Background

Currently, the most prevalent technologies used in detecting toxic chemicals are gas chromatography (GC), high liquid chromatography (HPLC), and atomic absorption spectroscopy (AAS) that provide accurate measurements by applying different detection principles. However, these well-established methods require both skilled personnel and expensive equipment and cannot practically measure all the toxic chemicals in soil (Brouwer, 1991; Eom et al., 2019a). In contrast, bioassays have been one of the most useful technologies for the detection of environmental toxicity. Bioassays depend on changes in the physiological responses of living organisms to toxic chemicals. Ecotoxicological tests (ET) more precisely identify the cumulative and synergistic effects of toxic contaminants even if they fail to clearly identify all toxic chemicals (Sisinno et al., 2007).

Several toxicity bioassays are based on measurements of growth inhibition, oxygen uptake, colony formation, or luminescence for screening toxicants in industrial effluents, sediments, and soils (Selivanovskaya et al., 2010). To date, few studies exist for direct contact assessment of toxicants in soil medium. To evaluate soil toxicity, both liquid phase (soil elutriates) and solid phase bioassays (direct contact tests) are commonly used (Hubálek et al., 2007). In liquid-phase bioassays, test organisms are exposed to the elutriate of toxicants previously in solid-phase after dissolution in water or organic solvents (Gälli et al., 1994; Tarradellas et al., 1996; Maxam et al., 2000). This approach provides limited information on solids-associated toxicity because, using aqueous elutriates, the elutriating process cannot accommodate the complexity of the solid-phase of soil (Ronnpagel et al., 1995; Selivanovskaya et al., 2010). Moreover, partial dissolution of toxicants in soil or synergistic effects between toxicants and an extractant can possibly underestimate or overestimate the toxicity of contaminants in soil (Ronnpagel et al., 1995; Tarradellas et al., 1996; Selivanovskaya et al., 2010).

On the other hand, direct contact toxicity tests can measure the total toxic response of diverse types of contaminants in a soil sample. A direct contact bioassay could enables the determination of actual toxicity of contaminants in a highly dynamic and complex system (soil or sediments) much better than aqueous elutriates of solids (Ronnpagel et al., 1995).

Recently, SOB bioassays have been successfully employed in water, wastewater, soil toxicity detection and assessment (Oh et al., 2011; Van Ginkel et al., 2011; Gurung and Oh, 2013; Ahmed et al., 2019, Eom et al., 2019a). Most toxicity assessment studies have been carried out in aqueous phase while few studies have investigated soil toxicity by SOB (Gurung and Oh, 2013; Ahmed et al., 2019, Eom et al., 2019b). SOB are chemoautotrophic bacteria which grow as a biofilm on the surface of elemental sulfur particles. They have the ability to oxidize sulfur (electron donor) to sulfuric acid in the presence of oxygen (electron acceptor) as shown in Eq. 1 (Oh et al., 2011; Hassan et al., 2013).