The global geographic distribution of subseafloor sedimentary microbes and the cause(s) of that distribution are largely unexplored. Here, we show that total microbial cell abundance in subseafloor sediment varies between sites by ca. five orders of magnitude. This variation is strongly correlated with mean sedimentation rate and distance from land. Based on these correlations, we estimate global subseafloor sedimentary microbial abundance to be 2.9·10 29 cells [corresponding to 4.1 petagram (Pg) C and ∼0.6% of Earth's total living biomass]. This estimate of subseafloor sedimentary microbial abundance is roughly equal to previous estimates of total microbial abundance in seawater and total microbial abundance in soil. It is much lower than previous estimates of subseafloor sedimentary microbial abundance. In consequence, we estimate Earth's total number of microbes and total living biomass to be, respectively, 50-78% and 10-45% lower than previous estimates.deep biosphere | cell enumeration | global microbial biomass | subsurface life B acteria and archaea drive many fundamental processes in marine sediment, including oxidation of organic matter, production of methane and other hydrocarbons, and removal of sulfate from the ocean (1-3). Previous studies of subseafloor sediment from ocean margins and the eastern equatorial Pacific Ocean reported high abundances of microbial cells (2). RNA studies indicate that many of these cells are active (4), have a diverse community composition (5, 6), and exhibit high diversity in their anaerobic metabolic activity (5). Cell counts from these environments generally show little variation between sites (2, 7) and decrease logarithmically with sediment depth, although there can be sharp peaks of high cell densities in zones of anaerobic methane-oxidation (2, 8).In 1998, Whitman et al. (9) (Fig. 1A). In these regions, dissolved oxygen penetrates deeply into the sediment and microbial activity is generally aerobic (5, 11). Metabolic activity per cell is extremely low among the anaerobes of both ocean margins and upwelling regions (12) and the aerobes of the open-ocean gyres (5, 11).The differences between cell counts from ocean margins and upwelling areas and cell counts from oceanic gyres raise three questions. First, how does the abundance of microbes in subseafloor sediment vary throughout the world ocean? Second, what property or properties are likely to control that variation? Third, how does this variation affect estimates of total subseafloor sedimentary biomass and Earth's total biomass? Materials and MethodsTo address these questions, we compiled our cell counts from the South Pacific Gyre (5), the North Pacific Gyre, and the eastern equatorial Pacific Ocean with previously published counts from ocean margins and the equatorial Pacific Ocean (Fig. 1B). We limited this compilation to sites with cell counts both above and below 1 m below sea floor (mbsf). To compare the data from different sites, we parameterized the cell distribution at each site by plotting cell abundance...

The deep subseafloor biosphere is among the least-understood habitats on Earth, even though the huge microbial biomass therein plays an important role for potential long-term controls on global biogeochemical cycles. We report here the vertical and geographical distribution of microbes and their phylogenetic diversities in deeply buried marine sediments of the Pacific Ocean Margins. During the Ocean Drilling Program Legs 201 and 204, we obtained sediment cores from the Peru and Cascadia Margins that varied with respect to the presence of dissolved methane and methane hydrate. To examine differences in prokaryotic distribution patterns in sediments with or without methane hydrates, we studied >2,800 clones possessing partial sequences (400 -500 bp) of the 16S rRNA gene and 348 representative clone sequences (Ϸ1 kbp) from the two geographically separated subseafloor environments. Archaea of the uncultivated Deep-Sea Archaeal Group were consistently the dominant phylotype in sediments associated with methane hydrate. Sediment cores lacking methane hydrates displayed few or no Deep-Sea Archaeal Group phylotypes. Bacterial communities in the methane hydrate-bearing sediments were dominated by members of the JS1 group, Planctomycetes, and Chloroflexi. Results from cluster and principal component analyses, which include previously reported data from the West and East Pacific Margins, suggest that, for these locations in the Pacific Ocean, prokaryotic communities from methane hydrate-bearing sediment cores are distinct from those in hydrate-free cores. The recognition of which microbial groups prevail under distinctive subseafloor environments is a significant step toward determining the role these communities play in Earth's essential biogeochemical processes.

Diverse microbial communities and numerous energy-yielding activities occur in deeply buried sediments of the eastern Pacific Ocean. Distributions of metabolic activities often deviate from the standard model. Rates of activities, cell concentrations, and populations of cultured bacteria vary consistently from one subseafloor environment to another. Net rates of major activities principally rely on electron acceptors and electron donors from the photosynthetic surface world. At open-ocean sites, nitrate and oxygen are supplied to the deepest sedimentary communities through the underlying basaltic aquifer. In turn, these sedimentary communities may supply dissolved electron donors and nutrients to the underlying crustal biosphere.