POTENTIAL ATTENUATION OF STABLE ISOTOPE LABEL DURING PREDATION OF 15N-TAGGED WILD-TYPE ESCHERICHIA COLI BY PROTOZOA

Currently, bacterial movement in karst aquifers is not well understood. Recent studies have examined use of stable isotopes to label non-pathogenic Escherichia coli as a particulate groundwater tracer in karst systems. A preliminary trace showed promise, but was conducted over a relatively short distance (500 m). Over longer distances, loss of the stable isotope signal is anticipated. One scenario that could result in loss of the isotope signal, is bacterial predation by protozoa. Filter-sterilized water from Royal Spring in Georgetown, Kentucky was inoculated with a mixture of either Tetrahymena pyriformis or Colpoda steinii and indigenous ¹⁵N-enriched E. coli, then stored in the dark at 14°C to simulate karst conditions. Samples were analyzed for nitrogen isotope composition and population counts of bacteria and protozoa on days zero and seven after inoculation. Protozoan populations increased in the presence of E. coli, while bacterial populations decreased. δ¹⁵N values increased in T. pyriformis that were fed enriched E. coli, with values lower than the bacteria themselves. Attenuation via predation is one explanation, which may be a concern for interpreting bacterial movement in future groundwater traces.