In nature, how are most microbial substrates found?

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Substrates that microbes encounter in nature are typically organized as biopolymers. In natural organic matter, carbon and energy are largely tied up in large, polymeric molecules such as polysaccharides (like cellulose and starch), proteins, nucleic acids, and complex lipids. Microorganisms use extracellular enzymes to break these polymers into smaller monomers that can be taken up and metabolized, which is why polymeric substrates are so common as growth sources. While simple monomers do exist and are used, they are not as abundant as the polymeric forms in many natural environments. Volatile gases and inorganic minerals play different roles: gases are often products of metabolism or present as atmospheric components, and inorganic minerals provide nutrients or energy only for specific chemolithotrophic paths, not the typical organic substrates for most microbes.