Anaerobic methane oxidation coupled to denitrification is the dominant methane sink in a deep lake

Anaerobic methane oxidation coupled to denitrification, also
known as “nitrate/nitrite-dependent anaerobic methane oxidation”
(n-damo), was discovered in 2006. Since then, only a few studies
have identified this process and the associated microorganisms in
natural environments. In aquatic sediments, the close proximity of
oxygen- and nitrate-consumption zones can mask n-damo as aerobic
methane oxidation. We therefore investigated the vertical distribution
and the abundance of denitrifying methanotrophs related
to Candidatus Methylomirabilis oxyfera with cultivation-independent
molecular techniques in the sediments of Lake Constance. Additionally,
the vertical distribution of methane oxidation and nitrate
consumption zones was inferred from high-resolution microsensor
profiles in undisturbed sediment cores. M. oxyfera-like bacteria
were virtually absent at shallow-water sites (littoral sediment) and
were very abundant at deep-water sites (profundal sediment). In
profundal sediment, the vertical distribution of M. oxyfera-like bacteria
showed a distinct peak in anoxic layers that coincided with the
zone of methane oxidation and nitrate consumption, a strong indication
for n-damo carried out by M. oxyfera-like bacteria. Both
potential n-damo rates calculated from cell densities (660–4,890
μmol CH4·m−2·d−1) and actual rates calculated from microsensor
profiles (31–437 μmol CH4·m−2·d−1) were sufficiently high to prevent
methane release from profundal sediment solely by this
process. Additionally, when nitrate was added to sediment cores
exposed to anoxic conditions, the n-damo zone reestablished well
below the sediment surface, completely preventing methane release
from the sediment. We conclude that the previously overlooked
n-damo process can be the major methane sink in stable
freshwater environments if nitrate is available in anoxic zones.