The diversity of the methyl-coenzyme reductase A (mcrA) and 16S rRNA genes was investigated in gas hydrate containing sediment from the Kazan mud volcano, eastern Mediterranean Sea. mcrA was detected only at 15 and 20 cm below seafloor (cmbsf) from a 40-cm long push core, while based on chemical profiles of methane, sulfate, and sulfide, possible anaerobic oxidation of methane (AOM) depth was inferred at 12–15 cmbsf. The phylogenetic relationships of the obtained mcrA, archaeal and bacterial 16S rRNA genes, showed that all the found sequences were found in both depths and at similar relative abundances. mcrA diversity was low. All sequences were related to the Methanosarcinales, with the most dominant (77.2%) sequences falling in group mcrA-e. The 16S rRNA-based archaeal diversity also revealed low diversity and clear dominance (72.8% of all archaeal phylotypes) of the Methanosarcinales and, in particular, ANME-2c. Bacteria showed higher diversity but 83.2% of the retrieved phylotypes from both sediment layers belonged to the δ-Proteobacteria. These phylotypes fell in the SEEP-SRB1 putative AOM group. In addition, the rest of the less abundant phylotypes were related to yet-uncultivated representatives of the Actinobacteria, Spirochaetales, and candidate divisions OP11 and WS3 from gas hydrate-bearing habitats. These phylotype patterns indicate that AOM is occurring in the 15 and 20 cmbsf sediment layers.
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