Selenium (Se) is an essential trace element required for various functions in animals including maintaining immunity, stimulation of growth and reproduction (Mansour et al., 2017), and has been widely used in the livestock industry (Tinggi, 2003). However, due to its relatively narrow safety range for animals in the organisms (Debruyn and Chapman, 2007), it can be highly toxic to aquatic organisms. Conclusive evidence for its toxicity at elevated concentrations to aquatic organisms has been obtained from the two heavily Se-contaminated aquatic ecosystems, i.e., the Belews Lake and Kesterson Reservoir, which have witnessed the extinction of a number of apex predatory species (fish and birds) in the aquatic food chain (Hamilton, 2004; Lemly, 2004). Meanwhile, laboratory experiments have documented a variety of adverse effects of Se in different aquatic organisms, including teratogenicity (Wang et al., 2020), neurotoxicity (Li et al., 2021), oxidative stress (Janz et al., 2010), the composition of microbial community (Liu et al., 2022), etc.
Elemental selenium is insoluble in water and is generally difficult to be oxidized and reduced, Se from natural and anthropogenic sources is present in the aquatic ecosystem mainly as highly soluble inorganic (i.e., selenite (Se(IV)) and selenate (Se(VI))) and organic Se (seleno-methionine (Se-Met), seleno-cystine (Se-Cys), and other Se-substituted analogs of organosulfur compounds) (Hyne et al., 2002; Maier et al., 1988). Inorganic Se derived from anthropogenic activities such as the coal fly ash and agricultural drainage (Schwartz et al., 2016) can adsorb onto surfaces of sediment minerals and organic matter once it is discharged into aquatic environments (Wang and Chen, 2003). Organic Se, such as Se-Met and Se-Cys, mainly biogenically derives from the death of organisms (Hyne et al., 2002) and is preserved in the sediments under moderately reduced conditions (Hamilton, 2004). It is well accepted that sediment is a major sink or source of Se (Canton and Van Derveer, 1997). Se in the sediments has been ubiquitously detected at ~ 1.0 µg/g ranging from < 0.08 to 39 µg/g (Lemly, 1997; May et al., 2008; Sun et al., 2021), with maximum values of 100 µg/g (Lemly, 1997) and 210 µg/g (Presser and Ohlendorf, 1987) in the sediments from the Belews Lake and Kesterson Reservoir, respectively. Meanwhile, Se in the sediments is readily bioaccumulated by benthic organisms via the dietary exposure route, and could be subsequently transferred to higher trophic level species (Orr et al., 2006). Se levels in aquatic organisms range from 0.2 to 70 µg/g (Hamilton, 2004; Jasonsmith et al., 2008), and could be up to 250 µg/g in Se-contaminated environments (Jasonsmith et al., 2008).
Se speciation in the sediments is a complicated process (Fujita et al., 2005), which is crucial for its bioavailability and the ensuing toxicity in animals. The toxicity of organic Se has been documented to be higher than inorganic Se in aquatic organisms (Davis, 1988; Maier et al., 1993; Xie et al., 2016), most probably because it is more readily bioavailable than the latter. For example, the level of total Se in the Lumbriculus variegatus exposed to 15 µg/L Se-Met for 2 weeks is ~ two orders of magnitude higher than that exposed to 15 µg/L Se(IV) and Se(VI) (Xie et al., 2016). However, the speciation and toxicity of different Se species in the sediments are largely unknown. The main objectives of the present study were to explore 1) the speciation of different Se species in the sediments for short and long duration; 2) the biodynamics of Se and acute lethal toxicity of Se(IV) and Se-Met in the oligochaete L. variegatus.