  Microorganisms can preferentially orient and move along gradients of a
chemo-attractant (i.e., chemotax) while colonies of many microorganisms can
collectively undergo complex dynamics in response to chemo-attractants that
they themselves produce. For colonies or groups of micro-swimmers we
investigate how an "auto-chemotactic" response that should lead to swimmer
aggregation is affected by the non-trivial fluid flows that are generated by
collective swimming. For this, we consider chemotaxis models based upon a
hydrodynamic theory of motile suspensions that are fully coupled to
chemo-attractant production, transport, and diffusion. Linear analysis of
isotropically ordered suspensions reveals both an aggregative instability due
to chemotaxis that occurs independently of swimmer type, and a hydrodynamic
instability when the swimmers are "pushers". Nonlinear simulations show
nonetheless that hydrodynamic interactions can significantly modify the
chemotactically-driven aggregation dynamics in suspensions of "pushers" or
"pullers". Different states of the dynamics resulting from these coupled
interactions in the colony are discussed.
