Profile of Microbial Community Structure and Presence of Endolithic Microorganisms Inside a Deep-Sea Rock

Microbial community structure in the depth profile of a deep-sea sedimentary rock collected from the Sanriku Escarpment in the Japan Trench at a depth of 6337 m were analyzed using enrichment culture methods and culture-independent molecular phylo-genetic techniques. The rock was subsampled at four depths (S1 to S4; from the surface to the inside), and carbon concentrations and colony-forming units (CFU) were determined under several culture conditions. Terminal-restriction fragment length polymorphism (T-RFLP) analysis of PCR-amplified 16S rRNA gene (rDNA) sequences indicated that a shift in bacterial and archaeal ribotype structures occurred in the sections at different depths from the surface. rDNA clone analysis revealed a significant change in microbial rDNA community structure. Bacterial community rDNA in sections S1 to S3 consisted of typical marine bacteria mainly members of the f and n -subclass of Proteobacteria, while the inner most section, S4, contained rDNA signatures for the g -subclass of Proteobacteria and the High G + C Gram-Positive Group. Major archaeal rDNA clones shifted from Marine Group I (S1) to Thermococcales (S2-S4). The changes in bacterial and archaeal rDNA community structure indicated the possible infiltration of seawater and microorganisms into the rock and strongly suggested the isolation of endolithic microbial communities over the geological history of the rock.