Proposal for new halophile classification system based on Statistical Rarity definition of extremophiles

Although current classification systems for halophilic and halotolerant microorganisms have contributed significantly to the field, there remain opportunities to enhance their theoretical foundation and broader adoption. In this study, we introduce a new classification system grounded in the Statistical Rarity definition of extremophiles. To support this proposal, a comprehensive bibliographic survey of 1,298 species described as 'halophile', 'halophilic', or 'halotolerant' from 1967 to 2021 was conducted. Of these, 1,196 species with complete optimal NaCl growth data were analyzed using the Counting Method to determine species frequency across NaCl concentrations. Quartile metrics were applied to define new subclasses: halotolerant (< 0.6 mol·L ^− 1 NaCl), slight halophile (0.6–1.2 mol·L ^− 1 ), moderate halophile (1.2–2.2 mol·L ^− 1 ), and extreme halophile (> 2.2 mol·L ^− 1 ). The proposed system reclassifies a significant portion of species and emphasizes biological relevance by correlating rarity of optimal growth conditions with extremity. This paradigm shift addresses inconsistencies in halophile classification and standardizes the definition of halophilic and halotolerant microorganisms, providing theoretical meaning to those terms.