Evaluation of Molecular Techniques in Characterization of Deep Terrestrial Biosphere

A suite of molecular methods targeting 16S rRNA genes (i.e., DGGE, clone and high-throughput [HTP] amplicon library sequencing) was used to profile the microbial communities in deep Fennoscandian crystalline bedrock fracture fluids. Variation among bacterial 16S rRNA genes was examined with two commonly used primer pairs: P1/P2 and U968f/U1401r. DGGE using U968f/ U1401r mostly detected β-, γ-proteobacteria and Firmicutes, while P1/P2 primers additionally detected other proteobacterial clades and candidate divisions. However, in combination with clone libraries the U968f/U1401r primers detected a higher bacterial diversity than DGGE alone. HTP amplicon sequencing with P1/P2 revealed an abundance of the DGGE bacterial groups as well as many other bacterial taxa likely representing minor components of these communities. Archaeal diversity was investigated via DGGE or HTP amplicon sequencingusing primers A344F/ 519RP. The majority of archaea detected with HTP amplicon sequencing belonged to uncultured Thermoplasmatales and Pendant 33/DHVE3, 4, 6 groups. DGGE of the same samples detected mostly SAGMEG and Methanosarcinales archaea, but almost none of those were revealed by HTP amplicon sequencing. Overall, our results show that the inferred diversity and composition of microbial communities in deep fracture fluids is highly dependent on analytical technique and that the method should be carefully selected with this in mind.