Effects of rhamnolipids on bacterial communities in a dioxin-contaminated soil and the gut of earthworms added to the soil

The biosurfactants rhamnolipids and the "soil ecosystem engineers" earthworms are often used to remediate contaminated soils. However, the effects of rhamnolipids on earthworm intestinal flora and microbial community in soil containing earthworms are not clearly understood. In our study, a 21-d microcosm experiment was carried out to reveal the effects of rhamnolipids on microbial abundance, composition, and metabolism, as well as contaminant degradation capacity. Both rhamnolipids and earthworms had positive effects on soil bacteria. Rhamnolipid-amended soil (RT) showed higher bacterial abundance and metabolic activity than earthworm-amended soil (ET), while the improvement in bacterial composition and contaminant degradation capacity by rhamnolipids was lower than that by earthworms. Notably, these effects were further amplified by the combined treatment of rhamnolipids and earthworms (RET). Specifically, the bacterial abundance (log-transferred) increased from 9.5 copies g(-1) in the control with no addition to 10.3, 10.6, and 11.1 copies g(-1) in ET, RT, and RET, respectively. Compared to ET, the relative abundance of the dominant phylum, Proteobacteria, increased from 41.66% to 51.67% in RET, and more pollutant-degrading bacteria were also enriched in RET. Therefore, the increases in bacterial abundance and contaminant-degrading bacteria led to the following ranking of soil dioxin removal rate: RET (77.28%) > ET (59.83%) > RT (24.65%) > control (4.71%). Moreover, the addition of rhamnolipids enhanced the abundance of bacterial functional genes involved in metabolism and environmental information processing. In addition, the composition and diversity of bacteria in the gut of earthworms were conspicuously affected by rhamnolipids, and the relative abundance of Microbacterium and Shewanella increased significantly (P < 0.05). Therefore, this study revealed that rhamnolipids remarkably influenced the abundance, composition, and metabolism of the microbial community in earthworm gut, further promoting the degradation rate of dioxin, providing theoretical support for optimizing the combined application of rhamnolipids and earthworms in soil bioremediation engineering and for the assessment of the ecological impact of rhamnolipids.