Toluene bioconversion into ectoines by halophile mixed microbial cultures

Toluene, which has been listed in the Pollutant Release and Transfer Register (PRTR) of many countries, is one of the most emitted pollutants to the atmosphere. This study demonstrates for the first time a new perspective in toluene treatment based on its continuous bioconversion into high-value chemicals, specifically ectoine and hydroxyectoine, which hold considerable commercial relevance in the cosmetic industry with market prices reaching 1000 € kg^-1. Specific ectoine and hydroxyectoine contents of 27.3 mg g_TSS^-1 were achieved together with toluene elimination capacities of 7.2 ± 1.9 g m^-3 h^-1 and a maximum biomass concentration of 1.8 g L^-1. Ectoine synthesis predominated initially, later shifting toward hydroxyectoine, reaching a combined amount of 71.2 mg L^-1 (ectoine:hydroxyectoine 32:68) by the end of the assay. Metagenomic analysis revealed key pathways and taxa involved in toluene degradation and ectoine and hydroxyectoine synthesis. Members of Paenibacillus, Rhodococcus and Microbacterium were identified as possessing the enzymes required for toluene degradation via the TOL pathway, while Gordonia, the most abundant genus, was primarily associated with the degradation of intermediates such as benzoate, muconate, or oxoadipate derivatives and their bioconversion into ectoine. These findings revealed a potential metabolically diverse consortium with functional complementarities, where metabolic synergies overcome species-specific limitations and promote the elimination and subsequent valorization of toluene into high-value products fostering sustainable industrial innovation.