Keystone bacteria in a thiosulfate-driven autotrophic denitrification microbial community

Keystone bacteria associated with a thiosulfate-driven autotrophic denitrification microbial community for treating high nitrate concentration wastewater were identified in this study. Five laboratory-scale anoxic bioreactors, designated as S0-S4, were fed with synthetic wastewater containing 120 mg NO– 3-N/L. Meanwhile, thiosulfate was used as the sole electron donor and provided varying ratios to these bioreactors, resulting in (S/N ratios of 0, 0.7, 2.4, 4.0, and 5.7), respectively. These bioreactors were run in a 104-day period, which was composed of eight operational cycles. Overall, bioreactors S1-4 could achieve 14.0, 56.5, 88.9, and 85.0 % nitrate removal rates, respectively. Approximately 30% of the electrons from thiosulfate were estimated to be directed to autotrophic denitrification across the bioreactors. The microbial ecology analysis revealed that Proteobacteria (42–66%), Bacteriodetes (15–30%), and Firmicutes (15–28%) were the major phyla residing in the bioreactors. Thiobacillus, Pseudomonas, Arenimonas, and Thauera were identified as predominant genera shared across the bioreactors. Thiobacillus was the dominant genus in thiosulfate-driven autotrophic denitrification, and contributed 42% to community in S4 reactor. Functional genes analysis further showed that only Thiobacillus and Thauera contained the complete nitrate reduction pathway. Higher S/N ratios directly promoted the oxidation of thiosulfate to sulfate by the sulfur oxidation (SOX) system. Due to Thiobacillus was absence of SoxCD for catalysis from SoxYZ-S-SH to SoxYZ-S-SO3, Thiobacillus and Thauera could work collaboratively on the SOX pathway as the keystone bacteria to achieve thiosulfate oxidation. But such collaboration might have led to extracellular electron transportation and casused low electron utilization for the autotrophic denitrification process.