Association of antibiotic resistance genes with fecal pollution sources and key environmental parameters in the coastal waters of eastern Guangdong, China

Antibiotic resistance genes (ARGs) in coastal ecosystems pose substantial threats to both environmental and human health. Thus, accurately identifying the sources of ARGs is critical for implementing targeted mitigation strategies. This study employed quantitative polymerase chain reaction to analyze seawater and sediment samples collected from the eastern coastal waters of Guangdong, China, during two sampling campaigns (September and December 2021). The aim was to investigate the spatiotemporal dynamics of ARGs, microbial source tracking (MST) markers, and physicochemical drivers. ARGs (sul1, sul2, tetW, ermF, mcr-1, bla_TEM, bla_NDM-1) exhibited near-ubiquitous detection (84 %–100 %), with sulfonamide resistance genes (sul1, sul2) dominating in seawater, while ermF and tetW dominated in sediments. Human-associated MST markers (BacHum and CPQ_056) demonstrated pronounced seasonal increases in seawater, which in turn was closely related to ARGs abundance. Principal component analysis revealed a temporal stratification of MST profiles attributed to intensified human fecal pollution in winter. Nutrients (NH₃-N and PO₄³⁻) and algal biomass (Chl-a) were identified as the key factors in winter, in contrast to the dominance of oxygen and pH in summer. Sediments played a crucial role as reservoirs for ARGs, with the sediment concentrations of ermF and tetW being 2–3 times those in seawater. The integrase gene intI1 consistently co-occurred with sul1 and sul2, which indicated that horizontal gene transfer (HGT) might serve as an important mechanism for disseminating antibiotic resistance in this region. Correlation and Mantel tests confirmed that human fecal inputs were the primary source of ARGs in the coastal area. Despite minimal contributions from ruminant-associated markers, the poultry-associated marker GFD exhibited persistent low-level presence, while the swine-associated marker P.ND5 demonstrated localized persistence in sediments. These findings underscore the urgency of integrating microbial source tracking into coastal management frameworks, with future research needed to evaluate the long-term efficacy of mitigation strategies and assess their scalability across anthropogenically stressed marine ecosystems worldwide.