Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 17 6 2025 12 13 912 928 Jagannath Sarkar Department of Biological Sciences, Bose Institute, Kolkata, India 2025 07 26 2025 10 21 Background and Objectives: Actinobacteria are ubiquitous across diverse environmental niches. Brevibacterium strains within this phylum are widely distributed in both marine and terrestrial ecosystems worldwide. Marine environments are defined by distinct physicochemical properties—high salinity, alkaline pH, fluctuating O2 levels, and dynamic nutrient availability—which set them apart from terrestrial habitats. The broad ecological range of Brevibacterium strains raises questions about genome-encoded metabolic features that have evolved to adapt in marine environments. Materials and Methods: Genomics of Brevibacterium strains from various marine provinces was analyzed, focusing on core genome and pan-genome structure. Results: Core genome and pan-genome derived phylograms reveal a distinct polyphyletic origin of marine strains, as evidenced by their phylogenetic proximity despite diverse species affiliations. Only 1.16% of gene clusters from the total nonredundant gene repertoire were part of the core genome. Core genome size is shaped by geographical distribution. Notably, when strains from localized regions are analyzed, the core genome expands, indicating specialized functional requirements of additional genes within that environment. In marine isolates, the core genome includes genes involved in nutrient uptake, osmoregulation, and resistance to sediment genotoxicity. Additionally, a marine province-specific core genome analysis reveals genomic adaptations essential for acclimatization across different environments, regardless of species-level taxonomy. Conclusion: Microbial genome evolution is shaped by ecological niche differentiation. The emergence and spread of habitats driven by tectonic plate movements may contribute to province-specific genomic divergence in Brevibacterium. This hypothesis merits further investigation, particularly as genomic data from deeper, geologically stable environments such as marine sediments become more accessible. https://ijm.tums.ac.ir/index.php/ijm/article/view/5671 https://ijm.tums.ac.ir/index.php/ijm/article/download/5671/1838