Escherichia coli and enterococci have been previously reported to differ in the mechanisms
and conditions that affect their sunlight-mediated inactivation in waste stabilization
ponds. This study was undertaken to further characterize these mechanisms, using
simulated sunlight and single strains of laboratory-grown E. coli and Enterococcus faecalis,
with a focus on characterizing the contribution of exogenous reactive oxygen species to the
inactivation process. We found that direct damage by UVB light (280e320 nm) was not a
significant inactivation mechanism for either organism. E. coli inactivation was strongly
dependent on dissolved oxygen concentrations and the presence of UVB wavelengths but
E. coli were not susceptible to inactivation by exogenous sensitizers present in waste stabilization
pond water. In contrast, E. faecalis inactivation in pond water occurred primarily
through exogenous mechanisms, with strong evidence that singlet oxygen is an important
transient reactive species. The exogenous mechanism could utilize wavelengths into the
visible spectrum and sensitizers were mainly colloidal, distributed between 0.2 and w1 mm
in size. Singlet oxygen is likely an important endogenous species in both E. faecalis and E.
coli inactivation due to sunlight. Although the two organisms had similar inactivation rates
in buffered, clear water, the inactivation rate of E. faecalis was 7 times greater than that of E.
coli in air-saturated pond water at circumneutral pH due to its susceptibility to exogenous
sensitizers and longer wavelengths.