12 Microbial biodiversity
Prof. Daizy Batish
1. To understand the hypotheses of origin of microorganisms
2. Classification: the place of microorganisms in the living world
3. Different groups of microorganisms
Introduction:
Microbiology, as the word itself depicts, is the study of all living organisms that are too microscopic to be visible with the naked eyes. They include virus, bacteria, fungi, algae, protozoa (collectively known as ‘Microbes’). It also includes the study of their natural distribution, their relationship to each other and to other living organisms (human beings, animals and plants) and also the study of their abilities to make physical and chemical changes in our environment, and their reactions to physical and chemical agents. Most of the microorganisms are unicellular, in which all the life process are performed by a single cell. In the higher form of life, organisms are composed of many cells that are arranged in tissues and organs to perform specific functions. Microorganisms comprise the greatest numbers of individual organism on Earth. However, researchers have identified only a small fraction of them and vast majority of the remaining microbial species is still unknown.
What is biodiversity? Most of the biologists define the biodiversity as the “totality of genes, species and ecosystems of a region”. In general, biodiversity is referred as the variety and abundance of all life form that live on Earth. ‘Species’ is the basic unit of biodiversity. According to Erwin, biodiversity is related to the species number and species richness plus the total activities that each organism undergoes during its existence through events in the life of its members and also its non-phenotypic genomic expression. Thus, to study the microbial biodiversity, it is important to study their interspecific and intraspecific interaction in a given habitat.
2. Hypotheses of origin of microorganisms: Two hypotheses were attempted to explain the origin of microorganisms.
a. Theory of Spontaneous Generation: This theory is also known as “Aristotelian abiogenesis”. Aristotle (384-322 B.C.) was among the early thinkers who believed that the life spontaneously arise from the non-living things. He taught that the life might originate spontaneously from the soils, plants, and the other unlike animals. Some common examples of spontaneous generations were that earthworms were created when dirt and water made mud, maggots arise from rotten meat, and mice are produced from bread left in a dark corner. The theory of spontaneous generation was debated for hundreds of years, and with the passage of time, many experiments were performed to disapprove this theory.
Francesco Redi (1626-1697): In 1668, Redi challenges the spontaneous generation theory. He published his first set of experiment in which he placed a piece of meat in a jar covered with gauge. By covering the jar, he made it impossible for the flies to lay egg inside the jar. Flies were attracted by the odour of meat, and flies laid eggs on the gauze covering. Thereafter, maggots were maggots were develops from eggs. In this experiment, he explained that the maggots developed from the flies eggs not from rotten meats. Thus, disapprove the theory of spontaneous generation of life.
b. Biogenesis: The concept of biogenesis was given by Rudolf Virchow in 1858. According to this concept living cell can only be borne out of pre-existing living cells. But he did not come up a convincing experiment to prove his concept of biogenesis. Later on, in 1861, Louis Pasteur (French Scientist) showed that microorganisms exist in the air and can contaminate the sterile solutions, but he also emphasized that air itself does not produce microbes and offers proof of biogenesis by using gooseneck neck flask. Finally, John Tyndall (1820-1893) conducted an experiment to prove that air carries the germs. If there are no germs in the air then there will be no microbial growth in sterile broth for indefinite periods.
3. Classification: the place of microorganisms in the living world
Until the eighteenth century, all the living organisms are placed into one of the two kingdoms, plant and animal. But there are some microorganisms that are plantlike, others that are animal like, while, some shares the characters of both plants and animals. Since there are some microorganisms that fall into neither the plants nor animals. So, it was proposed to establish a new kingdom to include those organisms that are neither plants nor animals.
In 1866, a German zoologist, E. H. Haeckel suggested a new kingdom, Protista, to include those organisms that are neither animals nor plants. The kingdom Protista includes bacteria, fungi, algae and protozoa. But there are some limitations of Haeckel’s kingdom Protista. For example, what are the criteria to distinguish bacteria from fungi? Viruses were excluded from this classification. Later on, R. H. Whittaker (1969) proposed a comprehensive system of classification, the five kingdom system. This system of classification was based on three level of cellular organization which evolved to accommodate three principal modes of nutrition (photosynthesis, absorption and ingestion).
