Kingdom Monera

Chemosynthetic autotrophic bacteria, covered in NCERT Class 11, Chapter 2 (Biological Classification), synthesise their own food but differ fundamentally from photoautotrophs in their energy source. Instead of trapping sunlight, they oxidise various inorganic substances such as nitrites, nitrates, ammonia, ferrous ions or sulphur compounds, and use the energy liberated from these oxidation reactions to fix carbon dioxide into organic molecules. In a sulphur spring, sulphur bacteria oxidise hydrogen sulphide or elemental sulphur to obtain this energy. Capturing light through bacteriochlorophyll describes photosynthetic bacteria, not chemosynthetic ones, so that option is incorrect. Engulfing organic particles by phagocytosis is a heterotrophic, holozoic mode seen in some protists, not in autotrophic bacteria. Absorbing decaying organic matter saprotrophically is the nutrition of decomposer bacteria and fungi, which are heterotrophs rather than autotrophs. Ecologically, these chemosynthesisers are indispensable because the oxidation reactions they drive recycle elements like nitrogen, phosphorus, iron and sulphur, replenishing soil and water with usable nutrient forms. A plausibility check confirms the internal consistency: an autotroph living in darkness underground cannot use light, so an energy source from oxidising inorganic chemicals is the only viable mechanism that lets it remain self-feeding.