Endogenous biohydrogen from a rhizobium-legume association drives microbial biodegradation of polychlorinated biphenyl in contaminated soil.

Endogenous hydrogen (H) is produced through rhizobium-legume associations in terrestrial ecosystems worldwide through dinitrogen fixation. In turn, this gas may alter rhizosphere microbial community structure and modulate biogeochemical cycles. However, very little is understood about the role that this H leaking to the rhizosphere plays in shaping the persistent organic pollutants degrading microbes in contaminated soils. Here, we combined DNA-stable isotope probing (DNA-SIP) with metagenomics to explore how endogenous H from the symbiotic rhizobium-alfalfa association drives the microbial biodegradation of tetrachlorobiphenyl PCB 77 in a contaminated soil. The results showed that PCB77 biodegradation efficiency increased significantly in soils treated with endogenous H. Based on metagenomes of C-enriched DNA fractions, endogenous H selected bacteria harboring PCB degradation genes. Functional gene annotation allowed the reconstruction of several complete pathways for PCB catabolism, with different taxa conducting successive metabolic steps of PCB metabolism. The enrichment through endogenous H of hydrogenotrophic Pseudomonas and Magnetospirillum encoding biphenyl oxidation genes drove PCB biodegradation. This study proves that endogenous H is a significant energy source for active PCB-degrading communities and suggests that elevated H can influence the microbial ecology and biogeochemistry of the legume rhizosphere.