In addition to increasing the number of active workers to boost metabolic output, you could also make them more efficient. One way to do this is by bringing enzymes that work in the same pathway together into an assembly (or disassembly) line. Such factories are common inside cells. You could even go a step further and enclose the factory in a dedicated building. Such structures, found in bacteria but not (as far as we know) archaea, are known as microcompartments–areas of the cytoplasm walled off by a protein shell (⇩). For instance, Acetonema longum like this use a factory called the propanediol utilization, or pdu, microcompartment to increase the efficiency of an enzymatic pathway that breaks down 1,2-propanediol. Illustrating the interlocking lives of microbial communities, 1,2-propanediol is itself the byproduct of the metabolism of other microbes, neighbors in A. longum’s environment in the gut of an animal.
Bacterial microcompartments are typically enclosed by a self-assembling shell formed by many copies of just a few proteins. The shell is icosahedral, consisting of hexameric units packed into flat planes, which are joined together by pentamers at the vertices. You can see this arrangement in this shell structure of a microcompartment called an encapsulin that Thermotoga maritima use to sequester soluble iron so that it does not react with reactive oxygen species and damage the cell [39].
