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

Abstract

Biosurfactant rhamnolipid and “soil ecosystem engineers” earthworms are often used to remediate contaminated soils. However, the effect of rhamnolipid on earthworm intestinal flora and microbial community in soil with earthworms is not understood. In our study, microcosm experiments were carried out to reveal the rhamnolipid on microbial abundance, composition, metabolism and contaminant degradation capacity. The results showed that both rhamnolipids and earthworms could have positive effects on soil bacteria. Bacteria in soil with rhamnosis have higher abundance and metabolic activity than in earthworm-only soils, while the improvement of rhamnolipid in bacterial composition and their contaminant degradation capacity were lower than earthworms. Notably, these changes were further amplified when rhamnolipids and earthworms coexisted. Specifically, the (log(copies/g) value increased from 9.50 (CK) to 10.25 (E), 10.50 (R) and 11.00 (RE). Comparing the soil with only earthworms (E), the relative abundance of dominant phyla Proteobacteria increased from 41.66% to 51.67% in the soil with both rhamnolipid and earthworms (RE), and more pollutant-degrading bacteria were also enriched in soil RE. Therefore, the increase in bacterial abundance and the enrichment of degrading bacteria lead that the removal rate of soil dioxins was earthworms + rhamnolipid (RE, 77.28%) > earthworms (E, 59.83%) > rhamnolipid (R, 24.65%) > control treatment (4.71%). Moreover, the addition of rhamnolipid enhanced the abundance of bacterial functional genes involved in “Metabolism” and “Environmental information processing”. In addition, the composition and diversity of bacteria in earthworms’ gut was conspicuously affected by rhamnolipid too, and the relative abundance of Microbacterium and Shewanella increased significantly (p<0.05). In conclusion, this study reveals that rhamnolipid will prominently influence the abundance, composition and metabolism of the microbial community in earthworms’ gut and soil, further promoting the degradation rate of dioxins. This study provides theoretical support for optimizing the combined application of rhamnolipid and earthworms in bioremediation engineering and the assessment of the ecological impacts of rhamnolipids.