Temporal Stability of the Microbial Community in Sewage-Polluted Seawater Exposed to Natural Sunlight Cycles and Marine Microbiota

Abstract

Billions of gallons of untreated wastewater enter the coastal ocean each year. Once sewage microorganisms are in the marine environment, they are exposed to environmental stressors, such as sunlight and predation. Previous research has investigated the fate of individual sewage microorganisms in seawater but not the entire sewage microbial community. The present study used next-generation sequencing (NGS) to examine how the microbial community in sewage-impacted seawater changes over 48 h when exposed to natural sunlight cycles and marine microbiota. We compared the results from microcosms composed of unfiltered seawater (containing naturally occurring marine microbiota) and filtered seawater (containing no marine microbiota) to investigate the effect of marine microbiota. We also compared the results from microcosms that were exposed to natural sunlight cycles with those from microcosms kept in the dark to investigate the effect of sunlight. The microbial community composition and the relative abundance of operational taxonomic units (OTUs) changed over 48 h in all microcosms. Exposure to sunlight had a significant effect on both community composition and OTU abundance. The effect of marine microbiota, however, was minimal. The proportion of sewage-derived microorganisms present in the microcosms decreased rapidly within 48 h, and the decrease was the most pronounced in the presence of both sunlight and marine microbiota, where the proportion decreased from 85% to 3% of the total microbial community. The results from this study demonstrate the strong effect that sunlight has on microbial community composition, as measured by NGS, and the importance of considering temporal effects in future applications of NGS to identify microbial pollution sources. G lobally, swimming in and the consumption of shellfish from coastal waters polluted with sewage result in over 120 million cases of gastrointestinal (GI) illnesses, 50 million cases of respiratory illnesses, and consumption of 800 million meals of potentially contaminated shellfish each year (1). Sewage in coastal waters comes from leaking sewer lines, malfunctioning sewage treatment plants or septic systems, and overflow from combined sewer systems during heavy rainfall (2). The United States Environmental Protection Agency (USEPA) (2) estimates that there are 23,000 to 75,000 sanitary sewage overflows per year in the United States, resulting in the discharge of 3 billion to 10 billion gallons of untreated wastewater. Understanding the fate of the sewage-derived microorganisms in coastal waters is critical to identifying sewage pollution and assessing the health risks posed by sewage-polluted waters.Throughout the world, coastal waters are monitored for fecal pollution using fecal indicator bacteria (FIB), such as Escherichia coli and enterococci (3-5). FIB enumeration methods are timeconsuming (ϳ24 h), and the concentrations of FIB in coastal waters vary over hourly time scales, complicating their use for health risk assessment (6, 7). In addition, FIB can be fou...

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