A Combined Digital PCR and Next Generation DNA-Sequencing Based Approach for Tracking Nearshore Pollutant Dynamics Along the Southwest United States/Mexico Border.

Amity G Zimmer-Faust,Xianyi Xiong,Madison Griffith,John F Griffith,Joshua A Steele,Christopher Staley,Margarita Diaz,Michael J Sadowsky

doi:10.3389/fmicb.2021.674214

Amity G Zimmer-Faust, Xianyi Xiong + Show 6 more

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https://doi.org/10.3389/fmicb.2021.674214

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Ocean currents, multiple fecal bacteria input sources, and jurisdictional boundaries can complicate pollution source tracking and associated mitigation and management efforts within the nearshore coastal environment. In this study, multiple microbial source tracking tools were employed to characterize the impact and reach of an ocean wastewater treatment facility discharge in Mexico northward along the coast and across the Southwest United States- Mexico Border. Water samples were evaluated for fecal indicator bacteria (FIB), Enterococcus by culture-based methods, and human-associated genetic marker (HF183) and Enterococcus by droplet digital polymerase chain reaction (ddPCR). In addition, 16S rRNA gene sequence analysis was performed and the SourceTracker algorithm was used to characterize the bacterial community of the wastewater treatment plume and its contribution to beach waters. Sampling dates were chosen based on ocean conditions associated with northern currents. Evidence of a gradient in human fecal pollution that extended north from the wastewater discharge across the United States/Mexico border from the point source was observed using human-associated genetic markers and microbial community analysis. The spatial extent of fecal contamination observed was largely dependent on swell and ocean conditions. These findings demonstrate the utility of a combination of molecular tools for understanding and tracking specific pollutant sources in dynamic coastal water environments.

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Once bacterial and chemical contaminants enter the nearshore coastal environment, complicated mixing, dilution, and transport processes make it increasingly difficult to identify their origin
Digital polymerase chain reaction (PCR) analysis of human-associated markers and 16S rRNA bacterial community sequencing both identified a significant gradient from the San Antonio de los Buenos (SADB) wastewater treatment plant (WTP) outfall at Punta Bandera moving south to north, when current direction was from the south, with sites closer to the discharge impacted more heavily
Conditions observed during Event 3 and Event 4 reflect the significant impact changing ocean condition can have on the impact of the SADB plume within the nearshore coastal zone

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Once bacterial and chemical contaminants enter the nearshore coastal environment, complicated mixing, dilution, and transport processes make it increasingly difficult to identify their origin. Microbial source tracking (MST) methods that differentiate amongst different specific animal and human fecal sources have been developed and their application has become more widespread. The MST marker gene approach has been used previously to effectively identify and differentiate between specific fecal sources in a variety of matrices, including estuarine (Riedel et al, 2015), fresh waters (Li et al, 2019), marine waters (Ervin et al, 2013), and sediments (Zimmer-Faust et al, 2017), resulting in recommendations for application of best management practices, specific infrastructure improvements, and regulatory support (USEPA, 2011; Verhougstraete et al, 2015; Goodwin et al, 2017)

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