A rhamnolipid biosurfactant increased bacterial population size but hindered hydrocarbon biodegradation in weathered contaminated soils

Surfactants are used to enhance the bioavailability of recalcitrant residual petroleum contamination during bioremediation. However, surfactants in some cases inhibit biodegradation, which is often attributed to their toxicity. Herein, we showthat a rhamnolipid biosurfactant likely served as a carbon source and exhibited physiological inhibition on petroleum biodegradation. The addition of biosurfactants in mixed, batch, slurry bioreactors with soils from a petroleum-contaminated site led to a dose-dependent shift in the microbial community with a decrease in diversity and increase in population size and delayed biodegradation. Microbial community analysis indicated the enrichment of Alphaproteobacteria affiliated taxa such as Sphingomonadaceae in systems amended with biosurfactant. The diversitywas significantly lower in systemswith higher doses of biosurfactants compared to systems without biosurfactant. Droplet Digital PCR indicated a 30-90 fold increase in 16S rRNA copy numbers in systemswith higher doses of biosurfactant than control systemswithout surfactant and nutrients, whereas the nutrient amendment alone led to a two-fold increase in population size. Total petroleum hydrocarbon analysis showed that the biodegradation extent was negatively impacted by rhamnolipid at the highest dose compared to lower doses (23% vs. 40%) or without the biosurfactant. Indigenous isolates cultivated from the oil-amended soil exhibited growth on rhamnolipid as a sole carbon source. A novel insight gained is how dose-dependent responses of microbial communities to biosurfactants alter the biodegradation time profile of hydrocarbons. The study highlights the significance ofmicrobial assessment prior to surfactant-mediated bioremediation practices. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

Surfactants are used to enhance bioavailability of petroleum contamination during bioremediation, but rhamnolipid biosurfactant can inhibit petroleum biodegradation by serving as a carbon source. Addition of biosurfactants led to dose-dependent shift in microbial community, reducing diversity and increasing population size, thus delaying biodegradation. The study highlights the importance of assessing microbial responses to surfactants before bioremediation practices.