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Effect of freshwater sediment characteristics on the persistence of fecal indicator bacteria and genetic markers within a Southern California Watershed.
Citation:
Zimmer-Faust, A., V. Thulsiraj, C. Marambio Jones, Y. Cao, J. Griffith, P. Holden, AND J. Jay. Effect of freshwater sediment characteristics on the persistence of fecal indicator bacteria and genetic markers within a Southern California Watershed. WATER RESEARCH. Elsevier Science Ltd, New York, NY, 119:1-11, (2017).
Impact/Purpose:
Fecal indicator bacteria (FIB) are used as proxies for the myriad of pathogens present in fecal matter, and their presence has been linked to adverse health effects. However, FIB do not provide information regarding the source of fecal contamination. Quantitative polymerase chain reaction (qPCR) quantification of source-associated DNA markers have greatly advanced microbial source tracking (MST) efforts. Molecular MST methods enable more successful source tracking by allowing for same-day water quality monitoring results and information regarding the source of the fecal contamination. However, DNA-based MST markers are poorly understood both in terms of how they decay in the environment and how their decay rates compare to the decay rates of FIB and pathogens. In this study, the relative aging of FIB and genetic markers for Enterococcus spp. (ENT1A), general Bacteroides (GenBac3), and human-associated Bacteroides (HF183) in six different freshwater sediments was evaluated. The main objectives were 1) to determine whether culturable FIB have decay rates comparable to genetic markers with respect to varied freshwater sediment characteristics, 2) to investigate the role of ambient microbiota on persistence of genetic markers and FIB in freshwater sediments, and 3) to assess the variability in sediment decay rates and implications for water quality within one Southern California Watershed.
Description:
In this study, the relative aging of FIB and genetic markers for Enterococcus spp. (ENT1A), general Bacteroides (GenBac3), and human-associated Bacteroides (HF183) in varying freshwater sediments was evaluated. Freshwater sediment was collected from four different sites within the upper and lower reach of the Topanga Canyon Watershed and two additional comparator sites within the Santa Monica Bay, for a total of six sites. Ambient and oven-dried (reduced microbiota) sediment was inoculated with 5% sewage and placed into pyrex beakers with artificial freshwater for a 21-day period. There were substantial differences in decay between the sediments tested, and decay rates were related to sediment characteristics. In the oven-dried sediments, decay rates were significantly higher and increases in culturable FIB and the GB3 and ENT1A marker levels were observed. In the ambient sediments, smaller particle size and higher levels of organic matter and nutrients (nitrogen and phosphorus) were associated with increased persistence of the GB3 marker and culturable FIB (cEC and cENT). The HF183 marker exhibited decay rates of -0.50 to -0.96 day-1, which was 2-5 times faster in certain ambient sediments than decay of FIB and the ENT1A and GB3 markers. The GB3 and ENT1A markers decayed at rates of between -0.07 to -0.28 and -0.10 to -0.44 day-1 and culturable E. coli (cEC) and enterococci (cENT) decayed at rates of between -0.22 and -0.80 and -0.03 and -0.40 day-1, respectively. A simplified two-box model using the HF183 marker and cENT decay rates generated from the microcosm experiments was applied to two reaches within the Topanga Canyon watershed in order to provide context for the variability in decay rates observed. The model predicted lower ambient concentrations of enterococci in sediment in the upper (76 MPN g-1) versus lower Topanga watershed (513 MPN g-1) and low ambient levels of the HF183 marker (below the LLOQ) in sediments in both lower and upper watershed sediments under baseflow conditions. These results were in accord with field observations; in situ sediment and water samples were collected within the Topanga watershed and processed for genetic markers (ENT1A and HF183) and culturable FIB. It is important to consider the variable influence that sediment characteristics can have on the persistence of genetic markers and FIB when evaluating indicators of fecal contamination in sediments, even within one watershed.
