  Microbial communities display extreme diversity. A variety of strains or
species coexist even when limited by a single resource. It has been argued that
metabolite secretion creates new niches and facilitates such diversity.
Nonetheless, it is still a controversial topic why cells secrete even essential
metabolites so often; in fact, even under isolation conditions, microbial cells
secrete various metabolites, including those essential for their growth. First,
we demonstrate that leaking essential metabolites can be advantageous. If the
intracellular chemical reactions include multibody reactions like catalytic
reactions, this advantageous leakage of essential metabolites is possible and
indeed typical for most metabolic networks via "flux control" and
"growth-dilution" mechanisms; the later is a result of the balance between
synthesis and growth-induced dilution with autocatalytic reactions.
Counterintuitively, the mechanisms can work even when the supplied resource is
scarce. Next, when such cells are crowded, the presence of another cell type,
which consumes the leaked chemicals is beneficial for both cell types, so that
their coexistence enhances the growth of both. The latter part of the paper is
devoted to the analysis of such unusual form of symbiosis: "consumer" cell
types benefit from the uptake of metabolites secreted by "leaker" cell types,
and such consumption reduces the concentration of metabolites accumulated in
the environment; this environmental change enables further secretion from the
leaker cell types. This situation leads to frequency-dependent coexistence of
several cell types, as supported by extensive simulations. A new look at the
diversity in a microbial ecosystem is thus presented.
