These results raise the possibility that those two OEUs represent
consortia of syntrophic organisms cooperating to carry out sulfate
reduction. We therefore checked whether any other OEUs contained
organisms with known mutualistic associations. We identified
two cases where OTUs within an OEU are probably part of a
consortium. In the first case, one OEU contained an OTU that
was 97.7% identical to the ammonia-oxidizing bacterium
Nitrosospira briensis27 and an OTU 99% identical to a nitrite-oxidizing
enrichment culture clone, Candidatus Nitrotoga arctica28. Nitrification
is a two-step process, typically carried out by different organisms29,30,
so it is likely that these two organisms interact to carry out nitrification.
In the second case, one OEU that aligns with the modelled
distribution of a methane oxidation metabolism contained an OTU
94.8% identical to Methylobacter tundripaludum (a methane oxidizer)
and an OTU 98% identical to a strain of Methylotenera versatilis
(a non-methane-oxidizer methylotroph). A study using stable