Unraveling the arsenite response mechanisms in the facultative anaerobe Aromatoleum sp. CIB

Arsenite is a highly toxic metalloid for living organisms; however, numerous microorganisms have evolved effective mechanisms to adapt to arsenic stress. The facultative anaerobic betaproteobacterium Aromatoleum sp. CIB exhibits a good level of arsenite tolerance, mediated by the coordinated action of multiple molecular systems involved in detoxification and cellular homeostasis. Here, we identified the arsSM genes, encoding enzymes responsible for the production of methylated arsenic species and putative precursors of arsenosugar biosynthesis. The organization of these genes within the arsSM operon suggests a role in arsenic homeostasis, potentially supported by their constitutive expression. This finding extends the known taxonomic distribution of arsSM-mediated arsenic transformation mechanisms beyond cyanobacteria and supports horizontal gene transfer as a likely route for arsSM operon acquisition. Moreover, we present the first comparative analysis of arsenite responses under aerobic and anaerobic conditions within a single bacterial species, revealing distinct physiological constraints and adaptive strategies. Transcriptomic profiling of Aromatoleum sp. CIB exposed to arsenite revealed a stronger global transcriptional response under aerobic conditions. Although canonical arsenic resistance genes (the ars cluster) were induced under both conditions, oxygen availability markedly intensified the global stress response, particularly oxidative stress-related pathways. In contrast, genes encoding protein-folding chaperones were preferentially upregulated under anaerobic conditions. These results indicate a context-dependent reorganization of cellular functions in response to arsenite stress, favoring survival over growth. Overall, this study highlights the complexity and flexibility of bacterial arsenic resistance and provides insights relevant to microbial ecology and arsenic biogeochemical cycling.