Beyond arsenite: Arsenite-oxidizing prokaryotes drive sulfur compound oxidation.

Arsenite-oxidizing prokaryotes (AOPs) contribute significantly to the biogeochemical processes governing arsenic cycling. Since AOPs can oxidize As(III) to As(V), markedly reducing arsenic toxicity and decreasing its migration potential, they have been widely utilized in the restoration of As(III)-contaminated environment and in the design of bioreactors for treating As(III)-contaminated groundwater. However, it is still unclear whether AOPs possess additional catalytic activities that may interfere with their bioremediation capacity, highlighting a critical knowledge gap that warrants further investigation. Because arsenic often coexists with sulfur, it was hypothesized that AOPs may also oxidize reduced sulfur compounds. To verify this hypothesis, an AOP-enriched culture was successfully established. Metagenomic analysis revealed that ∼96.1% of the AOP metagenome-assembled genomes (MAGs) contained at least one pathway for the oxidation of reduced sulfur compounds, including sulfide, thiosulfate, or sulfite. Functional assays using both the AOP community and three cultivable AOP strains demonstrated that AOPs actively catalyzed S2- oxidation coupled with NO3- reduction to NH4+ under anaerobic conditions, leading to complete S2- oxidation to SO42- and a marked decrease in pH from ∼7 to ∼3. In addition, AOP also directly degraded arsenopyrite, releasing As(V) and SO42- and causing acidification (pH 2.5). The findings from this study, for the first time, reveal that AOP possesses not only As(III) oxidation capability but also reduced sulfur compound oxidation activity that may lead to environmental acidification under anaerobic conditions, highlighting the need for extreme caution when applying AOPs in arsenic bioremediation.