By A.P. Mentzer Updated Aug 30, 2022
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While often associated with disease, bacteria are indispensable to ecosystems. By consuming and transforming organic and inorganic molecules, they release nutrients from dead matter, digest food in animal guts, fix atmospheric nitrogen into ammonia, supply plants with essential elements, and even oxygenate the air. Two key factors govern bacterial nutrition: whether they can synthesize their own food (autotrophy) or depend on external organic molecules (heterotrophy), and the source of energy that powers their metabolic reactions.
Autotrophs build their own organic compounds from carbon dioxide, while heterotrophs must ingest pre‑formed organic material such as glucose. Both strategies rely on either light or chemical energy to drive the necessary biochemical pathways.
Phototrophs harness sunlight. Photoheterotrophs use light for energy but still require organic molecules as their carbon source. Photoautotrophs, like cyanobacteria, convert sunlight and CO₂ into carbohydrates through photosynthesis, playing a critical role in oxygen production and primary production in aquatic environments.
Chemotrophs obtain energy from redox reactions involving inorganic compounds. Chemoheterotrophs consume organic or inorganic substrates for both energy and carbon, whereas chemoautotrophs generate carbohydrates from CO₂ using chemical energy—a process known as chemosynthesis. This pathway sustains life in habitats where sunlight is absent, such as deep‑sea vents.
Bacterial cells are bounded by a cell envelope comprising an inner cytoplasmic membrane and a rigid outer cell wall that confers shape. Unlike eukaryotes, they lack membrane‑bound organelles and nuclei, precluding endocytosis and phagocytosis. Nutrient acquisition is mediated by passive diffusion, facilitated diffusion through membrane proteins, and active transport that consumes ATP to move molecules against concentration gradients. Many bacteria secrete enzymes outside the cell to pre‑break down complex substrates, enabling efficient uptake of the resulting small molecules.