Figure 1. A schematic representation of Whittaker’s five-kingdom system of classification.
4. Different groups of microorganisms:
There are many groups that are included under microorganisms like bacteria, algae, fungi, protozoa and viruses. Although viruses are neither protists nor cellular organisms. Still, they are included under microorganisms because of their pathogenic nature and the techniques used for virus identification are microbiological in nature. The major groups of microoganisms are given below:
Bacteria: Bacteriology is the study of bacteria. A bacterium is a unicellular prokaryotic microorganism. It lacks nucleus and membrane bound organelles. Bacteria adapt to become well-suited to their environment. Therefore, they come in various shapes and forms (Figure 2). Some important characteristics features are discussed below:
1. Size: Size of bacteria ranges from 0.5 to 1.0 µm in diameter.
2. Shape and Arrangement: Bacterial cells are spherical (cocci; singular, coccus) as shown in figure 2; straight rods (bacilli; singular, bacillus) or rods that are helically curved (spirilla; singular, spirillum); and some bacterial cells are pleomorphic (that can exhibit a variety of shapes). Bacterial cells are usually arranged in specific manners that are characteristics to a particular species.
Figure 2. Characteristic arrangement of Cocci
In addition to common bacterial shapes, many other shapes also occurs: lobed spheres (e.g., Sulfolobus); disks arranged like stacks of coins (e.g., Caryophanon); pear shaped cells (e.g., Pasteuria) and many others.
3. Bacterial structures:
Source: http://cdn.biologydiscussion.com/wp-content/uploads/2016/07/clip_image004-106.jpg
Capsule: Some bacterial species have a special outer protective covering, a capsule, made up of polysaccharides. Capsule made up of a single kind of sugar are termed as homopolysaccharides (e.g., Straptococcusmutans), while capsule made up of several kind of sugars are termed as heteropolysaccharides (e.g.,Klebsiella pneumonia). A few capsules are polypeptide. For example, Capsule of Bacillus anthracis is entirely composed of a polymer of glutamic acid. Capsule serves a number of functions, (1) provides protection against temporary drying by binding water, (2) blocks bacteriophage attachment, (3) are antiphagocytic in nature, (4) promotes bacterial attachment to the surface, etc.
Cell wall: Each bacterial cell is enclosed by a rigid cell wall composed of peptidoglycan. It gives shapes to the cell. Its main function is to prevent the cell from expanding and eventually bursting because of water uptake. It also helps to anchor appendages like the pili and flagella. Cell wall composition varies widely amongst bacteria. For example, the walls of Gram-positive bacteria are generally thinner (10 to15 nm) as compared to Gram-positive bacteria (20 to 25 nm).
Cytoplasmic membrane: Cytoplasmic membrane is a selectively permeable membrane that is composed primarily of phospholipids and proteins. It completely surrounds the bacterial cytoplasm. It acts as a hydrophobic barrier to the passage of most water-soluble molecules. However, specific proteins present in the cytoplasmic membrane allow/facilitate the passage of small molecules through the membrane. In addition to transportation, plasma membrane also contains various enzymes that play very important role in respiratory metabolism and in synthesis of capsular and cell-wall component.
Cytoplasm: The cell material bounded by plasma membrane is cytoplasm and it can be divided in to (a) Cytoplasmic area that is rich in RNA-protein bodies called ribosomes; (b) the chromatin area; (c) and the fluid portion that dissolves substances. No double membrane organelles are present. No nuclear membrane is present. 70S ribosomes are present unlike eukaryotes where 80S ribosomes are present.
Nucleoid: Nucleoid is also termed as chromatin body or nuclear equivalent or bacterial chromosome. Nucleoid is a single circular double stranded DNA molecule devoid of highly conserved histone molecule. It is not enclosed by nuclear membrane. Generally, the number of Nucleoid per bacterial cell is one. But there are some exceptions, where, the number of nucleoid may go even to four or more.
