Characterize the dynamic of the bacterial communities mediating iron and arsenic removal in these two bioreactors, and to explore the influence of environmental and operational drivers on their diversity and activity.. Diversity and activity in semi-passive Fe- and As-oxidation bioreactors treating arsenic-rich acid mine drainage

Semi-passive bioreactors based on iron (Fe) and arsenic (As) oxidation and co-precipitation are promising for the sustainable treatment of As-rich acid mine drainages (AMD). Two semi-passive iron oxidation bioreactors filled with distinct bacterial biomass carriers (plastic or a mix of wood and pozzolana) were monitored during one year. The objectives of the present work was to characterize the dynamic of the bacterial communities mediating iron and arsenic removal in these two bioreactors, and to explore the influence of environmental and operational drivers on their diversity and activity. The diversity of the total and active communities (both suspended and attached to the biomass carriers) was analyzed by 16S rRNA gene metabarcoding. Bacterial communities were dominated by several iron-oxidizing bacteria, in variable proportions, and shifts in the bacterial communities were attributed to both operational and physiochemical parameters including the nature of the biomass carrier, the water pH, temperature, As and Fe concentrations. The better resilience to disturbances observed for the bioreactor filled with wood and pozzolana was related to a higher bacterial richness. The bioreactor filled with the plastic biomass carrier was characterized by a more variable bacterial composition although these variations did not significantly impact the global treatment performance. These results confirmed the contribution of biological arsenite oxidation to the arsenic removal from the AMD. Our results showed that both the resilience and the functional redundancy of the suspended and attached bacterial communities developed in the two bioreactors conferred robustness and stability to the treatment systems, in terms of Fe and As oxidation and removal.