Recent studies on bacterial populations and processes in subseafloor sediments: A review

Indirect chemical evidence suggests the presence of bacteria in subseafloor sediments; however, until recently this had not been confirmed by comprehensive microbiological studies, applied either to a range of depth intervals at a single site or to a global range of sites. This paper summarizes the results of a detailed study of bacterial populations and activities at 14 sites, sampled by the Ocean Drilling Program since 1986, in the Pacific and Atlantic Oceans and the Mediterranean Sea. Surprisingly large bacterial populations are present in subsurface marine sediments, and although bacteria generally decrease with increasing depth, bacterial biomass to the average ocean sediment depth (500 m) is equivalent to about 10% of the total surface biosphere. Populations are significantly correlated with sediment depth but much less so with age or porosity. Bacterial populations and activity can increase where geochemical conditions change in deeper layers, such as brine incursion or the presence of thermogenic methane. This situation emphatically demonstrates the viability of bacteria in deep sediments, which is further supported by (1) the presence of intact prokaryotic DNA with high molecular weight, (2) some unique 16S rRNA gene sequences, (3) a range of different culturable bacteria and bacteria activities, and (4) isolated pure cultures adapted for deep sediment conditions. An extreme example of elevated subsurface bacterial populations occurs in gas-hydrate sediments, where bacterial processes are stimulated around the base of the deep gas-hydrate zone. For some activities, such as methanogenesis, anaerobic methane oxidation, and acetate metabolism, potential rates in the subsurface are greater than those near the sediment surface. Subsurface bacteria also occur in hydrothermal sediments with large temperature gradients (up to 12 degrees C/m) and with population numbers similar to nonhydrothermal sites at temperatures from psychrophilic to mesophilic. At greater depths and temperatures, populations decline rapidly, but they are still significant up to hyperthermophilic temperatures and are even stimulated by subsurface seawater flow. These results suggest that temperature alone does not limit bacteria in non-hydrothermal sediments until about 4 km, and evidence exists that bacterial processes may even be sustained by interaction with thermogenic processes as temperatures increase during deep burial. Experiments demonstrate that in the presence of readily degradable organic substrates, actively growing bacteria can move faster than sediment deposition; hence, these bacteria are not necessarily trapped and buried. However, bacterial growth decreases with depth to such an extent that subsurface bacteria would not be able to keep up with sedimentation rate and hence would be buried. In some circumstances, such as in sapropel layers with high organic matter in the Mediterranean, bacteria may be buried within a specific deposition horizon. Subsurface bacteria can utilize old and recalcitrant organic matter, but only very slowly, and they seem to have a strategy of high biomass and low growth rate, commensurate with their geological habitat of generally low energy flux.