Global distribution of microbial abundance and biomass in subseafloor sediment

Abstract

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.

Fig. 1.

Subseafloor sedimentary cell counts used for this study. (A) Counted cell concentration vs. depth (mbsf) for the sites used in this study. (B) Site locations overlain on a map of time-averaged sea surface chlorophyll-a (34).

Fig. 2.

Distributions of cell abundance at 1 mbsf (b) and the power-law rate of decrease of cell abundance with depth (m) relative to sedimentation rate and distance from land. (A) Distribution of residuals for b. (B) Distribution of residuals for m. The histograms show the distributions of the actual residuals. The blue lines are the probability density functions for normal distributions with the appropriate SDs. (C) Distribution of b vs. sedimentation rate and distance from land. (D) Distribution of m vs. sedimentation rate and distance from land. Colored fields in C and D mark the actual range of combinations of sedimentation rate and distance from land in the world ocean. Note that data used for this model (shown as dots in C and D) occur throughout this range of actual combinations. Dot colors indicate actual values of b and m for each site.

Fig. 3.

Global distribution of subseafloor sedimentary cell abundance. (A) Geographic distribution of sedimentation rate (27). (B) Geographic distribution of distance from shore (35). (C) Geographic distribution of integrated number of cells (derived from b, m, and sediment thickness). Dot colors indicate numbers of cells calculated for actual sites (log₁₀ cells/km²).