Impact of chromium on the soil invertebrate model Enchytraeus crypticus (Oligochaeta) in standard reproduction and full life cycle tests

Chromium (Cr) has many applications in industry and day-to-day life, entering the terrestrial environment mostly from anthropogenic sources. Despite the fact that Cr is essential, it can be harmful in biota at high concentrations. Full life cycle tests (FLCt) are scarce, representing a gap for soil ecotoxicology. Hence, the effects of Cr were studied using the soil organism Enchytraeus crypticus in the 46-day FLCt and the standard 21-day enchytraeid reproduction test (ERT). FLCt hatching (day-11) and time to reach maturity (day 22–25) were the most sensitive endpoints, representing a delay, partly recovered with time. Reproduction was reduced to similar levels in both the ERT and the FLCt (EC₅₀∼320 mg Cr/kg), but survival was more affected in the ERT (LC₅₀ = 377 mg Cr/kg) than in the FLC (LC₅₀ = 467 mg Cr/kg). This could be due to the induction of stress response mechanisms at earlier life stages (cocoons/juveniles), providing protection to toxicity in a later stage (adults). FLCt results provided considerably more details and data points, and the additional endpoints allowed to explain the source of observed effects, e.g. a direct impact on fecundity and not only due to adult mortality. The estimated population growth curves confirmed the significant effect of 320 mg Cr/kg and onwards, reflecting the impact of Cr on both cocoon production and juvenile performance, with follow-up consequences for reproduction. An Adverse Outcome Pathway was drafted. The FLCt design is recommended as an upgrade of the current standard ERT, which could be integrated into the existing OECD ERT test guideline.