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

Subsurface bacteria also occur in hydrothermal sediments with large temperature gradients (up to 12 degree C/m) and with population numbers similar to non-hydrothermal 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 4km, 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.Original Abstract: Les bacteries souterraines existent aussi dans les sediments d'origine hydrothermale ou regnent des gradients de temperature eleves (jusqu'a 12 degree C/m) et ou sont presentes des populations semblables en nombre a celles des sites non hydrothermaux, a temperatures psychrophiles a mesophiles. A de plus grandes profondeurs et a des temperatures plus elevees, les populations diminuent tres rapidement, mais elles sont encore significativement presentes jusqu'aux temperatures hyperthermophiles et sont meme stimulees par l'ecoulement d'eau marine souterraine. Ces resultats laissent penser que la temperature seule ne limite pas les bacteries dans les sediments non hydrothermaux jusqu'a environ 4km; selon certains indices, les processus bacteriens peuvent meme etre entretenus par des interactions avec les processus lies a la temperature lorsque les temperatures augmentent au cours de l'enfouissement profond du sediment. Ces experiences demontrent qu'en presence de substrats organiques degradables les bacteries a c