Characterization and spatial distribution of methanogens and methanogenic biosignatures in hypersaline microbial mats of Baja California

Well-developed hypersaline cyanobacterial mats from Guerrero Negro, Baja California Sur, sustain active methanogenesis in the presence of high rates of sulfate reduction. Very little is known about the diversity and distribution of the microorganisms responsible for methane production in these unique ecosystems. Applying a combination of 16S rRNA and metabolic gene surveys, fluorescence in situ hybridization, and lipid biomarker analysis, we characterized the diversity and spatial relationships of methanogens and other archaea in the mat incubation experiments stimulated with methanogenic substrates. The phylogenetic and chemotaxonomic diversity established within mat microcosms was compared with the archaeal diversity and lipid biomarker profiles associated with different depth horizons in the in situ mat. Both archaeal 16S rRNA and methyl coenzyme M reductase gene (mcrA) analysis revealed an enrichment of diverse methanogens belonging to the Methanosarcinales in response to trimethylamine addition. Corresponding with DNA-based detection methods, an increase in lipid biomarkers commonly synthesized by methanogenic archaea was observed, including archaeol and sn-2-hydroxyarchaeol polar lipids, and the free, irregular acyclic isoprenoids, 2,6,10,15,19-pentamethylicosene (PMI) and 2,6,11,15-tetramethylhexadecane (crocetane). Hydrogen enrichment of a novel putative archaeal polar C₃₀ isoprenoid, a dehydrosqualane, was also documented. Both DNA and lipid biomarker evidence indicate a shift in the dominant methanogenic genera corresponding with depth in the mat. Specifically, incubations of surface layers near the photic zone predominantly supported Methanolobus spp. and PMI, while Methanococcoides and hydroxyarchaeol were preferentially recovered from microcosms of unconsolidated sediments underlying the mat. Together, this work supports the existence of small but robust methylotrophic methanogen assemblages that are vertically stratified within the benthic hypersaline mat and can be distinguished by both their DNA signatures and unique isoprenoid biomarkers.