Microbial community assemblies for the biodegradation of mixed hydrocarbon pollutants in industrial contaminated soil

Background information: Bioaugmentation with assemblies of microbial communities displaying complementary biodegradation capabilities may represent a promising approach to address the bioremediation of industrial sites contaminated with complex mixtures of petroleum hydrocarbons and their derivatives. Moreover, bioaugmentation with autochthonous specialized microorganisms already adapted to the environmental conditions of the affected site may further favor the viability, survival and activity of introduced microbes (Nwankwegu et al., 2022). In this work, we investigated at laboratory scale the bioremediation efficiency of autochthonous bioaugmentation approach with a microbial assembly of total petroleum hydrocarbons (TPH) and polycyclic aromatic hydrocarbons (PAH) degrading communities on the unsaturated and saturated soil from an historically contaminated industrial site. Main results: Two aerobic microbial communities were enriched from soil and groundwater samples collected from a contaminated industrial site in mineral medium supplemented either with a mixture of diesel and IFO180 fuels (TPH enrichment), or a mixture of naphthalene, phenanthrene and anthracene (PAH enrichment) as sole carbon and energy source. The two cultures were then assembled and inoculated (≈1.25x108 CFU/Kgsoil each) in the unsaturated soil (solid-phase reactors) and in the saturated soil and groundwater (slurry-phase reactors) from the original contaminated site, supplemented with nitrogen and phosphorous sources and incubated under aerobic conditions. In the unsaturated soil, bioaugmentation promoted a remarkable stimulation of naphthalene (100% vs 37% in not inoculated controls after 30 days) and phenanthrene (88% vs 15% after 120 days) biodegradation. Conversely, no significant effect of bioaugmentation was observed on naphthalene and phenanthrene biodegradation in the saturated soil and groundwater, where 100% naphthalene and 70% phenanthrene degradation were achieved after 30 and 120 days, respectively, both in the inoculated and the control reactors. Remarkably, no significant TPH degradation was observed both in the control and the inoculated unsaturated and saturated soil reactors. This suggested that either limited TPH bioavailability or toxic effects of the matrix or of possible PAH degradation intermediates/products may have affected the activity of the TPH degrading fraction of the inoculated microbial assembly. Cultivation tests of the two enrichments in mineral medium supplemented with both TPH and PAHs highlighted that, while the degradation activity of the PAH enrichment is not affected by the presence of TPH, the degradation activity of the TPH enrichment is remarkably inhibited by the presence of PAH, even in co-culture with the PAH degrading enrichment and even when naphthalene degradation has been completed. Conclusions: Autochthonous bioaugmentation of a soil historically contaminated by a complex hydrocarbon mixture with an assembly of TPH and PAH degrading cultures promoted naphthalene and phenanthrene degradation. The lack of TPH degradation after bioaugmentation was attributed to a potential toxic effect of PAH, in particular of phenanthrene and/or anthracene. Sequential bioaugmentation may allow to eliminate inhibition and achieve degradation of all target pollutants.