Leg 201 synthesis: Controls on microbial communities in deeply buried sediments

Leg 201 of the Ocean Drilling Program was the first ocean drilling expedition dedicated to the study of life deep beneath the seafloor, a challenge that has since become an integral part of the Integrated Ocean Drilling Program. Seven sites were drilled in the Peru margin and the eastern equatorial Pacific, ranging in water depth from 150 to 5300 m and in sediment depth from 0 to 420 meters below seafloor (mbsf), corresponding to sediment ages of Holocene to late Eocene. Continuous contamination tests with perfluorocarbon and fluorescent microbead tracers were conducted during drilling, and improved shipboard procedures were developed for anaerobic and aseptic handling of sediment samples. Sufficiently uncontaminated sediment was thus obtained from all sites for detailed microbiological and biogeochemical analyses. Microbial cells were detected and enumerated in cores from the sediment surface down to oceanic basalt; the oldest sediment was ∼35 Ma. The prokaryotic population size varies in relation to geochemical zonations and to sediment properties determined by oceanographic conditions at the time of deposition. Cell densities increase from the Pacific open-ocean sites to the upwelling region of the Peruvian \u27shelf, where densities at a sulfate-methane transition zone reach 1010 cells/mL, the highest yet recorded in subsurface sediments. Molecular screening of the subsurface microbial communities reveals a large diversity of bacterial and archaeal phylogenetic lineages, most with no or very few cultured representatives. Only a few sequences related to classical sulfate-reducing bacteria and methanogenic archaea are represented in the clone libraries, even where sulfate reduction and methanogenesis are the predominant biogeochemical processes in the sediment. Quantification by deoxyribonucleic acid (DNA)/ribonucleic acid (RNA)-based techniques show that bacteria predominate over archaea and that a significant fraction of all cells contain ribosomes and thus can be confirmed as living and active microorganisms. Cultivation yielded a wide variety of bacterial isolates, including Proteobacteria, Actinobacteria, and Firmicutes, but no archaeal isolates. In situ temperatures range from 2°C at the sediment surface to 26°C at depth, and, accordingly, most isolates were mesophiles or moderate psychrophiles, but some thermophiles were also cultured. Diffusion-reaction modeling of pore water gradients was combined with direct radiotracer experiments to quantify major microbial processes such as sulfate reduction and methanogenesis. The results demonstrate the dependence of these processes on geochemical zonations and availability of energy sources. The generally low concentrations of key bacterial metabolites (acetate, formate, and H2) indicate continuing bacterial activity in the deep subsurface. Authigenic minerals such as pyrite, dolomite, or biogenic magnetite form as a result of the biogeochemical processes