Divalent metal addition restores sulfide-inhibited N2O reduction in Pseudomonas aeruginosa

Hydrogen sulfide (H2S) inhibits the last step of the denitrification process, i.e. the reduction of nitrous oxide (N2O) to dinitrogen gas (N-2), both in natural environments (marine sediments) and industrial processes (activated sludge, methanogenic sludge, BioDeNOx process). In a previously published study, we showed that the inhibitory effect of sulfide to N2O reduction in mixed microbial communities is reversible and can be counteracted by dosing trace amounts of copper. It remained, however, unclear if this was due to copper sulfide precipitation or a retrofitting of the copper containing N2O-reductase (N2OR). The present study aimed to elucidate the mechanism of the restoration of sulfide-inhibited N2O reducing activity by metal addition to a pure Pseudomonas aeruginosa culture. This was done by using other metals (zinc, cobalt and iron) in comparison with copper. Zinc and cobalt clearly alleviated the sulfide inhibition of N2OR to the same extent as copper and the activity restoration was extremely fast (within 15 min, Fig. 3) for zinc, cobalt and copper. This suggests that the alleviation of the inhibitory effect of sulfide is due to metal sulfide precipitation and thus not exclusively limited to Cu. This work also underlines the importance of metal speciation: supply of iron did not restore the N2OR activity because it was precipitated by the phosphates present in the medium and thus could not precipitate the sulfide.