Geomicrobiology of deep, low organic carbon sediments in the Woodlark Basin, Pacific Ocean.

Abstract The distribution of bacterial populations and activity were determined at two Ocean Drilling Program sites (1109 and 1115) in the Woodlark extensional Basin, near Papua New Guinea, Pacific Ocean. These relatively deep water (1 150-2 211 m), low organic carbon ( approximately 0.4%), low organic matter sedimentation sites, with average thermal gradients ( approximately 30 degrees C/km) are representative of large areas of ocean sediments. At a third site, 1118, with a higher thermal gradient (63 degrees C/km), only bacterial distributions and pore water acetate (bioavailable) concentrations were determined. Active bacterial populations were present to all depths at Sites 1109 and 1115, maximum depth 801 mbsf (metres below seafloor), and this represents the deepest and oldest marine sediments ( approximately 15 Ma) in which the presence of bacteria has been demonstrated using a combination of different approaches (direct microscopic and viable counts, radiotracer turnover and geochemical analysis). In addition, direct counts and geochemical data at Site 1118 provide strong evidence for significant bacterial populations to at least 842 mbsf (3.2x10(5)/cm(3)). Similar to previously studied subseafloor sediments, maximum bacterial populations and activity occurred in the upper approximately 20 m, with much lower rates (up to 10 000 times) in deeper layers. However, a depth integration of data demonstrated that the majority of bacterial cells and activity (sulfate reduction, methanogenesis, thymidine incorporation into DNA and acetate oxidation) occurred in the subsurface, below 20 m. For sulfate reduction this was dependent on the depth of sulfate penetration. Acetate was an important substrate for methanogenesis. Despite being degraded, acetate concentrations in subsurface layers remained relatively constant (<10 muM) and therefore, there must also have been deep acetate formation, which is consistent with the presence of acetogenic bacteria. These results extend the significance of deep acetate formation for the maintenance of subsurface bacterial populations to sediments representative of large areas of the ocean.