Diversity and functional potential of indigenous sulfur-oxidizing bacteria in anaerobic digesters and a Thiopaq bioreactor.

Sulfides accumulation during anaerobic digestion negatively affects biogas quality, process stability, and infrastructure integrity, making efficient desulfurization strategies essential. This study investigated sulfur-oxidizing bacterial communities associated with anaerobic digesters and a Thiopaq® bioreactor at the Marrakech wastewater treatment plant. High-throughput 16S rRNA gene sequencing revealed a marked ecological contrast between the two systems. While the Thiopaq® bioreactor was largely dominated by a single Thioalkalibacteraceae lineage, anaerobic sludge environments harbored a phylogenetically diverse assemblage of sulfur-oxidizing bacteria spanning multiple proteobacterial families, including Thiobacillaceae, Rhodobacteraceae, Hyphomicrobiaceae, Paracoccaceae, Comamonadaceae, Rhodocyclaceae, Zoogloeaceae, Azonexaceae, and Burkholderiaceae. Phylogenetic reconstruction and culture-based isolation showed only partial overlap between sequencing-derived operational taxonomic units and cultivated strains, highlighting the complementarity of molecular and physiological approaches. Functional assays demonstrated that several indigenous isolates were capable of oxidizing sulfide produced by sulfate-reducing bacteria under both microaerophilic and nitrate-reducing conditions. Sulfide concentrations decreased from 14.87 mM to 0.03 mM, corresponding to a removal efficiency of 99.8%, under denitrifying conditions and accompanied by elemental sulfur formation. These findings indicate that locally adapted sulfur-oxidizing bacteria constitute a functional reservoir that can be exploited for improved sulfide management. The results support the development of site-specific strategies, including controlled microaeration within anaerobic digesters and the use of indigenous inocula for biological desulfurization, as alternatives to exclusive reliance on commercial strains.