Environmental Engineering Reference
In-Depth Information
methane concentrations at the two sites as shown above are not significantly
different and cell counts were in the same range.
Interestingly, in the sediments of most methane seeps ANME-1 and ANME-
2 archaea occurred in a consortium with sulfate reducers of theδ-proteobacteria
groups
Desulfosarcina/Desulfococcus
[4, 30, 32]. However, no cell consortia
in the water column could be observed and the methane oxidizers occurred
rather as single cells. Investigations by 16S rDNA based methods are ongoing
to further resolve the question which organisms are involved in the anaerobic
oxidation of methane in the Black Sea water column.
From the carbon isotopic composition of the methane it is obvious that the
chemocline is especially interesting when looking for methane sinks. At the
central station (7605) and at the north-western station (7617) a strong isotopic
enrichment of the methane was measured just below the chemocline (Fig.
4) providing strong evidence for enhanced microbial activity, i.e., methane
oxidation. From tracer experiments by Reeburgh et al. [37] and own results
we know that in the Black Sea water column rates of anaerobic oxidation of
methane are 100 times higher than aerobic oxidation rates. However, since
the strongest isotopic fractionation occurs where oxygen is already present,
although in small amounts, we cannot exclude that aerobic methanotrophs
contribute to the methane consumption. Higher sampling resolution at the
chemocline is necessary to support this conclusion.
5. CONCLUSIONS
We have evaluated the different methane sources to the Black Sea water
column and conclude that a significantly fraction of the methane derives from
methane seeps located mainly on the shelf but also on the upper and lower
slope. Significant microbial methane oxidation was indicated by increasing
δ
13
C
CH
4
values especially at the chemocline, but also in the oxic water column.
Methanotrophic bacteria of type I are mainly responsible for aerobic methane
oxidation in the oxic water column. Methane oxidation rates above the methane
seep in the shallow water are approximately 30 times higher compared to a
reference station. Methane concentrations and methane oxidation rates from a
deep seep and a deep reference station were similar, but an additional input of
methane via seepage was indicated by neon depletion in the water column due
to gas ebullition. The carbon isotopic composition of methane in the anoxic
water column indicates a methane source deriving from the deeper shelf/slope.
Anaerobic methane oxidizers (ANME-1 and ANME-2 group) were detected
over the deep seep and above the reference station. Detailed investigations
are ongoing to better resolve the communities involved in anaerobic methane
oxidation. To further understand the methane budget in the Black Sea it is
important to determine more specifically methane oxidation rates especially
