Quantification of Prokaryotic Gene Expression in Shallow Marine Subsurface Sediments of Aarhus Bay, Denmark

The community abundance and diversity of mRNA transcripts for sulfate-reducing prokaryotes were investigated in parallel with rate measurements of sulfate reduction activity in marine subsurface sediments of Aarhus Bay, Denmark. Solid phase and porewater chemistry and sulfate reduction rates were determined on subsamples from sediment cores to 5 m below the sediment surface. Molecular analysis of the dissimilatory (bi) sulfite reductase (dsrAB) mRNA transcripts and 16S rRNA were performed by quantitative reverse transcription real-time PCR and traditional clonal analysis. The distribution of dsrA transcripts was directly linked to both sulfate reduction rates and rRNA content. Quantitative analysis of dsrA gene transcripts indicated the presence of active sulfate reduction at 465 cm below the sediment-water interface, where high methane concentrations persist in regions of near sulfate depletion. These results suggest an abundance of active bacteria in zones of high sulfate reduction and a marked decrease in zones of low sulfate reduction rates. Archaeal communities showed a constant distribution in the sediment profile in zones of intermediate and low sulfate reduction rates, indicating their predominance in sediments where low sulfate and high methane concentrations were observed. Substantiated by biogeochemical and rRNA analysis, the analysis of mRNA gene transcripts serves as a versatile molecular proxy for the study of sulfate reducing communities in marine subsurface sediments.