THE SUBSURFACE BIOSPHERE: DIVERSITY, EXTENT AND RELATIONSHIP TO THE PALAEOENVIRONMENT

Marine sediments are one of the most extensive microbial habitats on Earth. This unique ecosystem hosts diverse microbial communities and could represent 0.6% of the Earth’s total biomass. It harbours representatives from the three domains of life, i.e., numerous endemic and/or as yet uncultured Archaea and Bacteria, protists and fungi belonging to Eukarya that contribute to early/late sediment diagenesis. The presence of life in sediments will be influenced by the sediment depth, pressure, temperature and age. In what form life dwells in the sediments, what are the main metabolisms present and what is the link beetween the microbial diversity and the sediment nature and/or the paleoenvironment, are questions that will be adressed during this talk. I will focus on two main study areas: the Canterbury Basin, out the coast of New Zealand and the N-W of the Mediteranean Sea (Ligurian Sea, ) out the coast of France - Nice. We used a multidisciplinary approach to assess the diversity and function of subsurface microbial communities and their survival within a sediment core from the seafloor surface to 2 Km below the sea floor, and we also explored their relationship to the palaeoenvironment in a specific context: the turbidits deposition. Our results suggest that diverse microorganisms persist down to nearly 2 Km below the seafloor of the Canterbury Basin (New Zealand) and extend the previously known depth limits of microbial evidence (i) from 159 to 1740 mbsf for Eukarya and (ii) from 518 to 1922 mbsf for Bacteria. Shifts in microbial community composition along a 2km-long core reflect vertical taxa zonation influenced by sediment nature. We also hypothesize that in some environmental contexts such as turbidits formation, subseafloor microbial diversity might represent an imprint of the paleoenvironmental deposition conditions influenced by past climatic events. Besides this main talk I will also present some aspects on my present research at the Oceanographic Observatory of Villefranche, that address the occurrence of a seasonality in a physico-chemical stable environment in the ocean, the mesopelagic (« twilight ») zone, a « sandwich layer » between euphotic surface waters and the deep ocean.