Scalar perturbations of a four-fluid cosmological model are investigat
ed for the case that two of the fluid components are charged. The norm
al modes of a mixture of ionized hydrogen, radiation, and dark matter
before hydrogen recombination are determined analytically for several
limiting cases. The corresponding modes in an electron-positron plasma
are also studied. Well-known growing and decaying modes of fluid cosm
ologies are recovered. Additionally, plasma oscillations both on scale
s larger and smaller than the Hubble horizon are found. All modes are
gauge-invariantly characterized in terms of thermodynamical variables
on comoving hypersurfaces. While the plasma oscillations decay and do
not essentially influence the gravitational instability scenario, the
latter is modified by the possibility of growing baryonic energy densi
ty perturbations as soon as the dark matter becomes dynamically domina
ting. At the decoupling time of matter and radiation all the matter, i
ncluding the baryonic one, may be more clumpy than the radiation. The
picture of baryons falling into the dark matter potential wells after
decoupling turns out to be superfluous.