Environmental Engineering Reference
In-Depth Information
4.4 Methane Sinks
The main sinks in the methane budget of the Black Sea are, as pointed
out earlier by Reeburgh et al. [37], central basin sediments, the oxidation of
methane in the oxic and anoxic water column (AOM), and evasion to the
atmosphere. One of the big questions as to sinks of methane in the Black
Sea is the identity of methane oxidizers in the anaerobic part of the water
column. It has been shown earlier that members of the order Methanosarcinales
together with sulfate reducing bacteria are involved in the anaerobic oxidation
of methane in the Black Sea [3, 30, 47]. At active gas seeps of the Black
Sea, methanotrophic mats were found containing high amounts of strongly
depleted archaeol and sn-2-hydroxyarchaeol. Interestingly we found archaeol
and sn-2-hydroxyarchaeol in the water column of the Black Sea, coinciding
with a strong fractionation against 13 C methane and high methane oxidation
rates, indicating that methane is oxidized by organisms related to methanogenic
archaea just below the chemocline. Unfortunately, due to the low concentration
of these compounds in the water column it was not possible to measure their
carbon isotopic composition, which would be needed for an ultimate proof that
these organisms are involved in AOM. Schouten et al. [43] and Wakeham et al.
[49] showed that 13 C depleted archaeal derived biphytanes occur in the anoxic
water column of the Black Sea providing evidence that methane consuming
archaea are present. Isotopically depleted phytane (δ
13 C = -51 ‰) released
after chemical (HI) treatment from an archaeal lipid precursor (e.g. archaeol,
hydroxy-archaeol) has been described in the anoxic water column of the Black
Sea (Wakeham et al. 2004). Thisδ
13 C value is, however, rather heavy compared
to values measured in sedimentary lipids extracted from venting sites (around
-100 ‰, [4]) and the possibility remains that these compounds originate from
methanogenic archaea.
Recently, Vetriani and co-workers [48] provided evidence based on 16S
rRNA sequences and T-RFLP in one Black Sea water sample at 305 m that
archaea phylogenetically related to the ANME-2 cluster are present. Using
16S rRNA-targeted oligonucleotide probes specific to both groups, we were
able to detect ANME-1 and ANME 2-related cells in the water column of the
Black Sea. Cell counts of samples from the water column above the methane
seep (072) revealed ANME-1 and ANME-2 cells in concentrations of up to
4 % of all DAPI stained cells. Above the reference site (064) only up to 2
% ANME-1 and ANME-2 were detected. We believe that these organisms
are at least partly responsible for anaerobic methane oxidation in the water
column of the Black Sea but it is likely that there are other groups yet to be
discovered. Incubation experiments revealed no difference in methane oxidation
rates between the reference and seep site. This may be due to the fact that actual
Search WWH ::




Custom Search