Flagella: Bacterial flagella are hair like helical appendages, each with a rotary motor at its base. They protrude through the cell wall and helps in movement. Their location on cells may be polar or lateral. Different species of bacteria can have different number and arrangement of flagella. Arrangement of flagella can be (a) Monotrichus e.g., Pseudomonas aeruginosa; (b) Lophotrichus e.g., Pseudomonas flouorescens; (c) Amphitrichouse.g., Aquaspirillumserpens; (d) Peritrichouse.g., Salmonella typhii.
Pili or Fimbriae: Pili are hollow, non-helical appendages. These are thinner and shorter than flagella. They help in bacterial mating (F pilus) and also help in attachment of bacterial cell to the epithelial cell lining the respiratory, genitourinary or intestinal and thus, help in establishment of infection.
4. Reproduction: Bacteria reproduce by vegetative, asexual and sexual method. Vegetative method includes Budding (e.g., Rhodopseudomonasacidophila); Fragmentation (e.g., Nocardiasp.) and Binary fission (e.g., Bacillus subtilis). Asexual reproduction includes Endospore formation (e.g., Clostridium sp.)and Conidia (e.g.,Streptomyces sp.), whereas, sexual reproduction includes
Transformation (e.g., Diplococcus pneumonia), Transduction and Conjugation(e.g., Escherichia coli).
5.Ecological significance of bacteria:Bacteria are the major decomposer of dead animals and plants on Earth, thereby act as natural scavengers. Some bacteria also play an important role in nitrogen fixation (e.g., Rhizobium spp), nitrification (e.g., Nitrosomonas, Nitrisococcus) and denitrification (e.g., Pseudomonas aeruginosa, Thiobacillusdenitrificans).
Fungi: The fungi are a group of eukaryotic organisms that includes microorganisms like yeasts and molds. Whereas yeasts are usually unicellular, molds are filamentous and multicellular. These are eukaryotic, heterotrophs, saprophytes/parasites, spore-bearing protists that lack chlorophyll. The science or study of fungi is mycology.
1. Size: Yeasts are generally longer than most bacteria. Size of yeasts generally ranges from 5 to 30 µm in width and more in length.
2.Morphological characteristics: Thallus or body of a fungi may consist of a single cell (as in yeasts) or filamentous and multicellular (as in molds). The thallus of a mold consists of two parts: the mycelium and the spores. Mycelium is a complex of several filaments called hyphae. Hyphae generally occurs in three forms, (a) Non-septate or Coenocytic; (b) Septate with uninucleate cells (Figure 4); (c) Septate with multinucleate cells. Yeasts have no flagella or other organelles of locomotion.
3.Reproduction: Fungi reproduce naturally by various means. (a) Asexual reproduction is accomplished by (1) Budding (e.g., Saccharomyces cerevisiae); (2) Fission (e.g., Schizosacchromycespombe); (3) Fragmentation; (4) Asexual spores (e.g., Aplanospore in Mucor; Chlamydospores of Ustilagotriticii; Conodiophores of Aspergillusspp.; Conidia of Helminthosporium;Blastospores of Candida albicans; Sporangiospores of Rhizopusspp. Sexual reproduction is carried out by fusion of the compatible nuclei of two parent cells. There are various methods of sexual reproduction (1) Planogametic copulation (e.g., Synchytrium); (2) Gametangial contact (e.g., Phytophthora, Albugo); (3) Gametangial copulation (e.g., Mucor, Rhizopus); (4) Somatogamy (e.g., Agaricus); (5) Spermatization (e.g., Pycniospores).
4.Ecological significance of fungi: Fungi are heterotrophic organisms. They act as saprophytes and decompose dead organic matter in to simpler chemical substances that are returned to the soil, thereby increasing its fertility. These are also important in industrial fermentation: for example, making of wine, production of antibiotics like penicillin. There are some parasitic fungi that cause diseases in plants, animals and in humans also.
