Deep-sea sediments become apparently more hostile to life with increasing depth as temperature and pressure rise, and organic matter becomes increasingly recalcitrant. Demonstrations of high bacterial populations in deep sediments(1,2) may thus appear enigmatic. How, then, can the continued presence of active bacterial populations in deep sediments that are over 10 million years old be explained? Although volatile fatty acids, particularly acetate, are important intermediates in the anaerobic degradation of organic matter(3,4), their concentrations are kept very low in sediments (< 15 mu M) by rapid bacterial consumption(5,6). Here we show that heating surface coastal marine sediments to simulate increasing temperature during burial produces an increase of over three orders of magnitude in acetate concentration and increases bacterial 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 confirmed that there was a notable generation of acetate associated with temperature increases during burial, This was supported by heating experiments 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 biosphere than has previously been considered.