  Interactions between dark matter and dark energy with a given equation of
state are known to modify the cosmic dynamics. On the other hand, the strength
of these interactions is subject to strong observational constraints. Here we
discuss a model in which the transition from decelerated to accelerated
expansion of the Universe arises as a pure interaction phenomenon. Various
cosmological scenarios that describe a present stage of accelerated expansion,
like the LCDM model or a (generalized) Chaplygin gas, follow as special cases
for different interaction rates. This unifying view on the homogeneous and
isotropic background level is accompanied by a non-adiabatic perturbation
dynamics which can be seen as a consequence of a fluctuating interaction rate.
