Benchmarking the sub-lethal chronic aquatic toxicity of an emerging biosurfactant (Sophorolipid) to a traditional amine oxide surfactant in a freshwater fish-pathogen system.

Surfactants, a broad category of amphiphilic compounds, are one of the largest categories of emerging contaminants within freshwaters, many of which have known aquatic toxicity profiles. This, together with a global push towards a greener economy that prioritises less carbon intensive methods of chemical production, has led to a rise in biosurfactants entering the market. However, their toxicity requires further assessment and benchmarking against traditional synthetic surfactants. This study benchmarks the sub-lethal chronic (31-day exposure) aquatic toxicity of an emerging biosurfactant, sophorolipid, the by-product of yeast (Candida bombicola) metabolism against a traditional surfactant, an amine oxide, widely used in household and industrial cleaning products at low concentrations of 0.2 and 0.7 mg L^-1 found in environmental settings. Utilising an established freshwater fish host-pathogen system (Poecilia reticulata-Gyrodactylus turnbulli), we assessed how these two surfactants impact fish feeding behaviour, disease susceptibility and survival. Both surfactants caused some significant reduction in disease burdens, with the synthetic amine oxide having a greater anti-pathogen effect than the sophorolipid. However, no sub-lethal effects of either surfactant were observed on feeding behaviour or survival of the fish, and pathogen infection was a much greater stressor to the fish in inhibiting feeding and survival overall. This study therefore indicates that chronic exposure of synthetic amine oxide and biobased sophorolipids does not cause sub-lethal effects at low concentrations.