Inverted microbial community stratification and spatial-temporal stability in hypersaline anaerobic sediments from the S'Avall solar salterns

The anaerobic hypersaline sediments of an ephemeral pond from the S'Avall solar salterns constituted an excellent study system because of their easy accessibility, as well as the analogy of their microbial assemblages with some known deep-sea hypersaline anaerobic brines. By means of shotgun metagenomics and 16S rRNA gene amplicon sequencing, the microbial composition of the sediment was shown to be stable in time and space. The communities were formed by prokaryote representatives with a clear inferred anaerobic metabolism, mainly related to the methane, sulfur and nitrate cycles. The most conspicuous finding was the inverted nature of the vertical stratification. Contrarily to what could be expected, a methanogenic archaeal metabolism was found to dominate in the upper layers, whereas Bacteria with fermentative and anaerobic respiration metabolisms increased with depth. We could demonstrate the methanogenic nature of the members of candidate lineages DHVE2 and MSBL1, which were present in high abundance in this system, and described, for the first time, viruses infecting these lineages. Members of the putatively active aerobic genera Salinibacter and Halorubrum were detected especially in the deepest layers for which we hypothesize that either oxygen could be sporadically available, or they could perform anaerobic metabolisms. We also report a novel repertoire of virus species thriving in these sediments, which had special relevance because of their lysogenic lifestyles.