  We consider a broad class of interactions between radiation and a light
scalar field, including both conformal and disformal couplings. Such a scalar
field potentially acts on cosmological scales as dark energy and could also
appear in modified gravity theories. We study the consequences of these
couplings on the mixing between the scalar field and photons in galaxy clusters
in the presence of a magnetic field. In particular we focus on the resulting
turbulence-induced irregularities in the X-ray and UV bands. We find new bounds
on the photon-to-scalar couplings, both conformal and disformal, which
complement laboratory experiments and other astrophysical constraints.
