Search

A historical perspective is given of the current paradigm that does not explicitly protect nonhuman biota from radiation but instead relies on the concept that if dose limits are set to protect humans, then the environment is automatically protected as well. We summarize recent international questioning of this paradigm and briefly present three frameworks for protecting biota that are being considered by the U.S. Department of Energy, the Canadian Nuclear Safety Commission, and the International Commission on Radiological Protection. We point out a controversial component in each of the three frameworks and suggest topics that need additional research. We emphasize that to properly address radiation protection of the environment, we need to understand how effects are integrated across different levels of biological organization. We caution that the proposed use of molecular end points to estimate ecological risks from radioactive contamination is applicable only if we understand the extent of the impact that molecular damage has on individual organisms and populations of exposed biota. To accomplish the latter, enhanced collaborations are required among the traditionally separate disciplines of radiation biology and radiation ecology.

Using a whole-chromosome FISH painting probe we previously developed for chromosome 1 of the yellow-bellied slider turtle (Trachemys scripta), we investigated the dose-rate effect for radiation-induced symmetrical translocations in T. scripta fibroblasts and lymphocytes. The dose rate below which no reduction in effect per unit dose is observed with further dose protraction was approximately 23 cGy h(-1). We estimated the whole-genome spontaneous background level of complete, apparently simple symmetrical translocations in T. scripta lymphocytes to be approximately 1.20 x 10(-3)/cell projected from aberrations occurring in chromosome 1. Similar spontaneous background levels reported for humans are some 6- to 25-fold higher, ranging from about 6 x 10(-3) to 3.4 x 10(-2) per cell. This relatively low background level for turtles would be a significant advantage for resolution of effects at low doses and dose rates. We also chronically irradiated turtles over a range of doses from 0-8 Gy delivered at approximately 5.5 cGy h(-1) and constructed a lymphocyte dose-response curve for complete, apparently simple symmetrical translocations suitable for use with animals chronically exposed to radiation in contaminated environments. The best-fitting calibration curve (not constrained through the zero dose estimate) was of the form Y(as) = c + aD + bD(2), where Y(as) was the number of apparently simple symmetrical translocations per cell, D was the dose (Gy), a = (0.0058 +/- 0.0009), b = (-0.00033 +/- 0.00011), and c = (0.0015 +/- 0.0013). With additional whole-chromosome probes to improve sensitivity, environmental biodosimetry using stable chromosome translocations could provide a practical and genetically relevant measurement end point for ecological risk assessments and biomonitoring programs.

Nonhuman organisms are being exposed to ionizing radiations at radionuclide-contaminated sites around the world. Direct methods are seldom available for measuring biologically relevant doses received by these organisms. Here we extend biological dosimetry techniques, which are much better developed for humans and a few other mammalian species, to a nonmammalian species. Turtles were chosen because a long-lived animal would best serve the need for low-level, chronic exposure conditions. We chose the yellow-bellied slider turtle (Trachemys scripta), which is known to have a maximum life span of at least 22 years. As reported elsewhere, we first isolated an embryonic fibroblast cell line and constructed whole-chromosome-specific DNA libraries for chromosome 1 by microdissection and PCR. A FISH painting probe was prepared and used to establish a dose-response curve for ionizing radiation-induced chromosome interchange aberrations in turtle fibroblasts. This was compared to the dose response for human fibroblasts treated under similar conditions in our laboratory. With respect to induction of chromosome interchange aberrations, human fibroblasts were approximately 1.7 times more sensitive than the T. scripta fibroblasts. To the extent that symmetrical interchanges are persistent over long periods, this approach could eventually provide a measure of the integrated lifetime dose these organisms receive from radionuclides in their environment and give a measure of the extent of relevant genetic damage over that time.

Some major principles and the status of knowledge concerning the transport of radionuclides through terrestrial ecosystems are reviewed. Fundamental processes which control the flow of radionuclides between ecosystem components such as air, soil, plants, and animals are described, with emphasis on deposition, resuspension, plant uptake, ingestion, and assimilation. Properties of radionuclides, organisms, and ecosystems are examined in relation to their influence on the accumulation of radioactive materials by plants and animals. The effects of the physicochemical nature of the radionuclide; morphology, physiology, and behavior of the organism; and soil, nutrient, and trophic characteristics of the ecosystem are highlighted. Observations in natural ecosystems on radionuclides such as 137Cs, 90Sr, 131I, 3H, and 239Pu are used to illustrate current concepts. An assessment of the degree to which the processes controlling radionuclide behavior are understood and of our ability to simulate and predict such behavior with computerized models is offered. Finally, brief comments are made on research needs.