A new enrichment strategy of dissimilatory iron-reducing bacteria for remediation of organic-contaminated river sediments: Process, performance, and mechanism

Dissimilatory iron-reducing bacteria (DIRB) have been widely applied to organic matter metabolism in sediments due to their specific biological properties, which are significantly affected by environmental factors. Until now, there has been no systematic study on applying these effects of environmental factors to DIRB enrichment. In this study, we identified the critical environmental factors through RDA, PLS-PM, and correlation analysis and then adjusted the proportion of these factors (C/Fe, C/N, and Fe/S) through single-factor and response surface experiments to enrich DIRB. The performance of the enriched DIRB in carbon metabolism of organic-rich sediments was analyzed by remediation experiment, and the metabolic mechanism was revealed through iron-reducing performance, community structure, and functional genes. The results showed that the carbon, nitrogen, phosphorus, and sulfur were crucial factors influencing the activity of DIRB in river sediments, and the enriched mud-bacteria mixture containing a large amount of DIRB (referred to as “DMB”) obtained by domestication under C/Fe = 0.492 and C/N = 13.659 showed the highest iron reduction rate of 55.51% ± 2.53%. The DMB exhibited better-sustained removal of total organic matter (TOM), which was higher than the control (MB) by 8.81%. Moreover, we discovered that the enriched DIRB: (1) had a better reduction effect on different Fe(III) forms, especially in carbon-limited and iron-limited conditions. (2) increased in abundance and established a beneficial symbiotic relationship with hydrolytic acidifying bacteria. (3) showed an increased abundance of functional pathways associated with organismal systems and signal processing (such as ABC transporters and Translation). These findings revealed the reasons for the improved efficiency of organic matter metabolism.