Shrinking majority of the deep biosphere

Abstract

Living microorganisms were first discovered in sediment cores from scientific ocean drilling in the late 1980s when microbiologists observed and counted DNA-stained microbial cells under the microscope. During the following decade, new observations were made from drill sites in the Atlantic and Pacific Oceans and the Mediterranean Sea (1, 2). The combined data on cell numbers from these sites showed a systematic, log-log linear decrease with depth, from approximately 109 cells⋅cm−3 near the sediment surface to 106 cells⋅cm−3 at several hundred meters below seafloor, where the sediment had been deposited many million years ago (2) (Fig. 1). In their 1998 study, Whitman et al. (3) used these data to make a grand estimate of the total number of prokaryotic cells on Earth. Their astonishing conclusion was that 55% of all microbial cells occurred in the deep seabed whereas most of the rest were found in the deep terrestrial subsurface. Even more staggering, the combined deep biospheres appeared to harbor one third of the total living biomass on Earth. The publication of Whitman et al. (3) sparked a rapidly growing interest in the deep biosphere, and the data from this study were repeatedly cited in the literature in the following years. Their biomass estimate was later supported by data on intact polar membrane lipids as indicators of living cells (4). Now, these numbers are being challenged. In PNAS, Kallmeyer et al. (5) conclude that the total cell number for the deep subseafloor biosphere is probably 12-fold lower than the earlier estimate whereas the total biomass may be 74-fold lower. Is the deep biosphere losing significance?