Large-scale comparative genomics of chitin metabolism in extremely halophilic archaea from the Halobacteria class.

Chitin is the second most abundant polysaccharide and can serve as a carbon and nitrogen source for microorganisms in various ecosystems, including hypersaline environments. However, chitin metabolism in extremely halophilic archaea of the class Halobacteria has not been systematically analyzed, and the pathways of N-acetylglucosamine (GlcNAc) utilization in these organisms remain poorly understood. In this study, we performed a large-scale comparative genomic analysis of the class Halobacteria to assess prevalence and organization of genes, presumably involved in chitin utilization. Potential chitinolytic species are unevenly distributed across the class Halobacteria and tend to cluster within only 7 out of its 117 recognized genera. Glycoside hydrolases from GH18 and GH3 families were found as predominant endochitinases and β-hexosaminidases, respectively. Most putative chitinolytic haloarchaea lacked the genes involved in GlcNAc catabolism in bacteria and hyperthermophilic archaeon Thermococcus kodakaraensis. However, many of them harbor enzymes of pathways previously proposed for Natrarchaeobius and nanohaloarchaeon Ca. "Nanohalobium constans". Analysis of gene neighborhoods demonstrated a conserved organization of the gene clusters associated with chitin metabolism in some genomes, resembling the clusters previously described for Natrarchaeobius species. Overall, our results indicate that Halomicrobium, Natrarchaeobius, Natrialba, Saliphagus, Halocatena, Haladaptatus, and Salinarchaeum genera are enriched in species with chitinolytic potential.