Effects of humic acid on pentachlorophenol biodegrading microorganisms elucidated by stable isotope probing and high-throughput sequencing approaches

Humic substances (HSs) are ubiquitous in soil and can be reversibly oxidized and reduced by acting as redox mediators. They participate in microbial metabolism and directly affect the biodegradation of organic pollutants. However, the effects of HSs on specific microbial taxa that are responsible for the degradation of organic pollutants remain unclear. In this research, the effects of humic acids (HAs), extracted from three types of soil, forest (CBHA), paddy (PSHA) and peat (YNHA) on the microbial community involved in anaerobic mineralization of pentachlorophenol (PCP) were investigated with high-throughput sequencing and stable isotope probing (SIP) approaches. The results showed that all HA samples accelerated the biotransformation processes of PCP; the largest rate was obtained with YNHA. Illumina sequencing revealed that Desulfovibrio and Clostridium were the dominant functional bacteria for the dechlorination of PCP. During the subsequent PCP degradation and mineralization processes, HAs affected the diversity and abundance of microbial communities, and several taxa were enriched in the C-13-heavy fractions compared with C-12-heavy fractions. In the absence of HA, Methanobacterium and Spartobacteria showed a significant increase in C-13-heavy fractions. Methanosarcina and OP11 were the dominant PCP degraders in microcosms when amended with CBHA, whereas Burkholderia and Methanobacterium were the key PCP degraders in PSHA- and YNHA-amended experimental microcosms. These results improve our knowledge of the diversity and ubiquitous nature of HS-utilizing microorganisms involved in PCP degradation, providing scientific support for the development of in situ bioremediation technologies for HS-rich soil contaminated by PCP.