New Species Provide Genomics-Based Insights Into Cobalamin Synthesis in Haloarchaea.

Hypersaline aquatic and terrestrial ecosystems display a cosmopolitan distribution. These environments teem with microbes and harbor a plethora of prokaryotic lineages that evaded ecological characterization due to the prior inability to cultivate them or to access their genomic information. In order to close the current knowledge gap, we performed two sampling and isolation campaigns in the saline soils of the Odiel Saltmarshes and the salterns of Isla Cristina (Huelva, Spain). From the isolated haloarchaeal strains subjected to high-throughput phylogenetic screening, two were chosen (F15B and F9-27) for physiological and genomic characterization due of their relatedness to the genus . Comparative genomic analyses were carried out between the isolated strains and the genomes of previously described species CECT 7525, F13-13 and environmentaly recovered metagenome-assembled representatives of the genus . The topology of the phylogenomic tree showed agreement with the phylogenetic ones based on 16S rRNA and genes, and together with average amino acid and nucleotide identities suggested the two strains as novel species within the genus. We propose the names sp. nov. (type strain F15B = CECT 9688 = CCM 8954) and sp. nov. (type strain F9-27 = CECT 9745 = CCM 8956) for these strains. Comparative genomic analyses within the genus highlighted a typical signature, characterized by acidic proteomes with low isoelectric points, and indicated heterotrophic aerobic lifestyles. Genome-scale metabolic reconstructions revealed that the newly proposed species encode all the necessary enzymatic reactions involved in cobalamin (vitamin B) biosynthesis. Based on the worldwide distribution of the genus and its abundance in hypersaline habitats we postulate that its members perform a critical function by being able to provide "expensive" commodities (i.e., vitamin B) to the halophilic microbial communities at large.