Given their narrow associations with human civilization, including urban, agricultural and industrial settings, freshwater systems worldwide are primary recipients of contaminants from anthropogenic origins, threatening biodiversity (Dudgeon 2019). Freshwater invertebrates are typically abundant in these environments. They are easily sampled, and several species can also be raised in the laboratory. Furthermore, they have the propensity to accumulate contaminants from their environments through both aqueous and dietary routes. These traits make them ideally suited as bioindicators of environmental contamination and for the study of the mechanisms of contaminant uptake and effects. Therefore, over the last decades, several studies have investigated the bioaccumulation and toxicity of a wide range of organic and inorganic contaminants. Knowledge of the relative importance of the aqueous and dietary exposure routes is key to understanding the processes involved in contaminant uptake and organismal and ecological consequences. Although the mechanisms of aqueous uptake have received much attention in recent literature, those associated with dietary uptake are far less known. This is the case for species commonly used for biomonitoring environmental contamination such as the amphipod Gammarus fossarum, and for metals of major concern for the Water Framework Directive (WFD) such as Ag, Cd and Zn. 

To address these knowledge gaps, Gestin et al (2024) investigated the assimilation efficiency (AE) of Ag, Cd and Zn from two contrasting types of food, one plant (alder leaves) and one invertebrate (Chironomus riparius larvae) for gammarids using a pulse-chase-feeding method in a laboratory setting. Food was radiolabeled and fed for a short period to gammarids (3 to 5 hours for alder leaves and 1 hour for chironomid larvae), after which they were left to depurate for 14 days, during which period they were fed with uncontaminated alder leaves. During the depuration period, gammarids were monitored to follow radioactivity using a gamma counter. A nonlinear least squares modelling approach was used to estimate assimilation efficiencies and elimination rates of the metals from each food source.  
From this data, the authors concluded that Cd was assimilated with a higher efficiency, followed by Zn, with Ag showing the lowest AE. Their data also showed that the AE of Cd and Zn was higher when gammarids were fed alder leaves compared to chironomid larvae. In contrast, elimination rates were not different among metals but varied between food types, with metals from chironomids being eliminated more slowly than those from alder leaves. Elimination rate and AE of Ag could not be determined for gammarids fed chironomid larvae, due to undetectable radioactivity. This study highlights that the assimilation and elimination rates of metals ingested from food depend on their chemical properties and on the way the metals are stored in prey. The data needs to be interpreted by taking into consideration that since chironomid larvae were live prey, they could internalize the metals and make it more difficult to accumulate for the gammarid consumer, compared to the inert matrix of dead alder leaves. This study will contribute to improving toxicokinetic models needed to improve regulatory guidelines for metals in freshwater systems. 

References

Dudgeon, D. (2019). Multiple threats imperil freshwater biodiversity in the Anthropocene. Current Biology 29(19):R960-R967. https://doi.org/10.1016/j.cub.2019.08.002

Gestin, O., Lopes, C., Delorme, N., Garnero, L., Geffard, O., Lacoue-Labarthe, T. (2024). Assimilation efficiencies and elimination rates of silver, cadmium and zinc accumulated by trophic pathway in Gammarus fossarum. bioRxiv, 2023.07.14.549054, ver.4 peer-reviewed and recommended by Peer Community In Ecotoxicology and Environmental Chemistry. https://doi.org/10.1101/2023.07.14.549054