Comparative Analysis of Microbial Communities in Six Urban Recreational Beach Sands and Seawater in the United States and Australia

Marine microbiomes play an important role in coastal marine environments. This study examined microbial communities in beach waters and sands at six recreational beaches to identify the fingerprints of anthropogenic influences. Samples were collected from four metropolitan areas in the U.S., within Miami, Florida; Seattle, Washington; Newport Beach, California and in Australia within Adelaide. Samples were analyzed for enterococci and fungi by culture. Reverse-transcription droplet digital PCR (RT-ddPCR) was performed for pepper mild mottle virus (PMMoV). Next-generation sequencing was carried out to elucidate the microbial diversity and predict antibiotic resistance. Enterococci concentrations surpassed the U.S. EPA marine water quality guideline value in the seawater samples from Seattle and Adelaide, and fungal concentration exceeded the WHO guideline in the sand of North Star Beach, California. Low levels of PMMoV were detected in seawater and sand from multiple locations. Chloroflexi and Acidobacteria were the most abundant bacterial phyla in sand, while Marinimicrobia and Cyanobacteria dominated in seawater. Beach sand had higher bacterial and fungal diversity than seawater, of which the most abundant fungal genera include taxa of potential pathogens. Predicted antibiotic resistance genes showed high levels of beta-lactam and multidrug resistance genes in all samples. This study contributes to the understanding of anthropogenic impact on the coastal environment, emphasizing the need for human health protection measures.