Bioaccumulation of the pathogenic bacterium Vibrio alginolyticus by the mussel Brachidontes pharaonis in the Gulf of Suez, Red Sea, Egypt

Introduction The bivalve Brachidontes pharaonis (Fisher, 1871) is a filter-feeding mussel widely used as a coastal bioindicator and may accumulate Vibrio alginolyticus , a ubiquitous marine bacterium with pathogenic potential, from surrounding seawater. Despite its ecological relevance in the Gulf of Suez, the dynamics of this host–bacterium association remain poorly characterized. This study therefore investigated whether B. pharaonis accumulates V. alginolyticus from Gulf of Suez seawater and evaluated the ecological significance of this host–habitat association. Methods Seawater and mussel samples were collected seasonally from three sites along the Gulf of Suez, Egypt, between 2021 and 2022. Results V. alginolyticus abundance in seawater was quantified using quantitative PCR (qPCR), while its presence in mussel tissues was determined by end-point PCR, and occurrence patterns in both compartments were analyzed in relation to physicochemical variables. V. alginolyticus was consistently detected in seawater across all sites, with concentrations ranging from 1.59 to 3.27 × 10 9 gene equivalent (GE)/µL, showing modest seasonal increases in summer and marked inter-site heterogeneity. The site with intermediate mussel abundance (Kabanon) exhibited the highest bacterial prevalence in tissue (73% overall, peaking at 57% in summer), with no associated mass mortality, suggesting a mild, non-pathogenic accumulation regime. Notably, bacterial dynamics in mussel tissue, mussel-free bacteria, and seawater bacteria appeared to respond divergently to physicochemical parameters, indicating potential compartment-specific drivers. Discussion These findings indicate that B. pharaonis accumulation of V. alginolyticus may be better explained by a host–habitat interaction than by a pattern proportional to mussel density, with prevalence approximately twofold higher at the intermediate-abundance site; however, this interpretation remains to be experimentally validated. The partial decoupling of mussel-associated assemblages from water-column communities points to host-selective filtration and microhabitat heterogeneity as possible ecological drivers, underscoring the value of B. pharaonis as a sentinel species for Vibrio monitoring in coastal ecosystem s.