Determining metal assimilation efficiency in aquatic invertebrates using enriched stable metal isotope tracers

We employ a novel approach that combines pulse-chase feeding and multi-labelled stable isotopes to determine gut passage time (GPT), gut retention time (GRT), food ingestion rate (IR) and assimilation efficiency (AE) of three trace elements for a freshwater gastropod. Lettuce isotopically enriched in ⁵³Cr, ⁶⁵Cu and ¹⁰⁶Cd was fed for 2 h to Lymnaea stagnalis. The release of tracers in feces and water was monitored for 48 h, during which unlabelled lettuce was provided ad libidum. The first defecation of ⁵³Cr occurred after 5 h of depuration (GPT), whereas 90% of the ingested ⁵³Cr was recovered in the feces after 22.5 h of depuration (GRT). ⁵³Chromium was not significantly accumulated in the soft tissues upon exposure. In contrast, ⁶⁵Cu and ¹⁰⁶Cd assimilation was detectable for most experimental snails, i.e., ^65/63Cu and ^106/114Cd ratios in exposed snails were higher than those for controls. Food IR during the labelled feeding phase was 0.16 ± 0.07 g g⁻¹ d⁻¹. IR was inferred from the amount of ⁵³Cr egested in the feces during depuration and the concentration of ⁵³Cr in the labelled lettuce. Assimilation efficiencies (±95% CI) determined using mass balance calculations were 84 ± 4% for Cu and 85 ± 3% for Cd. The ratio method yields similar AE estimates. Expanding the application of this novel stable isotope tracer technique to other metals in a wide variety of species will provide unique opportunities to evaluate the interplay between digestive processes and dietary influx of metals. Understanding the biological processes that modulate dietborne metal uptake is crucial to assess the toxicity of dietborne metals.