Coenzyme F430, the
prosthetic group of methyl coenzyme M reductase
(MCR), is a key compound in methane metabolism. We applied coenzyme
F430 as a function-specific biomarker of methanogenesis to subsurface
marine sediments collected below the sulfate reduction zone to investigate
the distribution and activity of methanogens. In addition, we examined
the kinetics of the epimerization of coenzyme F430, which is the first
stage of the degradation process after cell death, at various temperatures
(4, 15, 34, 60 °C) and pH (5, 7, 9) conditions, which cover in situ conditions of drilled sediments used in this study.
The degradation experiments revealed that the kinetics of the epimerization
well follow the thermodynamic laws, and the half-life of coenzyme
F430 is decreasing from 304 days to 11 h with increasing the in situ temperature. It indicates that the native F430 detected
in the sediments is derived from living methanogens, because the abiotic
degradation of F430 is much faster than the sedimentation rate and
will not be fossilized. Based on coenzyme F430 analysis and degradation
experiments, the native form of F430 detected in subseafloor sediments
off the Shimokita Peninsula originates from living methanogen cells,
which is protected from degradation in cells but disappears soon after
cell death. The biomass of methanogens calculated from in
situ F430 concentration and F430 contents in cultivable methanogen
species decreases by 2 orders of magnitude up to a sediment depth
of 2.5 km, with a maximum value at ∼70 m below the seafloor
(mbsf), while the proportion of methanogens to the total prokaryotic
cell abundance increases with the depth, which is 1 to 2 orders of
magnitude higher than expected previously. Our results indicate the
presence of undetectable methanogens using conventional techniques.