Heterotrophic Archaea dominate sedimentary subsurface ecosystems off Peru

Area of Science:

• Deep biosphere microbiology
• Geomicrobiology
• Biogeochemical cycling

Background:

• Sedimentary microbial communities play key roles in biogeochemical processes.
• Anaerobic methane oxidation coupled to sulfate reduction is a significant process in marine sediments.
• Previous studies indicated elevated prokaryotic cell numbers in methane-consuming sediment layers.

Purpose of the Study:

• To investigate the active microbial community members involved in anaerobic methane oxidation.
• To determine the carbon assimilation pathways of Archaea in these deep subsurface ecosystems.
• To understand the metabolic functions of uncultured Archaea groups.

Main Methods:

• Analysis of archaeal ribosomal RNA (rRNA) to identify active community members.
• Stable isotope probing of whole archaeal cells and membrane lipids.
• Reconstruction of carbon flow using isotopic compositions of various carbon pools.

Main Results:

• Active archaeal communities were dominated by uncultured Marine Benthic Group B and Miscellaneous Crenarchaeotal Group, not known methanotrophs.
• These Archaea assimilated sedimentary organic compounds other than methane.
• Methane oxidation occurred without direct assimilation of methane-carbon by these dominant groups.

Conclusions:

• Marine Benthic Group B and Miscellaneous Crenarchaeotal Group likely oxidize methane without assimilating its carbon.
• Subsurface microbial communities exhibit extremely low maintenance energies and long turnover times (100-2,000 years).
• This study offers insights into the metabolic capabilities of cosmopolitan uncultured Archaea.