The capabilities of alternating cellular automata (ACA) to accept formal languages are investigated. Several notions of alternation in cellular automata have been proposed. Here we study so-called nonuniform ACAs. Our investigations center on space bounded real-time computations. In particular, we prove that there is no difference in acceptance power regardless of whether one-way or two-way communication lines are provided. Moreover, the relations between real-time ACAs and deterministic (CA) and nondeterministic (NCA) cellular automata are investigated. It is proved that even the real-time ACAs gain exponential speed-up against nondeterministic NCAs. Comparing ACAs with deterministic CAs it is shown that real-time ACAs are strictly more powerful than real-time CAs.