The sulfate-reducing bacterial populations of a stratified marine water column, Mariager Fjord, Denmark,
were investigated by molecular and culture-dependent approaches in parallel. Denaturing gradient gel electrophoresis
(DGGE) of PCR-amplified 16S rRNA and DNA encoding rRNA (rDNA) isolated from the water
column indicated specific bacterial populations in different water column layers and revealed a highly differentiated
pattern of rRNA- and rDNA-derived PCR amplificates, probably reflecting active and resting bacterial
populations. Hybridization of DGGE patterns with rRNA probes indicated the increased presence and activity
(by at least 1 order of magnitude) of sulfate-reducing bacteria within and below the chemocline. Parallel to this
molecular approach, an approach involving most-probable-number (MPN) counts was used, and it found a
similar distribution of cultivable sulfate-reducing bacteria in the water column of Mariager Fjord, Approximately
25 cells and 250 cells per ml above and below the chemocline, respectively, were found. Desulfovibrioand
Desulfobulbus-related strains occurred in the oxic zone.