Evolutionary ecology of microbial populations inhabiting deep sea sediments associated with cold seeps

Deep sea cold seep sediments host abundant and diverse bacterial and archaeal populations that significantly influence biogeochemical cycles. While numerous studies have revealed the community structure and functional capabilities of cold seep microbiomes, little is known about their genetic heterogeneity within species. Here, we examined intraspecies diversity patterns of 39 abundant species identified in sediment layers down to 4.3 mbsf across six cold seep sites from around the world. These species were predicted to participate in methane oxidation and sulfate reduction, and based on their metabolic capabilities, grouped as aerobic methane-oxidizing bacteria (MOB), anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria (SRB). These physiologically and phylogenetically diverse MOB, ANME and SRB display different degrees of intrapopulation sequence divergence and different evolutionary trajectories. Populations were in general characterized by low rates of homologous recombination and strong purifying selection with most of the nucleotide variation being synonymous. Functional genes related to methane ( pmoA and mcrA ) and sulfate ( dsrA ) metabolisms were found to be under strong purifying selection in the vast majority of species investigated, although examples of active positive selection were also observed. These genes differed in evolutionary trajectories across phylogenetic clades but are functionally conserved across cold seep sites. Intrapopulation diversification of MOB, ANME and SRB species as well as their mcrA and dsrA genes was observed to be depth-dependent and undergo divergent selection pressures throughout the sediment column. These results highlight the role of the interplay between ecological processes and the evolution of key bacteria and archaea in deep sea cold seep sediments and shed light on how microbial populations adapt in the subseafloor biosphere.