Occurrence and possible sources of antibiotic resistance genes in seawater of the South China Sea

Antibiotic resistance genes (ARGs) might have great effect on ecological security and human health. Oceans are important reservoirs that receive tremendous amounts of pollutants globally. However, information on the proliferation of ARGs in seawater is still limited. This study performed field sampling to investigate the occurrence and distribution of ARGs in seawater of the South China Sea, which is the deepest and largest sea in China. The results showed that the total absolute abundances of ARGs in seawater samples ranged from 2.1 x 103 to 2.3 x 104 copies/mL, with an of 5.0 x 103 copies/mL and a range of 2.2 x 103-1.8 x 104 copies/mL for those with mobile genetic elements (MGEs). Genes resistant to multidrug, aminoglycoside, tetracycline, and fluoroquinolone antibiotics accounted for 77.3%-88.6% of total ARGs in seawater. Proteobacteria and Cyanobacteria represented 32.1%-56.2% and 30.4%-49.5% of microbial community, respectively. Prochlorococcus_MIT9313 and Clade_la were the prevalent genera in seawater of the South China Sea. Complex co-occurrence relationship existed among ARGs, MGEs, and bacteria. Anthropogenic activities had critical influence on ARGs and MGEs. Hospital wastewater, wastewater treatment plant effluent, sewage, aquaculture tailwater, and runoff were determined as the important sources of ARGs in seawater of the South China Sea based on positive matrix factorization analysis.