Metagenomics has revealed hundreds to thousands of microbial species

coexisting in almost all microbiota. It is increasingly appreciated

that microbial communities condition their own environments.

To better understand the role of this environmental conditioning in

promoting diversity, we physically model the population dynamics of

microbes that compete for steadily supplied resources. In a model

where cells require multiple nutrients, we find that population dynamics

generally leads to the coexistence of different metabolic types,

which satisfy an extended competitive exclusion principle. Moreover,

we establish that these consortia of metabolic types act as cartels,

whereby population dynamics pins down resource concentrations at

values for which no other strategy can invade. Strikingly, these cartels

also yield maximum biomass, constituting a microbial example

of Adam Smith’s “invisible hand” leading to collective optimal usage

of resources. Curiously, in a model where only total resource acquisition

is considered, diversity can arbitrarily exceed that predicted by

the competitive exclusion principle.