Effect of eucalyptus saponin and sophorolipid amendment on soil and sediment microbial communities and seed germination: potential application for PAH bioremediation

Purpose The potential toxic effects due to the application of sophorolipid and the novel saponin biosurfactant extracted from the leaves of Eucalyptus camaldulensis for the purpose of bioremediating PAH-contaminated soils and sediments were studied. Methods The phytotoxic effects of sophorolipid and eucalyptus saponin were determined based on the seed germination assays carried out using the seeds of Lactuca sativa L and compared with commonly used rhamnolipid and Tween 20 surfactants. Further, biosurfactant-induced changes in soil/sediment bacterial structure and diversity were investigated by conducting Miseq amplicon sequencing of the bacterial genes. Results Germination indices (GI) demonstrated the non-phytotoxic effects (GI > 80%) of saponin and sophorolipid biosurfactants (100–500 mg/L), while rhamnolipid demonstrated greater phytotoxicity than Tween 20 at high concentrations (500 mg/L). Saponin-amended soil resulted in greater bacterial diversity and richness compared to controls, while sophorolipid produced the opposite effect. These significant variations were not observed in sediment samples. Incubation of biosurfactants for 20 and 40 days did not result in significant changes in bacterial diversity and structure in any of the samples. Increased abundance of some of the PAH-degrading bacteria was noted at OTU level, in the presence of saponin and sophorolipid. Saponin had less impact on native soil/sediment bacteria relative to sophorolipid based on the prevalence of the significantly shifted OTUs. Conclusion As saponin and sophorolipid were shown to have no adverse impacts on the microbiome, and non-phytotoxic effects, their sustainable applications to remediate PAH-contaminated soils and sediments can be recommended.