chlorophyll in the cell.
• The vegetative phase lacks a cell wall but a cellulose-rich cell wall is present in spores.
• At a particular stage, they have an amoeboid body structure. 
• They are usually located in decaying areas and are found on lawns and moist fields. 
• They reproduce in both modes i.e. Sexually and Asexually. Sexually using the gametes and Asexually with the help of spores. 
• Slime moulds represent both protozoa and true fungi. They are similar to protozoa in the amoeboid plasmodial stage and resemble fungi in abundant spore formation. 

Question 2: Give a few examples of cellular and acellular slime moulds

Answer:

• Cellular slime moulds: Dictyostelium, Polysphondylium, Acytostelium
• Acellular slime moulds: Physarum, Physarella, Fuligo, Dictydium, Lyсоgala, Tubifera.

Question 3: What are slime moulds?

Answer: 

They are simple organisms that contain an acellular mass of gelatinous protoplasm containing nuclei in it. After reaching a particular size it forms a large number of spore cases. They are named after their gelatinous and slimy appearance. 

Question 4: Discuss the habitat of slime moulds. 

Answer:

They are usually found in places where decaying leaves, twigs, logs of wood and other decaying material are present. They grow in damp places rich in decomposing vegetation matter. Also found in the forest areas and during rainy seasons. These moulds also occur in humus containing the upper layer of damp soil.  

Question 5: What is the significance of Slime moulds?

Answer: 

Mostly they feed on microorganisms that live on any type of dead plant and animals matter. They greatly contribute to the decomposition of dead remains of plants and animals. They act as biological decomposers and recyclers of nutrients in the food web. They consume bacteria, yeasts and fungi as food.