New constraints significantly reducing previous estimates of the global distribution of microbial abundance and biomass in sediments beneath the seafloor were set forth by German and U.S. researchers in October 2012 .
Their work shows total microbial cell abundances in subseafloor sediment varying among study sites by approximately five orders of magnitude. The variation strongly correlates to mean sedimentation rate and distance from land. Using these correlations, they estimate global seafloor sedimentary microbial abundance to be 2.9 X 1029 cells - a number much lower than previous estimates. Their estimates of Earth's total number of microbes and total living biomass are, respectively, 50 to78% and 10 to 45% lower than the previous estimates.
The correlations presented in this study align with the strong influence of organic matter burial rates on subseafloor cell abundance. Direct and indirect factors that affect organic burial rate include the productivity of the overlying ocean, water depth, organic matter flux from land, and sedimentation rate. Organic burial rates have been previously estimated for the world oceans based on these factors. The results of this study vary from previous higher estimates for two primary reasons: 1) Its database is more geographically diverse than previous studies and it specifically include gyre areas with extremely low cell abundances. 2) Unlike some of the previous studies, they used estimates of world ocean sediment thickness derived from geophysical data. That said, the study recognizes that the cell abundances in many other oceanic environments still remain uncertain‚ some uncertainties are very large and others have not yet been quantified.
Figure: Global distribution of subseafloor sedimentary cell abundance in comparison to sedimentation rates and distance from land.