Cell counts decrease with sediment depth. Typical explanations consider limiting factors such as
water availability and chemistry, carbon source, nutrients, energy and temperature, and overlook
the role of pore size. Our analyses consider sediment self-compaction, the evolution of pore size with
depth, and the probability of pores larger than the microbial size to compute the volume fraction of
life-compatible pores. We evaluate cell counts vs. depth profiles gathered at 116 sites worldwide.
Results confirm the critical role of pore size on cell counts in the subsurface and explain much of the
data spread (from ~ 9 orders of magnitude range in cell counts to ~ 2 orders). Cells colonize pores often
forming dense biofilms, thus, cell counts in pores are orders of magnitude higher than in the water
column. Similar arguments apply to rocks.