Microbes are single-celled organisms found in every environment imaginable, from volcanic vents to the soil in your backyard. Their incredible diversity is matched by the astonishing variety of their diets, which collectively forms the nutritional base for all life on Earth. Unlike animals, microbes have evolved metabolic pathways to process a menu that spans light, complex biological molecules, and inert chemical compounds. This ability makes them the planet’s ultimate recyclers and chemical processors, driving global nutrient cycles.
Energy and Carbon: The Fundamental Food Groups
The classification of a microbe’s diet hinges on two fundamental needs: an energy source to fuel its metabolism and a carbon source to build its cellular structures. Organisms that capture light for energy are called phototrophs, while those that derive energy from breaking down chemical compounds are known as chemotrophs. This distinction establishes the first part of their nutritional identity.
The second part depends on the source of carbon used for growth. Autotrophs use an inorganic source, specifically carbon dioxide ($\text{CO}_2$), to synthesize all their necessary organic compounds. In contrast, heterotrophs must consume pre-formed organic compounds, such as sugars, proteins, or fats, to acquire the carbon they need.
By combining these two factors, scientists define the four main nutritional groups. For instance, a chemoheterotroph, the most common microbial group, gets both its energy and its carbon from organic molecules, similar to humans. A photoautotroph, like cyanobacteria, uses light for energy and $\text{CO}_2$ for carbon. Chemotrophs are further split into chemoorganotrophs (using organic chemicals) and chemolithotrophs (using inorganic chemicals).
Decomposers and Consumers: Eating Organic Material
The most familiar microbial diet involves the consumption of large, complex organic matter, carried out primarily by chemoheterotrophs. These organisms are the primary agents of decomposition, recycling the carbon and nutrients locked within dead plants, animals, and waste products. They target large biomolecules like cellulose, proteins, fats, and nucleic acids.
Since these food molecules are too large to pass through the cell membrane, the organism employs extracellular digestion. The microbe secretes powerful digestive enzymes, such as proteases, lipases, and glycanases, directly into its external environment. These enzymes break down complex polymers into smaller, absorbable units like simple sugars and amino acids.
This digestive process facilitates the decay visible in the environment, returning nutrients to the soil and water. Examples include the spoilage of food, the decomposition of a fallen log, or the breakdown of kitchen scraps in a compost pile. Without this widespread consumption of organic matter, Earth’s surface would be buried in undecomposed biological material, disrupting nutrient cycles.
The Extremists: Dining on Rocks and Chemicals
Some of the most fascinating microbial diets belong to the chemolithotrophs, which derive their energy from inorganic compounds. These organisms thrive where sunlight and organic matter are scarce, such as deep-sea vents, acidic mines, and subterranean rock layers. They gain energy by oxidizing reduced inorganic chemicals, using them as electron donors in their metabolic processes.
Specific groups of bacteria and archaea consume sulfur compounds, oxidizing hydrogen sulfide or elemental sulfur to produce sulfate. This process can fuel entire ecosystems deep beneath the ocean surface. Other microbes are iron-oxidizing, drawing energy from converting ferrous iron ($\text{Fe}^{2+}$) into ferric iron ($\text{Fe}^{3+}$), a reaction often responsible for reddish-orange deposits in acidic streams. Methane-consuming bacteria, or methanotrophs, use the greenhouse gas methane ($\text{CH}_4$) as their sole source of carbon and energy, regulating atmospheric gas levels.
The metabolic flexibility of these microbes extends to synthetic materials, offering a potential solution for environmental pollution. Certain fungi and bacteria have been shown to degrade complex polymers such as low-density polyethylene (LDPE) and polyurethane (PUR) foam. They achieve this by secreting enzymes, such as PETase, which break down the tough chemical chains of plastics like polyethylene terephthalate (PET) into smaller, consumable molecules.
Shared Meals: Microbes in Symbiotic Relationships
A distinct category of microbial nutrition involves obtaining food directly from a living host organism in a symbiotic relationship. These interactions range from mutualistic, where both partners benefit, to parasitic, where the microbe consumes host resources detrimentally. In mutualistic associations, the microbe gains access to a stable food supply that the host cannot fully utilize.
A primary example is the human gut microbiome, where trillions of bacteria feed on complex carbohydrates and undigested dietary fiber. The microbes break down these fibers through fermentation, producing short-chain fatty acids that serve as an energy source for the host’s colon cells. Similarly, mycorrhizal fungi partner with plant roots, receiving simple sugars synthesized by the plant. In exchange, the fungus’s extensive network increases the plant’s ability to absorb essential nutrients, such as phosphate, from the soil.
Parasitic microbes feed by consuming the host’s tissues or circulating nutrients, often leading to disease. Pathogenic bacteria and fungi tap into the host’s nutrient supply, diverting sugars, amino acids, and other organic compounds for their own growth. These microbes rely on the continuous metabolic output of a larger organism as their specialized food source.