Phenanthrene is among the 16 priority pollutants thus its mitigation in the environment has been a global concern. It serves as a model compound for the biodegradation study of polyaromatic hydrocarbons (PAHs) because it has both the Bay- and K-region found in most PAH pollutants. Like other PAH pollutants, different means are available for its remediation in the environment, including microbial biodegradation. The objective of this review is to provide evidence of 2,2-diphenic acid as a biomarker for phenanthrene biodegradation. Through the adoption of critical review, it has been shown that diverse species of bacteria and fungi can utilize phenanthrenes as source of carbon and energy. This has been shown in published pathways of phenanthrene biodegradation implicating various intermediary metabolites, including 2,2’-diphenic acid, which is a downline metabolite of 9,10-dihydroxyphenanthrene. Though the 2,2’-diphenic acid has been widely demonstrated to produce carbon (iv) oxide and linked to phthalate, only a few have traced salicylic acid as its downstream molecule. 2,2’-diphenic acid mounts equivalent position to 1-hydroxy-2-naphthoic acid, metabolite that ends the upper pathway of phenanthrene degradation. The presence of 2,2-diphenic acid would indicate phenanthrene degradation by bacteria and fungi, hence can serve as a dependable biomarker of phenanthrene metabolism in a laboratory treatability study and natural habitats

Keywords: 1-hydroxy-2-naphthoic acid; Bacteria; Fungi; Intermediary metabolites; Phenanthrene