Protozoa: Protozoa are unicellular, eukaryotic microorganisms with no cell walls. The protozoa are often described as the pinnacle of unicellular complexity. Unlike bacteria, protozoa can have different intracellular organelles performing specific tasks. Some species of protozoa have structures that are analogous to mouth, GI tract and anus. Science dealing with the study of protozoa is called protozoology. Some characteristic features of protozoa are given below:
Source: http://cdn.biologydiscussion.com/wp-content/uploads/2014/09/clip_image00224.jpg
1. Size: Size and shape of protozoa shows considerable variation. For example, Leishmania donovani measures 1 to 4 µm in length while, Amoeba proteus (Figure 5) measures 600 µm.
2.Morphological characteristics: Like eukaryotic cells, protozoan cells also consist of cytoplasm, nucleus etc. The protozoa cells have at least one eukaryotic nucleus. However, some protozoa have multiple nuclei, e.g., Ciliates. The cytoplasm is separated from surrounding by a unit membrane called plasma lemma. In addition to plasma lemma, many protozoa may possess an extra covering of membranes that is modified for movement, support and protection is known as pellicle. Protozoa have a feeding structure. These food gathering structures are diverse, e.g., pseudopodia of amoebas, tentacular feeding tubes of suctorians, mouths of many ciliates called cytostome etc. like other eukaryotic cells, protozoa have membrane system. It also includes endoplasmic reticulum, ribosomes, golgi complex, mitochondria, kinetosomes, food vacuoles, contractile vacuoles etc. protozoa may have three type of specialized organelles that helps in movement: Pseudopodia (e.g., Amoeba); Flagella (e.g., Trypanosoma brucei) and Cilia (e.g., Paramecium bursaria).
3.Reproduction: Protozoa reproduces by (a) Asexual reproduction; (b) Sexual reproduction. Asexual reproduction includes (1) Binary fission (e.g., amoeba);(2) Multiple fission (e.g., Plasmodium) and (3) budding (e.g., Ephelota). Sexual reproduction generally includes both isogamy and anisogamy. For example, in Plasmodium vivax, anisogamy results in the formation of ookinetes which gives rise to a large number of sporozoites.
4.Ecological significance of protozoa: Protozoa plays an important role in the food chain of aquatic communities. For example, photosynthetic phytoplankton and zooplanktons. That are also involved in the final decomposition of organic matter, thus, plays important role in ecological balance. They also help in aerobic (e.g., Bodo, Paramecium) and anaerobic (e.g., Saprodinium, Epalxis) biological sewerage treatment.
Algae: Algae are oxygenic, phototropic, eukaryotic microorganisms. Most are aquatic and lack many of the cell and tissue types, such as stomata, xylem and phloem which are characteristic features of land plants. Algae have chlorophyll as theirprimary photosynthetic pigment and lack a sterile covering of cells around their reproductive cells. The science dealing with the study of algae is known as Phycology. Some characteristic features of algae are given below:
1. Size and morphology: Algae have a wide range of size and size. Size ranges from 1 µm to several feet. Many species occurs as single cells that may be spherical, rod-shaped, club-shaped or spindle-shaped. Others are multicellular including filamentous (e.g., Spirogyra), parenchymatous (e.g., Macrocystic) etc. In most species the cell wall is thin and rigid. The cell walls of many algae are surrounded by a flexible, gelatinous outer matrix secreted through cell wall. Like eukaryotic cell, algal cell contain nucleus and other membrane bound organelles.in algae, there are three kinds of photosynthetic pigments like chlorophyll, carotenoids (carotenes and xanthophylls) and biliproteins. There are five type of chlorophyll pigment that is present in algae. Presence of a particular chlorophyll pigment serve as a major characteristic of the algal division. For example, Chlorophyll d appears only in the division Rhodophyta. The motile algae have flagella occurring singly, in pairs or in cluster. Mainly two types of flagella are there: whiplash (smooth); tinsel (hairy) while some algae are non-motile.
