Bacterial Sulfur Sensors Control Denitrification in Wastewater Treatment

This study reveals an unexpected connection between sulfur sensing and nitrogen removal in bacteria critical for environmental health. Researchers at the University of Bonn investigated two related proteins (sHdrR and SoxR) in Hyphomicrobium denitrificans, a bacterium that removes nitrates from wastewater and drinking water systems.

Using RNA sequencing and targeted gene expression analysis, the team found that these sulfur-sensing proteins regulate far more than anticipated. SoxR controls 170 genes while sHdrR affects 165 genes, with 138 genes overlapping between the two regulators. Most surprisingly, both proteins significantly impact denitrification - the bacterial process that converts harmful nitrates to harmless nitrogen gas.

The proteins work by sensing sulfane sulfur compounds through conserved cysteine residues. When sulfur is present, the proteins change shape and release their grip on DNA, allowing gene expression to proceed. This mechanism coordinates the bacterium's ability to use both sulfur compounds and nitrates as energy sources.

Phylogenetic analysis revealed these regulatory systems are widespread across diverse bacterial groups, suggesting this sulfur-nitrogen regulatory connection may be common in environmental bacteria. The researchers identified similar proteins in soil and water bacteria worldwide, indicating this regulatory mechanism has broad ecological significance.

These findings have practical implications for wastewater treatment optimization. Since H. denitrificans constitutes up to 0.2% of bacteria in freshwater and soil samples and is highly abundant in sewage treatment plants, understanding how sulfur availability affects its nitrate-removing capacity could improve treatment efficiency. The discovery also suggests that sulfur compound levels in treatment systems may need consideration when optimizing denitrification processes.