Salient features of Five Kingdoms

Carl Linnaeus originally classified all living organisms into just two kingdoms — Plantae and Animalia — based on features such as mode of nutrition and whether the organism was migratory or sedentary. This broad two-kingdom scheme eventually proved inadequate because it could not properly accommodate organisms that show characters of both kingdoms at once — bacteria, fungi and Euglena being good examples. To resolve this, R. H. Whittaker proposed the Five Kingdom system of classification in 1969, a scheme that better reflects the phylogenetic relationships between organisms. The five kingdoms are: (1) Monera, (2) Protista, (3) Plantae, (4) Fungi, and (5) Animalia.

1. Kingdom Monera — Unicellular organisms with prokaryotic cell organisation. Monerans are omnipresent, found in essentially every kind of habitat, including extreme environments not generally inhabited by other living beings. Nutritionally, a few are photoautotrophic or chemoautotrophic, but the majority are heterotrophic. They lack a well-defined nucleus: their DNA exists as a simple, circular, double-stranded ring without a surrounding nuclear membrane, called the nucleoid, and they often carry additional small circular DNA molecules called plasmids. Their cell wall is made of peptidoglycan (also called murein), a polymer of sugars and amino acids. Membrane-bound organelles such as mitochondria, chloroplasts and endoplasmic reticulum are absent, and their ribosomes (70S) are smaller than those of eukaryotic cells. Reproduction is mostly asexual — by binary fission or budding — with sexual reproduction, by conjugation, occurring only rarely. Morphologically, bacteria are grouped into four basic shapes: the spherical coccus, the rod-shaped bacillus, the comma/kidney-shaped vibrio, and the spring-like spiral spirillum (with intermediate forms such as coccobacillus and spirochete also seen — Fig 2.3). On the basis of evolution, bacteria are further divided into Archaebacteria and Eubacteria.

a. Archaebacteria — Distinguished from other bacteria by distinct cellular features, Archaebacteria are mostly found in extreme environments and are therefore termed extremophiles — from volcanic craters to salty lakes and hot springs. Bacteria able to withstand high salinity are called halophiles, while those tolerating extreme temperature are called thermophiles. A very common example is the methanogenic bacteria found in the gut of ruminants such as cows and buffaloes, which are also exploited to produce methane in biogas plants. Their cell wall lacks peptidoglycan, and they reproduce by binary fission.

b. Eubacteria — Commonly called "true bacteria," these have a peptidoglycan cell wall and include both autotrophs and heterotrophs. Autotrophic eubacteria can be photosynthetic (e.g. Chlorobium, the green sulphur bacterium, and Chromatium) or chemosynthetic (e.g. sulphur bacteria). Cyanobacteria (blue-green algae, Fig 2.4) are mostly multicellular, filamentous forms living in fresh water; their body is covered by a mucilaginous sheath, their genetic material is typically prokaryotic, and their photosynthetic pigments include chlorophyll-a, chlorophyll-b, carotenes and xanthophylls. Their filaments carry specialised cells called heterocysts, which fix atmospheric nitrogen. Heterotrophic eubacteria are the most abundant group; most are decomposers, breaking down large molecules into simpler ones or minerals. They can be anaerobes that curdle milk (Lactobacilli), fix nitrogen (Azotobacter), produce antibiotics (Streptomyces), or take part in composting and oil degradation. Some, however, are pathogens, causing diseases such as typhoid, cholera, tuberculosis and tetanus.

A further noteworthy Monera group is Mycoplasma — the smallest of all living forms. They lack a cell wall entirely, many forms are pathogenic, and because they have no cell wall, they are resistant to the common antibiotics that target cell-wall synthesis.