Source:http://cdn.biologydiscussion.com/wp-content/uploads/2016/08/clip_image031_thumb2-2.jpg
2. Reproduction: Reproduction in algae can be vegetative, asexual (Figure 6) and sexual. Vegetative reproduction includes: Binary fission (e.g., Euglena), Fragmentation (e.g., Sargassum). Asexual reproduction occurs by the formation of different type of spores. Spores can be motile or non-motile. Different types of spores are Zoospores (e.g., Chlamydomonas, Ulothrix); Synzoospores (e.g., Vaucheria); Aplanospores (e.g., Microspora); Hypnospores (e.g.,Pediastram, Sphaerella); Autospores (e.g., Scenedesmus, Chlorella); Akinetes (e.g., Gloeotrichia); Exospores (e.g., Chamaesiphon); Endospores (e.g., Dermocarpa) and Tetraspores (e.g., Polysiphonia,). In sexual reproduction, there is a fusion of gamete to form zygotes.Depending upon the physiological behaviour, structure, sexual reproduction is of following types: Autogamy (e.g., Diatoms); Hologamy (e.g., Chlamydomonas);Isogamy (e.g., Ulothrix); Anisogamy (e.g., Zygnema, Spirogyra);Oogamy (e.g., Oedogonium, Laminaria, Sargassum).
3. Ecological importance of algae: In aquatic area, algae are the main oxygen producers. They play an important role in ecological food chain. Algae are also used as food in several countries as they are enriched with many proteins, vitamins, minerals and lipids, for example: Aonori from Monostroma. They are also used as fertilzers (e.g., Nostoc, Oscillatoria, Scytonema) fodder (e.g., Rhodymenia,palmate, Laminariasaccharina), and also in antibiotic production (e.g., antibiotic Chlorellin is obtained from Chlorella). They also help in nitrogen fixation, for example: Blue green algae.
Virus: Viruses are the infectious agents that are incapable of independent growth in artificial medium, and they replicate only inside the living cells of other organisms. They reproduce in the host cells by replication. Viruses largely lack metabolic machinery to generate energy or to synthesize proteins. They are totally depending on the host cell to carry out their viral activity. Thus, viruses are referred as obligate intracellular parasites. The science dealing with the study of virus is known as Virology. Some major characteristic features of viruses are described below:
1. Structure and composition: When virus is not present in the living cells, then the virus exist in the form of independent particles and serves as vehicle for the entry in to another host cell. These structurally complete infectious viruses are known as Virion. These viral particles are consisting of following parts: (a) Genetic material; (b) Capsid; (c) Envelope. Viral genetic material is composed of nucleic acid. They contain either DNA or RNA. The genome of viruses can be double-stranded or single-stranded. This nucleic acid is protected by protective coat of protein called capsid, which is made up of capsomeres. Some viruses can have a membrane like structure known as envelope; it is made up of lipoproteins. There are some proteins that are associated with nucleic acid, known as nucleoprotein. Generally, there are following main morphological viruses based on symmetry:
- Replication of viruses: Multiplication in viruses takes place by replication. During replication, viral proteins and the nucleic acid are reproduced by their host cells as they are lacking of metabolic machinery. These viral protein and nucleic acid components are assembled into virus particle and then released from the host cells. The mechanism of release varies with the type of virus. Some virus infection, host cell lyses to release the virion particles, while some viruses are released by budding through the special area of the host-cell membrane.
Source:http://cdn.yourarticlelibrary.com/wp-content/uploads/2013/08/clip_image0081.jpg
- Economic importance of viruses: viruses are important in preparing vaccines by using partial or completely dead virus, and also by using their infectious parts. For example: Pox, Mumps, Polio vaccines. Sometimes viruses are used to control disease. For example: dysentery is controlled by the use of T2 bacteriophage.
| you can view video on Microbial biodiversity | |