Sulphur metabolism as a key factor in the evolution of environmental adaptation of Acidihalobacter

This study compares predicted sulphur metabolism genes across four Acidihalobacter type strains and two metagenome-assembled genomes (MAGs), revealing genomic differences that appear to correspond to ecological specialization. Phylogenomic analysis separates the species into two clades: clade I includes Acidihalobacter ferrooxydans from a geothermal region in Italy and the two MAGs derived from deep-sea hydrothermal vents in the Pacific Ocean, while clade II comprises Acidihalobacter aeolianus and Acidihalobacter prosperus from a geothermal region in Italy and Acidihalobacter yilgarnensis from a saline and acidic drainage in Australia. Variations in sulphide/quinone oxidoreductases (SQRs) across the species, in particular in Ah. ferrooxydans and Ah. yilgarnensis , likely relate to the availability and speciation of sulphur substrates, which are strictly governed by local redox potential ( E h ) and metal redox cycling in their respective habitats. Notably, only Ah. ferrooxydans (clade I) lacks the canonical sulphur/thiosulphate oxidation (Sox) system for thiosulphate oxidation found in clade II and instead encodes components of an alternative S 4 I pathway. We hypothesize that this difference reflects an adaptation to dynamic microniches going from highly reduced (sulphide-rich) to oxidized metastable sulphur intermediates. In contrast, the retention of the Sox system in clade II suggests a distinct strategy permitting greater metabolic versatility under fluctuating E h -pH conditions. Differences in clade I terminal oxidases ( cbb 3 -type cytochrome, bc 1 complex) and regulatory elements appear to support further adaptation to environments with elevated H 2 S, setting this clade apart from clade II members. These adaptations, mainly evidenced by gene redundancy, gene loss and horizontal gene transfer, seem to reflect a unique ecological microniche and evolutionary trajectory for Ah. ferrooxydans distinct from other members of the genus, particularly from a sulphur-based energy metabolism perspective.