DEEP MARINE BIOSPHERE FUELED BY INCREASING ORGANIC-MATTER AVAILABILITY DURING BURIAL AND HEATING

Deep-sea sediments become apparently more hostile to life with increas ing depth as temperature and pressure rise, and organic matter becomes increasingly recalcitrant. Demonstrations of high bacterial populatio ns in deep sediments(1,2) may thus appear enigmatic. How, then, can th e continued presence of active bacterial populations in deep sediments that are over 10 million years old be explained? Although volatile fa tty acids, particularly acetate, are important intermediates in the an aerobic degradation of organic matter(3,4), their concentrations are k ept very low in sediments (< 15 mu M) by rapid bacterial consumption(5 ,6). Here we show that heating surface coastal marine sediments to sim ulate increasing temperature during burial produces an increase of ove r three orders of magnitude in acetate concentration and increases bac terial activity. We found that pore-water acetate concentration at two sites in the Atlantic Ocean increased at depths below about 150 m and was associated with a significant stimulation in bacterial activity, Comparing these acetate concentrations to in situ temperatures confirm ed that there was a notable generation of acetate associated with temp erature increases during burial, This was supported by heating experim ents with deep sediments. Thus, acetate generation from organic matter during burial may explain the presence of a deep bacterial biosphere in marine sediments, and could underpin an even deeper and hotter bios phere than has previously been considered.