Cover photograph (Copyright © 2005, American Society for Microbiology. All Rights Reserved.): Micrograph showing a consortium, consisting of physically associated archaea and sulfate reducers, anaerobically oxidizing methane. Anaerobic oxidation of methane is the major biological "sink" of the greenhouse gas methane in marine sediments and serves as an important means of control of methane emission to the hydrosphere. The consortia are embedded in the matrix of a thick microbial mat from a cold methane seep in the Black Sea. They were detected using 16S rRNA-targeted oligonucleotide probes specific for archaea of the ANME-2c cluster (red fluorescence) and sulfate reducers of the Desulfosarcina/Desulfococcus group (green fluorescence), respectively. (See related article on page 467.)