n order to reduce sample numbers, PCR amplicons from replicates of sediment were pooled before loading on DGGE. Prior to pooling, a DGGE was run on the individual samples to ensure acceptable repro- ducibility (results not shown). DGGE was performed with 8% (w/v) acrylamide gel which contained a linear chemical gradient ranging from 35% to 60% denaturant (100% denaturant 7 M urea, and 40% formamide). Similar amounts of PCR products were loaded into the gel and electrophoresed in 1× TAE buffer at 60 °C and 60 V for 16 h, using a Dcode Universal Mutation Detection System (Bio-Rad Laborato- ries, Richmond Calif). After electrophoresis, the gel was stained with silver nitrate modified from Sanguinetti et al. (1994) and the image obtained with a scanner.