Acetogenesis in Deep Subseafloor Sediments of The Juan de Fuca Ridge Flank: A Synthesis of Geochemical, Thermodynamic, and Gene-based Evidence

Abstract

In deep subsurface sediments of the Juan de Fuca Ridge Flank, porewater acetate that is depleted in ¹³ C relative to sedimentary organic matter indicates an acetogenic component to total acetate production. Thermodynamic calculations indicate common fermentation products or lignin monomers as potential substrates for acetogenesis. The classic autotrophic reaction may contribute as well, provided that dihydrogen (H ₂ ) concentrations are not drawn down to the thermodynamic thresholds of the energetically more favorable processes of sulfate reduction and methanogenesis. A high diversity of novel formyl tetrahydrofolate synthetase (fhs) genes throughout the upper half of the sediment column indicates the genetic potential for acetogenesis. Our results suggest that a substantial fraction of the acetate produced in marine sediment porewaters may derive from acetogenesis, in addition to the conventionally invoked sources fermentation and sulfate reduction.

Keywords:

• aquatic sediments
• biogeochemical cycling
• community structure
• deep biosphere
• subsurface microbiology

We would like to thank Mitchell L. Sogin and Leslie Graham of the Josephine Bay Paul Center at the MBL, Woods Hole, for sequencing support, and Barbara MacGregor for helpful comments. Moreover, we would like to thank the IODP Expedition 301 shipboard scientists, technicians, and ship crew for terrific support during and after the cruise. This research used samples provided by the Integrated Ocean Drilling Program (IODP), which is sponsored by the US National Science Foundation and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. The study was funded by the National Science Foundation (NSF), the Deutsche Forschungsgemeinschaft (project grant Hi 616/5-1 to K.-U.H. and V.B.H.), the Japan Agency for Marine-Earth Science & Technology Multidisciplinary Research Award 2007 (to F.I.), the Japan Society to Promote Science (JSPS), the National Aeronautics and Space Agency (NASA) Astrobiology Institutes “Subsurface Biospheres” and “Environmental Genomes,” a Schlanger Ocean Drilling Fellowship (to M.A.L.), an NSF-East Asia and Pacific Summer Institute fellowship (to M.A.L.), and a Dissertation Completion Fellowship by the University of North Carolina at Chapel Hill (to M.A.L.).