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- AOM coupled to denitrification
CH 4 oxidation coupled to denitrification was first demon-
strated in an anaerobic enrichment culture from a freshwater
contaminated aquifer containing high nitrate concentrations
(Smith et al. 1991 ). Evidence of this process was later sup-
ported by experimental observations (Islas-Lima et al. 2004 ).
Moreover Raghoebarsing et al. ( 2006 ) subsequently demon-
strated AOM coupled to nitrite reduction (Eq. 16.2) in
enrichment cultures obtained from sediments of two fresh-
water ecosystems with high nitrate concentrations (up to
1 mM).Box 16.4: Prokaryotic Lipids as Indicators of
Methanotrophic Communities
The combined study of the distribution and stable carbon
isotopic composition (δ^13 C values) of diagnostic lipids
derived from bacteria and archaea in environmental sam-
ples is a powerful mean for demonstrating the implication
of prokaryotes into the past and present methane cycle.Due to the strongly depleted δ 13 C values of biogenic (but
also thermogenic) methane, the biomass and cellular con-
stituents of prokaryotes using methane as a carbon source
consequently appear^13 C-depleted. For example, δ 13 C
values as low as −100 ‰ of different hopanoid com-
pounds derived from aerobic bacteria (see an example of
structure), classically serve as indicators of aerobic oxi-dation of methane (i.e., by aerobic or microaerophilic
methanotrophs).
Examples of lipid structures produced by aerobic and
anaerobic methanotrophs
The strongly depleted carbon isotopic composition
(−130‰ < δ^13 C < −50‰) of lipid biomarkers specifically
synthesized by Archaea (see examples of structures) has
provided the first irrefutable evidence of the involvement
of these organisms in the anaerobic oxidation of methane
(AOM) in habitats where the process was previously
inferred. The stable carbon isotope composition of other
lipid biomarkers (see an example of structure) suggested
to derive from the bacterial partners of ANME involved
in sulfate-dependent AOM further supported a syntrophic
association between ANME and SRB. These bacteriallipids are usually less depleted (−90‰ < δ^13 C < −40‰)
than the archaeal lipids. Differences in biomarker pro-
files from one AOM site to the other typify distinct anaer-
obic microbial consortia involved in AOM. In such
settings, the diversity of^13 C-depleted biomarkers of pro-
karyotic origin are indicative of methane carbon transfer-
ring via ANME to several SRB populations and then,
eventually, to other surrounding prokaryotic and/or
eukaryotic populations. Our understanding of AOM, as
well as demonstrations of its occurrence in different eco-
systems and of its direct or indirect involvement in bio-
geochemical processes (e.g., precipitation of carbonates
or iron sulfide nodules), are steadily increasing, and
partly rely on the analysis of lipid biomarker^13 C
composition.A.-C. Lehours et al.