  Microbial communities are ubiquitous in nature and come in a multitude of
forms, ranging from communities dominated by a handful of species to
communities containing a wide variety of metabolically distinct organisms. This
huge range in diversity is not a curiosity - microbial diversity has been
linked to outcomes of substantial ecological and medical importance. However,
the mechanisms underlying microbial diversity are still under debate, as simple
mathematical models only permit as many species to coexist as there are
resources. A plethora of mechanisms have been proposed to explain the origins
of microbial diversity, but many of these analyses omit a key property of real
microbial ecosystems: the propensity of the microbes themselves to change their
growth properties within and across generations. In order to explore the impact
of this key property on microbial diversity, we expand upon a recently
developed model of microbial diversity in fluctuating environments. We
implement changes in growth strategy in two distinct ways. First, we consider
the regulation of a cell's enzyme levels within short, ecological times, and
second we consider evolutionary changes driven by mutations across generations.
Interestingly, we find that these two types of microbial responses to the
environment can have drastically different outcomes. Enzyme regulation may
collapse diversity over long enough times while, conversely,
strategy-randomizing mutations can produce a "rich-get-poorer" effect that
promotes diversity. This work makes explicit, using a simple serial-dilutions
framework, the conflicting ways that microbial adaptation and evolution can
affect community diversity.
