ABSTRACT

CBET 1334359/1335673
Alexandria Boehm/Kara Nelson
Stanford University/University of California-Berkeley

Fecal indicator bacteria (FIB), such as Enterococcus and Escherichia coli, are used to assess beach water quality and serve as proxies for human pathogens. FIB concentrations in natural waters vary diurnally with concentrations and are often below assay detection limits in mid-afternoon and orders of magnitude higher at night, which has several implications. First, the time the sample is collected dramatically impacts the measured concentration, which could make the difference between compliance and noncompliance with water quality standards. Second, it is not known whether the concentrations of actual pathogens, and thus associated health risk, also experience such fluctuations. Therefore, it is critical to obtain information on the processes that control the diurnal fluctuations for FIB and human pathogens of concern. Sunlight is believed to be the major cause of the diurnal fluctuations in FIB. However, the dominant mechanisms through which sunlight damages microorganisms are not well understood. At least three mechanisms have been described: endogenous direct damage to cellular components by ultraviolet wavelengths, and indirect endogenous and exogenous photoinactivation caused by reactive species generated inside and outside the cell, respectively, when photons are absorbed by sensitizer molecules. Research to date has primarily focused on FIB photoinactivation and has generally been highly empirical and site-specific so that it is not possible to generalize to predict sunlight inactivation rates in other environmental contexts or for other organisms. Additionally, there is a striking lack of data on the photoinactivation of bacterial pathogens. The objectives of this project are to characterize the susceptibility of FIB and a suite of pathogenic bacteria to endogenous and exogenous photoinactivation and develop a quantitative model for photoinactivation. Laboratory experiments will be used to develop a mechanistic understanding of processes that control inactivation, and to understand the nature of observed differences between organisms. Field and laboratory data will be incorporated into a model to predict inactivation rates, and the model will be tested using a microcosm study. The model will use environmental parameters as inputs to estimate the inactivation of bacteria by sunlight and will be useful for estimating inactivation rates for a wide range of organisms and waters without the need for site- and organism- specific studies.

The project will advance knowledge in several ways. The research will yield essential insights into the fate of FIB and bacterial pathogens in the environment, a high priority research need to protect human health and improve coastal water quality. The work will have immediate implications for the management of recreational water for the protection of human health. The improved understanding of sunlight-mediated inactivation mechanisms and the new modeling approach will also be directly useful for engineered and natural systems in which sunlight plays a major role in disinfection, including solar disinfection of drinking water (SODIS) and wastewater treatment in ponds and wetlands. The results from the proposed work will be shared with policy makers and beach managers and will result in the improved protection of human health. The investigators will integrate the results into their classroom instruction. Graduate and undergraduate students will participate in the research. The investigators will develop new curriculum on the impact of sunlight on the treatment of stormwater runoff for high school students and a module on water and environmental engineering for elementary school students.

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