2. Kingdom Protista — This group covers all unicellular but eukaryotic organisms, and shows links to all three other eukaryotic kingdoms (Plantae, Fungi and Animalia).

a. Plant-like protists (Chrysophytes) — Also called phyto-planktons, these are microscopic and mostly photosynthetic, forming major producers in the ocean. Most are diatoms, whose body wall is made of two soap-box-like, tightly fitting silica shells (Fig 2.5); accumulated diatom shells form "diatomaceous earth," a granular material used in polishing and filtration. Dinoflagellates are another plant-like protist group — mostly marine and photosynthetic, with a cell wall of stiff cellulosic plates and a pair of flagella. Their photosynthetic pigments can be yellow, green, brown, blue or red. Gonyaulax (Fig 2.6) is a dinoflagellate responsible for the famous "red tide," which turns the sea red.

b. Animal-like protists (Protozoans) — These lack a cell wall, are heterotrophic, and are believed to be primitive animal forms. Amoeboid protozoans move using pseudopodia; Amoeba is free-living, while Entamoeba is an endoparasite that causes amoebic dysentery. Flagellated protozoans move using flagella; Trypanosoma is a common flagellated pathogen that causes sleeping sickness. Ciliate protozoans, such as Paramoecium (Fig 2.8), move using cilia, and in Paramoecium a cavity called the gullet opens onto the cell surface. Plasmodium is a sporozoan protozoan that causes malaria, forming spores during one stage of its life cycle.

c. Fungus-like protists — These commonly belong to the group Myxomycetes (slime moulds), saprophytic organisms found on decaying leaves. Their individual cells aggregate to form a large mass called a plasmodium (not to be confused with the malaria parasite genus Plasmodium). The spores produced by this plasmodium are very tough and can survive even harsh conditions; Stemonitis is an example.

Also classed under Protista are the Euglenoids (e.g. Euglena, Fig 2.7), which lack a cell wall but are protected by a tough, proteinaceous covering called a pellicle. They possess two flagella of unequal length — one short, one long. In the absence of light they behave as heterotrophs, but they carry pigments similar to those of higher plants and can photosynthesise when light is available.

3. Kingdom Plantae — Dominated by autotrophs, this kingdom also includes semi-autotrophic insectivorous plants such as the Venus fly trap, pitcher plant and bladderwort, as well as fully heterotrophic parasitic members such as Cuscuta. Members are multicellular, with eukaryotic cells containing chlorophyll, and their cell walls are mostly made of cellulose. Plants show alternation of generation — their life cycle has two distinct phases. Kingdom Plantae is divided into two major groups, Cryptogamae (Cryptogams) and Phanerogamae (Phanerogams), studied in greater detail in the next chapter.

4. Kingdom Fungi — Eukaryotic heterotrophs that digest their food extracellularly, typically found in warm, humid places. Their body is simple, either unicellular or built of long thread-like structures called hyphae; large fungi such as mushrooms have a compact mass of these hyphae, while unicellular forms have a protoplast containing many nuclei (e.g. Rhizopus, and Saccharomyces, the unicellular yeast). In filamentous fungi, the body — called the mycelium — is made up of hyphae, which may or may not have cross-walls (septa); non-septate, multinucleate hyphae are called coenocytic hyphae. The fungal cell wall is composed of chitin, a polysaccharide (sometimes called fungal cellulose). Nutrition is entirely heterotrophic — mostly saprophytic, though some are parasitic or predatory — and reproduction occurs both sexually and asexually, with asexual reproduction by fragmentation, fission or budding. Some fungi are symbiotic, living with algae as lichens, or with the roots of higher plants as mycorrhiza. Fungi are both useful and harmful: mushrooms are eaten as food, yeast is used in baking and brewing, and Penicillium is well known for antibiotic production; harmful fungi cause diseases in plants and animals, such as Puccinia (rust). Fungi are further classified by their structure, mode of spore formation and fruiting bodies:

a. Phycomycetes ("algal fungi") — Mycelium made of aseptate, coenocytic hyphae, commonly growing in moist and damp habitats, on decaying organic matter, in water, or as plant parasites. Examples: Mucor, Rhizopus (bread mould, Fig 2.9), Albugo (a parasite on mustard).

b. Ascomycetes ("sac fungi") — Mostly multicellular, with branched, septate hyphae; rare unicellular forms include yeast. They can be decomposers, parasites, or dung-growing (coprophilous). Morels and truffles, both sac fungi, are eaten as delicacies, and Neurospora is widely used in genetic and biochemical research. Examples: Aspergillus (Fig 2.10), Penicillium, Claviceps, Neurospora, Saccharomyces. …