This kingdom comprises bacteria, the most abundant microorganisms on Earth. They are found in nearly every environment, including extreme habitats like hot springs, deserts, and deep oceans.
Structure and Shape: Bacteria have a simple structure but exhibit complex behavior and extensive metabolic diversity. They are grouped into four categories based on shape:
Coccus: Spherical
Bacillus: Rod-shaped
Vibrium: Comma-shaped
Spirillum: Spiral
Nutrition:
Autotrophic: Synthesize their own food. This includes photosynthetic autotrophs and chemosynthetic autotrophs.
Heterotrophic: The vast majority, depending on other organisms or dead organic matter for food.
Reproduction: Primarily reproduce by fission. Under unfavorable conditions, they produce spores. They can also engage in a primitive type of DNA transfer.
Sub-groups of Monera
Archaebacteria: These bacteria are unique for their ability to survive in harsh environments. Their distinct cell wall structure is key to their survival.
Halophiles: Live in extreme salty areas.
Thermoacidophiles: Live in hot springs.
Methanogens: Live in marshy areas and the guts of ruminant animals (cows, buffaloes), where they produce methane (biogas).
Eubacteria (“True Bacteria”): Characterized by a rigid cell wall and a flagellum (if motile).
Cyanobacteria (Blue-green algae): Contain chlorophyll a and are photosynthetic autotrophs. They can be unicellular, colonial, or filamentous and are found in various aquatic and terrestrial environments. Some form “blooms” in polluted water and can fix atmospheric nitrogen in specialized cells called heterocysts (e.g., Nostoc, Anabaena).
Chemosynthetic Autotrophic Bacteria: Oxidize inorganic substances like nitrates, nitrites, and ammonia to produce ATP. They play a crucial role in nutrient recycling (nitrogen, phosphorous, iron, sulphur).
Heterotrophic Bacteria: The most abundant type. Many are important decomposers. They have significant impacts on humans, being used to make curd, produce antibiotics, and fix nitrogen in legumes. Some are pathogens that cause diseases like cholera, typhoid, tetanus, and citrus canker.
Mycoplasma: Organisms that completely lack a cell wall. They are the smallest known living cells, can survive without oxygen, and many are pathogenic in plants and animals.