In the last article, we have discussed the Kingdom Monera. In this article, we shall study the Kingdom Protista.

All single-celled eukaryotes are placed under Kingdom Protista (Greek – protistos – First of all), but the boundaries of this kingdom are not well defined. We include Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and Protozoans under Protista. Members of Protista are primarily aquatic. This kingdom forms a link with the others dealing with plants, animals, and fungi.

Characteristics of Kingdom Protista:

• They are with or without a cell wall. If the cell wall is present it is composed of cellulose.
• Being eukaryotes, the protistan cell body contains a well-defined nucleus with the nuclear envelope, nucleoplasm, nucleolus and helical DNA, chromosomes and other membrane-bound organelles such as mitochondria, chloroplast, ER, Golgi Complex etc.
• Some have flagella or cilia.
• They show the photosynthetic, holozoic, saprophytic or parasitic mode of nutrition.
• Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation.

Classification of Protists:

Plant Like Protist:

Chrysophytes:

• This group includes diatoms and golden algae (desmids).
• They are found in freshwater as well as in marine environments.
• They are microscopic and float passively in water currents (plankton).  Most of them are photosynthetic. Diatoms are the chief ‘producers’ in the oceans.
• In diatoms, the cell walls form two thin overlapping shells, which fit together as in a soapbox. The walls are embedded with silica and thus the walls are indestructible. Thus, diatoms have left behind a large amount of cell wall deposits in their habitat; this accumulation over billions of years is referred to as ‘diatomaceous earth’.  Being gritty this soil is used in polishing, filtration of oils and syrups.

Dinoflagellates:

• These organisms are mostly marine and photosynthetic.
• They appear yellow, green, brown, blue or red depending on the main pigments present in their cells.
• The cell wall has stiff cellulose plates on the outer surface.
• Most of them have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates.
• Very often, red dinoflagellates (Example: Gonyaulax) undergo such rapid multiplication that they make the sea appear red (red tides). Toxins released by such large numbers may even kill other marine animals such as fishes.

Fungi Like Protist:

• Slime moulds are saprophytic protists.
• The body moves along decaying twigs and leaves engulfing organic material.
• Under suitable conditions, they form an aggregation called Plasmodium which may grow and spread over several feet.
• During unfavourable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips. The spores possess true walls.
• They are extremely resistant and survive for many years, even under adverse conditions.
• The spores are dispersed by air currents.

Euglenoids (Plant or Animal):

• Majority of them are freshwater organisms found in stagnant water.
• Instead of a cell wall, they have a protein-rich layer called pellicle which makes their body flexible.
• They have two flagella, a short and a long one.
• Though they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms.
• The pigments of euglenoids are identical to those present in higher plants.
• Example: Euglena

Animal-Like Protist Protozoans:

All protozoans are heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals. There are four major groups of protozoans.

Amoeboid protozoans:

 These organisms live in freshwater, seawater or moist soil. They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba. Marine forms have silica shells on their surface. Some of them such as Entamoeba are parasites.

Flagellated protozoans:

The members of this group are either free-living or parasitic. They have flagella. The parasitic forms cause diseases such as sleeping sickness. Example: Trypanosoma.

Ciliated protozoans:

These are aquatic, actively moving organisms because of the presence of thousands of cilia. They have a cavity (gullet) that opens to the outside of the cell surface. The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet. Example: Paramoecium

Sporozoans:

 This includes diverse organisms that have an infectious spore-like stage in their life cycle. The most notorious is Plasmodium (malarial parasite) which causes malaria, a disease which has a staggering effect on the human population.

Science > Biology > Classification of Microbes, Protists, and Fungi > Kingdom Protista

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Microbes are omnipresent i.e. they exist everywhere: in soil, air, water and also in the human body and the bodies of plants and other animals! They also exist in places where no other life-form exists. Bacteria, fungi, protozoa, viroids are examples of microorganisms. The general presumption is that the microbes are harmful to us because some of them cause diseases to both plants and animals including humans. However, there are many microorganisms, who are useful to us in many ways. In this article, we shall study the use of microbes in household activities.

Edible mushrooms are cultivated and consumed as food worldwide. Algae are used as thickening agents and in ice creams. They are nutritionally important in vegetarian diets. Yeasts are used as a common flavoring agent of food and for food production. Bacteria are used to produce dairy products like cheese and yogurt.

Fermentation:

The process of anaerobic respiration in which the complex molecules incompletely brakes into simple ones by the microbial action is called fermentation. There are many types of fermentation that are distinguished by the end products formed from pyruvate or its derivatives. The two fermentations most commonly used by humans to produce commercial foods are ethanol fermentation (used in beer and bread) and lactic acid fermentation (used to flavor and preserve dairy and vegetables). While there are a number of products from fermentation, the most common are ethanol, lactic acid, carbon dioxide, and hydrogen gas (H₂). These products are used commercially in foods, vitamins, pharmaceuticals, or as industrial chemicals. 

