CsTFA density gradient centrifugation profiles of bacterial rRNA extracted from sludges MP (A), TP (B), MBF (C), and JET (D). Open squares, RNA from unlabeled sludge; closed triangles, RNA from first 13C-palmitate incubation; closed circles, RNA from second 13C-palmitate incubation. The amount of bacterial rRNA content in each gradient fraction was quantified by quantitative RT-PCR using a bacterial domain-specific primer set.