Methane supersaturation in near-shore coastal waters overlying permeable sediments have been frequently observed, however the drivers and production pathways of this methane are yet unknown (1). Permeable (sandy) sediments are dynamic systems with frequent fluxes in oxygen concentration as well as being rich in sulfate which has typically been thought to inhibit growth and activity of obligately anaerobic archaeal methanogens. Recent research into marine methane emissions generally have focussed on non-archaeal methane production. However, evidence presented here indicates that archaeal methanogens may be able to survive and thrive in these sediments, using unconventional methanogenic substrates.
We undertook a range of ex situ incubation experiments on surface sands including selective inhibition of archaeal methanogenesis and specific substrate additions, as well as metagenomic analysis and in situ correlational studies. Results show that archaeal methanogens were indeed the primary source of methane production in surface sediments and production was primarily driven by methylotrophic methanogenesis. It is proposed that in the environment marine algae and seagrasses provide a significant source of these methylated substrates and may therefore cause large methane emissions from sandy coasts. This may have implications as a potential link between increasing ocean eutrophication and greenhouse gas emissions.