Fermentation of dough is done for making dosa, idli. CO₂ produced in the process gets trapped in gluten and makes idli puffy. Microorganisms involved in the process are Bacillus Candida, Saccharomyces cerevisiae. For making bread the microbe used in Baker‘s yeast (Saccharomyces cerevisiae). Toddy is made from the sap of palm using the process of fermentation.

The products obtained in the dairy industry by microbe activities are cheese, yogurt, buttermilk, paneer. Microbes used are species of Streptomyces, Penicillium, Lactobacillus

Bread Making:

The flour, salt, and yeast are mixed. Then the oil and water are mixed with it. The dough is kneaded until the dough becomes smooth. Then the dough is allowed to ferment overnight. The dough is moulded into balls and put on a baking tray. The oven is preheated to 220˚C and the balls are baked for 25-30 minutes until they turn golden brown, Then they are allowed to cool on wire mesh.

Cheese Making:

The process is used in cheese making (eg. Swiss cheese by Propionibacterium sharmanii, Roquefort cheese by fungi). Different varieties of cheese are known by their characteristic texture, flavour, and taste, the specificity coming from the microbes used. The bacterium Propionibacterium sharmanii is used in ‘Swiss cheese’ to give it its characteristic holes by producing a large amount of carbon dioxide. ‘Roquefort cheese’ is ripened by growing certain fungi on them to give them their specific flavour.

Curd Making:

The increased acidity causes the milk proteins (casein) to tangle into solid masses or curds. Milk that has been left to sour (raw milk alone or pasteurized milk with added lactic acid bacteria) will also naturally produce curds.

A small amount of curd added to the fresh milk as inoculum or starter contains millions of LAB, which at suitable temperatures multiply, thus converting milk to curd. During growth, the LAB produces acids that coagulate and partially digest the milk proteins. The microorganism involved is Lactobacillus Lactic acid Bacteria (LAB). The process increases nutritional value. It checks disease-causing microbes in our stomach. It increases Vitamin B₁₂

Yogurt Making:

Milk contains the sugar lactose. and some bacteria will ferment lactose to produce lactic acid. Yogurt is produced by batch culture, where pasteurized milk has the bacteria Lactobacillus bulgaricus and Streptococcus thermophilus added to it. The mixture is maintained at a temperature of around 40°C. The bacteria will produce lactic acid by respiration, and this lowers the pH. When it reaches a target the product is harvested.

Microbes as the Source of Food:

With the continuous rise in the global population, the demand for food has also increased. The conventional methods of agriculture and animal husbandry are not sufficient to fulfill the nutritional requirements of the growing population. This resulted in malnutrition due to deficiency of protein in food.

Some microbes or their fruiting bodies are directly used as a source of food. The microorganisms utilize the carbon and nitrogen present in these materials and convert them into high-quality proteins which can be used as a supplement in both human and animal feed.  They are rich in protein. Single-cell protein (SCP) is the protein extracted, or the cells themselves, from cultivated microbial biomass. The microbes involved are bacteria, fungi, and algae. SCP has high protein content. They can be grown on waste products such as whey, which makes it more of an economically- feasible protein source.

SCP can be obtained from bacteria (Bacillus subtilis), fungi (Candida and Saccharomyces cerevisiae), algae (Chlorella).  Bacterial sources of SCP are Pseudomonas fluorescens, Lactobacillus, Bacillus megaterium. The fungal sources of SCP are Aspergillus fumigatus, Aspergillus niger, Rhizopus cyclopean. Yeasts used as source of SCP are Saccharomyces cerevisiae, Candida tropicalis, Candida utilis. The algal sources of SCP are Spirulina sps, Chlorella pyrenoidosa, Chondrus crispus.

Mushrooms:

Fungus like mushrooms and truffles (bacidomycetes) are directly used as food. They are sugar-free, fat-free. They have low-calorie value. They are rich in proteins, vitamins, minerals, and amino acids. Some common edible mushrooms are white button mushroom (Agaricus bisporus), Paddy straw mushroom (Volvariella Volvacea), Oyster mushroom (Pleurotus florida). Toadstools are poisonous mushrooms.

Algae, which include seaweeds and other aquatic plants, contain chlorophyll and require carbon dioxide and continuous sunlight. Those without chlorophyll, particularly yeasts and fungi, show more promise for commercial production.

Next Topic: Industrial Uses of Microbes

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