Marine sediments cover more than two-thirds of Earth. Microbial cells and prokaryotic activity appear to be widespread in those sediments. Intact cells (Parkes et al., 2000) and intact membrane lipids (Zink et al., 2003; Sturt et al., 2004) provide evidence of prokaryotic populations in sediments as deep as 800 meters below seafloor (mbsf). Prokaryotic activity, in the form of sulfate reduction and/or methanogenesis, occurs in sediments throughout the world's oceans (D'Hondt et al., 2002). The prokaryotes of subseafloor sediments have been estimated to constitute one-half to five-sixths of Earth's prokaryotic biomass (Whitman et al.,1998) and one-tenth to one-third of Earth's total living biomass (Parkes et al., 2000; Whitman et al., 1998). Fundamental aspects of this subseafloor ecosystem are poorly known. What is the phylogenetic composition of subseafloor prokaryotic communities? What are their functional genes and their metabolic activities that allow these prokaryotes to grow and survive in the subsurface?
Leg 201 was the first Ocean Drilling Program (ODP) expedition dedicated to the study of life deep beneath the seafloor (D'Hondt, Jørgensen, Miller, et al., 2003). Leg 201 combined detailed geochemical analyses, cell counts, cultivations, and molecular screening of subsurface microbial communities. The objective of this multidisciplinary approach was a comprehensive census of subsurface microbial life (diversity, density, and activity) in the context of geochemical controls that shape microbial community composition and activity. Sampling sites were selected that represented low-activity sediments from central oceanic basins in the Pacific, as well as organic-rich sediments on the Peru margin and in the Peru Trench influenced by the Peruvian upwelling system.
Subsurface sediment cores of 120–420 m depth were obtained from three deepwater, open-ocean sites with organic-poor sediments in the Eastern eastern equatorial Pacific (Sites 1225 and 1226) and the Peru Basin (Site 1231), from three shallow sites with organic-rich sediments on the Peruvian continental shelf (Sites 1227, 1228, and 1229), and from an organic-rich deep-sea sediment site in the Peru Trench (Site 1230) (D'Hondt, Jørgensen, Miller, et al., 2003). Sites 1227–1230 had a total organic carbon (TOC) content in the range of 2–8 wt% (2–4 wt% for Site 1230), with outliers near 10 wt% (Meister et al., this volume), cell densities in the upper range of previously recorded acridine orange direct count (AODC) profiles for ODP sediments (Parkes et al., 2000; D'Hondt, Jørgensen, Miller, et al., 2003), and steep sulfate and methane gradients indicating downhole sulfate depletion and methane buildup caused by organic carbon decomposition by sulfate reduction and methanogenesis (D'Hondt et al., 2004). The open-ocean Sites 1225 and 1231 had cell densities near or below the average of AODC profiles for ODP sediments (Parkes et al. 2000; D'Hondt, Jørgensen, Miller, et al., 2003). Their organic C content was at least an order of magnitude lower than at Peru margin sites (0.05–0.2 wt%; some datapoints at Site 1231 as much as 0.7 wt%) (Meister et al., this volume). Their near-linear profiles of sulfate and methane indicate little sulfate depletion or methane accumulation; instead, mutually overlapping broad zones with high concentrations of dissolved metals (Mn and Fe) indicate active microbial metal respiration (D'Hondt et al., 2004). All these characteristics contrast strongly with those of Peru margin sites (D'Hondt et al., 2004). Therefore, Leg 201 offered the opportunity to examine subsurface microbial communities from a wide range of geochemically distinct marine subsurface sediments. This paper reviews and summarizes the available data sets for microbial community composition in Leg 201 sediments.