A Micro-Algal Solid-Phase Test To Assess the Toxic Potential of Freshwater Sediments

Abstract

Abstract To assess the degree of toxicity of freshwater sediments which may arise from both readily available and ad(ab)sorbed contaminants, we have developed a novel "direct contact" solid-phase assay with the widely used Chlorophyte Selenastrum capricornutum. With this procedure, algal cells were exposed for 4 or 24 h to serial dilutions of test sediments. The capacity of exposed cell esteras-es to cleave the non-polar stain fluorescein diacetate and liberate fluorescein, a polar and fluorescent by-product, then becomes a criterion to determine the extent to which the algae have been intoxicated by the sediment. Individual cell fluorescence (from fluorescein) is rapidly and precisely quantified with the help of flow cytometry to determine a toxicity endpoint which relates to both esterase inhibition and cell membrane integrity. The algal solid-phase assay (ASPA) was appraised with certified reference material sediments prepared by the National Water Research Institute in Burlington as well as with some naturally contaminated sediments originating from various locations in the Québec portion of the Saint-Lawrence River. ASPA toxicity responses (IC50s) generated for sediments span over more than two orders of magnitude, indicating its apparent discriminatory power to demarcate their toxic potential on the basis of their contamination level. Correlation analysis, conducted with selected certified reference material sediments, suggests that ASPA responses may be partly linked to PAH and total PCB hold content. While further validation studies are planned for ASPA, it appears to promise as a useful phytotoxicity screening tool to assess the toxic potential of freshwater sediments.