Seabed and Beach Sediments as Dynamic Genetic Interfaces

Coastal marine sediments and beach sands receive microbial and genetic inputs from wastewater discharge, urban runoff, aquaculture, wildlife, and recreational activity, yet their role as coupled microbial–genetic interfaces linking environmental processes and human exposure remains incompletely synthesized. This review integrates quantitative evidence from culture-based studies, qPCR surveys, metagenomic analyses, and multi-year monitoring investigations focused on coastal sediments and sands. Reported antibiotic resistance gene (ARG) concentrations in coastal sediments reach 2.2 × 109 copies g−1 (wet weight) for sul1 in wastewater-impacted systems, with total ARG abundances commonly ranging from 1.59 × 107 to 2.88 × 108 copies g−1 in effluent-receiving zones and tetM reported at 1.43 × 107 copies g−1. Beach sands contain measurable resistance markers, including intI1 at 9–3823 copies g−1 and blaTEM up to 14 copies g−1 in wet sand. Viable fecal indicator bacteria and pathogens have been cultured directly from sands, including Staphylococcus aureus at 0–8710 CFU g−1 and methicillin-resistant S. aureus at 0–605 CFU g−1. Collectively, the evidence indicates that coastal sediments and sands function as structured microbial and genetic reservoirs requiring integrated assessment of benthic retention, hydrodynamic redistribution, and exposure-relevant interpretation.