this study expand the understanding of redox fluctuations in association with the degradation of recalcitrant substrates in soil and the corresponding microbiome.

The biodegradation of polychlorinated-p-dioxins and dibenzofurans (PCDD/Fs) has been recently demonstrated in a single reactor under hypoxic conditions. Maintaining hypoxic conditions through periodic aerations results in a marked fluctuation of reduction–oxidation (redox) potential. To further assess the effects of redox fluctuations, we operated two fed-batch continuously stirred tank reactors (CSTRs) with sophisticated redox controls at different anoxic/oxic fluctuations to reduce PCDD/Fs in contaminated soil. The results of long-term reactor operation showed that the CSTR with redox fluctuations at a narrow range (-111±56 mV) (CSTR_A) revealed a higher substrate hydrolysis activity level and PCDD/F degradation rate than did the CSTR with a redox potential that fluctuated at a broad range (1±114 mV) (CSTR_B). In accordance with analyses of bacterial 16S rRNA genes, the designated hypoxic conditions with added compost supported survival of bacterial populations at a density of approximately 109 cells/g slurry for both reactors. The evolved core microbiome was dominated by anoxic/oxic fluctuation-adapted Bacteroidetes, Alphaproteobacteria, and Actinobacteria, with higher diversity and complexity in addition to more abundant functionality of hydrolysis, including hydrolytic dehalogenases, and a higher degradation of dioxin-like compounds in CSTR_A than in CSTR_B. Taken together, the overall results of this study expand the understanding of redox fluctuations in association with the degradation of recalcitrant substrates in soil and the corresponding microbiome