Identification of the dominant sulfate‐reducing bacterial partner of anaerobic methanotrophs of the ANME‐2 clade

Abstract

The anaerobic oxidation of methane (AOM) with sulfate as terminal electron acceptor is mediated by consortia of methanotrophic archaea (ANME) and sulfate‐reducing bacteria (SRB). Whereas three clades of ANME have been repeatedly studied with respect to phylogeny, key genes and genomic capabilities, little is known about their sulfate‐reducing partner. In order to identify the partner of anaerobic methanotrophs of the ANME‐2 clade, bacterial 16S rRNA gene libraries were constructed from cultures highly enriched for ANME‐2a and ANME‐2c in consortia with Deltaproteobacteria of the Desulfosarcina/Desulfococcus group (DSS). Phylogenetic analysis of those and publicly available sequences from AOM sites supported the hypothesis by Knittel and colleagues that the DSS partner belongs to the diverse SEEP‐SRB1 cluster. Six subclusters of SEEP‐SRB1, SEEP‐SRB1a to SEEP‐SRB1f, were proposed and specific oligonucleotide probes were designed. Using fluorescence in situ hybridization on samples from six different AOM sites, SEEP‐SRB1a was identified as sulfate‐reducing partner in up to 95% of total ANME‐2 consortia. SEEP‐SRB1a cells exhibited a rod‐shaped, vibrioid, or coccoid morphology and were found to be associated with subgroups ANME‐2a and ANME‐2c. Moreover, SEEP‐SRB1a was also detected in 8% to 23% of ANME‐3 consortia in Haakon Mosby Mud Volcano sediments, previously described to be predominantly associated with SRB of the Desulfobulbus group. SEEP‐SRB1a contributed to only 0.3% to 0.7% of all single cells in almost all samples indicating that these bacteria are highly adapted to a symbiotic relationship with ANME‐2.