Microbial life in deep subseafloor coal beds.

Deep beneath the seafloor, microbial communities thrive on the leftovers of organic material that in the past settled down from the surface layers of the ocean to the sediment. As the organic matter was buried deeper and deeper over geological time it became increasingly recalcitrant to microbial degradation. Microbial cells that still persist in these ancient deposits appear to survive at the limit of starvation. However, it is estimated that half of all bacteria and most of all archaea in the ocean live under such nutrient-poor conditions in the deep seabed (1). This deep biosphere extends around the globe and, on a long geological time scale, interacts with the chemistry of the sea water. Thus, the nutrients in the ocean and the oxygen in the atmosphere are affected by the efficiency with which subseafloor microorganisms degrade and remineralize the buried organic matter. Microbiologists who study this deep biosphere have been searching for the limits to life in the subsurface and have set new records in finding even deeper and older microbial communities. Trembath-Reichert et al. now report in PNAS (2) that microorganisms live in 20 million-y-old coal beds buried 2 km beneath the seafloor and that the organisms are able to take up nutrients and grow when kept for years under seminatural conditions in the laboratory. Trembath-Reichert et al. (2) studied deep lignite coal beds that had been discovered by seismic profiling at 1,200-m water depth off the east coast of Japan. A forearc basin is here formed by the subduction of the Pacific Ocean plate, which has gradually also pulled down the continental plate. During the early Miocene, organic remains of rich coastal forests were buried in warm backswamps that later subsided into the cold ocean and formed lignite seams, which were subsequently overlaid by 2-km-thick marine shales. Expedition 337 of the Integrated Ocean Drilling Program (IODP) used the Japanese riser drillship, D/V Chikyu, in 2012 to penetrate 2.5 km down into the seabed. They succeeded in recovering sediment core material from the 40–60 °C warm coal beds and shales for microbiological and geochemical studies (3). Among the main objectives was to detect whether deeply buried hydrocarbon reservoirs, such as coalbeds, may act as a geobiological reactor that sustains subsurface life for millions of years. Signatures of life were already indicated by the biogenic isotope signature of methane from that depth and by the relatively low concentrations, relative to methane, of ethane and other hydrocarbons of preferentially thermogenic origin. The recovery of uncontaminated core material from 2-km depth in the seabed is a truly challenging endeavor. The seabed material used for microbiological studies consisted of crushed pieces of lignite and shale that first needed to be rigorously tested for bacterial contamination from drilling fluid or from handling. This test is critical for the later confidence in the microbiological data. Testing was done by the addition of a perfluorocarbon contamination tracer in the drilling fluid and by checking for seawateror humanderived bacterial genes (3). The least-contaminated samples were selected, and then even greater methodological challenges started.