Selenium accumulation, speciation and bioaccessibility in selenium-enriched earthworm (Eisenia fetida)

Selenium (Se) is an essential micronutrient for humans and animals. Earthworm (Eisenia fetida) has been used as animal feed due to its high protein contents and bioactive components. In this laboratory study, earthworm substrates were treated with sodium selenite of 0, 20, or 40 mg kg−1, and further Se accumulation, speciation and bioaccessibility in earthworm tissues were determined. The results showed that in the Se treatments of 20 and 40 mg kg−1 the earthworm biomass and reproduction was not adversely affected, and total Se concentrations in earthworm tissues were 97.78 and 151.56 mg kg−1 dry weight, respectively. Se speciation in E. fetida was extracted with protease XIV, gastric and gastrointestinal enzymes and was analyzed by high performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS). Se (IV) was largely biotransformed into organoselenium metabolites, including about 45.8% in SeMet, 29.53% in SeCys2 and 7.3% in unidentified organic Se components. A small fraction of <2.23% was remained in inorganic Se (IV) form. Gastrointestinal digestion was the most efficient way to release Se from earthworm tissues, and Se bioaccessibility based on in vitro simulated gastrointestinal digestion testing was up to 90%. This study suggests that Se-enriched E. fetida could become a good source of Se supplementation in animal feed.