Let's start by looking at the basics of how microbes make their livings. First, as with all living things, microbes are made up C, N, P, O, S, H, K, Fe, Mg, and Ca. Interestingly, many microbes don't require Na, which is required by most plants and animals. Modes of microbial nutrition. Microbes and all other living things can be classified based on how they obtain their carbon and energy.
Autotrophs can fix carbon dioxide and turn it into organic molecules.
Bacteria, Archaea and Fungi feed by absorbing chemicals from their environment. Most of what we will discuss below applies to Bacteria, Archaea and Fungi. To better understand the size limit for absorptive feeders we can do a little simple geometry. These amazing creatures can also store elemental sulfur granuoles in their cytoplasm as inclusion bodies. Fossing, H., et al.