Wawer et a/. were the first to use DGGE analysis of [NiFe] hydrogenase gene fragments of Death Vibrio species, an important group of sulfate-reducing bacteria. By comparative analysis of PCR products obtained from genomic DNA and mRNA extracted from bioreactor sam- ples incubated with hydrogen, the substrate for the [NiFe] hydrogenase enzyme, the authors could demonstrate the presence of different Desu~ohio populations, but only the preferential expression of the [NiFe] hydrogenase gene by one population. It was concluded that this popu- lation might be better adapted to growth on hydrogen than other Desulfovibrio populations suggesting a niche differen- tiation of closely related bacterial populations performing different functions in the community. As more sequences for other functional genes become available in the future, we will soon be able to use PCR-DGGE/TGGE to relate community structure and function.