In nature, microbial substrates are predominantly found as:

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Most organic matter available to microbes in nature is in the form of polymers, the large macromolecules produced by living things. These biopolymers include cellulose and starch (polysaccharides), proteins, lipids, and nucleic acids. Microbes rely on extracellular enzymes to break these polymers down into smaller units that they can take up and metabolize. Because polymeric material is so abundant in environments like soil, water, and sediment, it constitutes the main substrate pool. Monomers do exist, but they’re typically results of depolymerization and are consumed quickly, so they aren’t the dominant form overall. Gases and ions are important substrates for specific microbial processes or nutrient needs, but they don’t represent the primary substrate form microbes rely on across most natural environments.