Caenorhabditis elegans as a model for assessing behavioral effects and soil bioavailability of environmental contaminants

Abstract

The effects of metal exposures on soil ecosystems are observed at all levels of biological organization from changes in enzyme activity at the cellular level to changes in nutrient cycling at the ecosystem level. A handful of standardized toxicity tests exist to assess the effects of toxicants on soil quality; however, knowledge of the effects on various species and functional groups of organisms would allow for more realistic regulations of soil use. Nematodes are the most abundant of soil mesofauna and play significant roles in nutrient cycling and availability via microbial grazing. The free-living nematode Caenorhabditis elegans is a popular toxicity test organism in aquatic medium, sediments, and soils. The studies in this dissertation were aimed at furthering the understanding of how the results of C. elegans toxicity tests may be affected by external factors such as soil physicochemical properties and food availability. Recently, a standardized guide on the use of C. elegans in soil toxicity tests has been published by the American Society for Testing and Materials. This procedure was used to quantify the lethality of 48-h exposures of five metals (Cd, Cu, Ni, Pb, and Zn) to C. elegans in soils with varying texture, organic matter (OM) content, and cation exchange capacity (CEC). The toxicity of the metals decreased as OM and CEC decreased. A sequential soil extraction procedure was then used to recover the metals from different fractions of the soil in an attempt to estimate the bioavailability of the metals. The sensitivity and usability of two other nematode species, Panagrellus redivivus and Pristionchus pacificus, were compared to C. elegans using toxicity tests in soil and aquatic medium with various endpoints including lethality, reproduction, and movement. The sensitivity of the nematodes varied with the endpoint quantified. In a separate study, food availability alone was determined to affect the movement and feeding of C. elegans after 24-h exposures in aquatic medium. When 24-h metal (Cu, Pb, and Cd) exposures were performed at different food availabilities, the toxicity of all three metals appeared to increase as food availability decreased implicating starvation as a component of decreased movement in C. elegans behavioral assays. Decreasing the exposure to 4 h minimized the complicating effects of starvation.