Summary Shewanella strains are characterized by versatile metabolic capabilities, resulting in their wide distribution in the ocean at different depths. Considering that particle sedimentation is an important dynamic process in the ocean, we hypothesized that hadal Shewanella species evolved from the upper ocean. In this study, we isolated three novel Shewanella strains from deep‐sea sediments in the Southwest Indian Ocean. Genome sequencing indicated that strains YLB‐06 and YLB‐08 represent two novel species in the genus Shewanella. Through phylogenomic analysis, we showed that speciation and genomic changes in marine Shewanella strains are related to water depth. We further confirmed the aforementioned hypothesis and revealed a two‐stage process of the evolutionary transition of Shewanella from the upper ocean to the hadal zone by comparative genomics and gene gain/loss analysis. Finally, the transcriptomic analysis demonstrated that recently obtained genes are strictly repressed and may thus play a minor role in the response to environmental changes.

Phosphorothioate (PT) modification by the dnd gene cluster is the first identified DNA backbone modification and constitute an epigenetic system with multiple functions, including antioxidant ability, restriction modification, and virus resistance. Despite these advantages for hosting dnd systems, they are surprisingly distributed sporadically among contemporary prokaryotic genomes. To address this ecological paradox, we systematically investigate the occurrence and phylogeny of dnd systems, and they are suggested to have originated in ancient Cyanobacteria after the Great Oxygenation Event. Interestingly, the occurrence of dnd systems and prophages is significantly negatively correlated. Further, we experimentally confirm that PT modification activates the filamentous phage SW1 by altering the binding affinity of repressor and the transcription level of its encoding gene. Competition assays, concurrent epigenomic and transcriptomic sequencing subsequently show that PT modification affects the expression of a variety of metabolic genes, which reduces the competitive fitness of the marine bacterium Shewanella piezotolerans WP3. Our findings strongly suggest that a series of negative effects on microorganisms caused by dnd systems limit horizontal gene transfer, thus leading to their sporadic distribution. Overall, our study reveals putative evolutionary scenario of the dnd system and provides novel insights into the physiological and ecological influences of PT modification.

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Reference genes are critical to obtain reliable results of reverse transcription real-time quantitative PCR (RT-qPCR), which is widely used for relative quantification of gene expression. In this study, we evaluated the validity of seven candidate reference genes for normalization in RT-qPCR analysis in the deep-sea bacterium Shewanella psychrophila WP2 under different environmental conditions. Among the set of genes investigated, gyrA, 16S rRNA and rho were identified as the most suitable reference genes for WP2 at different temperatures, hydrostatic pressures and salinities, respectively. Notably, the rho gene is conserved in Shewanella genus and other deep-sea bacteria, thus, could be used as a versatile reference gene for RT-qPCR analysis of these microorganisms under extreme environmental conditions.

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