The role of the Ocean and the biological pump in the regulation of CO2 in the context of the global warming is now recognized, however data used in CO2 flux modelization mainly come from the surface layer (euphotic layer 0-200 m). Everyday, nearly 50% of the surface production (representative of the biological pump) is exported in the deeper zone (Ducklow & Carlson, 1992) as dissolved and particulate organic material which constitute the main vector for exportation of carbon and energy in the intermediary and deep waters, down to sediment. All the way down, most of the particulate organic carbon (POC) is mineralized and only a small fraction reaches deep sediment. Substrate quality influences the energetic yield, thus the dynamic (it goes without saying the composition) of microbial communities. Despite their crucial importance, there are only few studies on mesopelagic (200-1000 m) and bathypelagic (<1000 m) zones; the OM composition (and quality) (Wakeham et al., 1997 ; Hedges et al., 2000; Benner, 2002) and the bacterial processes involved in its mineralization such as regulation of genes involved in the degradation of certain organic compounds are not well characterized in this « unknown » part of the ocean (Vezzi et al., 2005).

Nowadays, most of the available information related to these processes comes from the epipelagic zone or/and do not takes into account the effect of the increasing hydrostatic pressure. It is however important to integrate this factor when one studies mineralization processes of oceanic OM. The LMGEM Laboratory (CNRS, Marseille, France) possesses not only the know-how but also high pressure devices (bottles and samplers, sinking particle simulator) that place the laboratory among the international leaders in the deep-sea task force.