Turda HCH-contaminated bulk soil, rhizosphere and root associated microbiome

Given the toxicity, volatility and persistance of the organochlorine pesticide hexachlorocyclohexane (HCH), reclamation of contaminated areas is a priority for the health and welfare of the neighboring human communities. Microbial diversity and functions at field scale, in relation to spontaneous vegetation in post-excavation situations, are essential indicators to consider in the development of bioaugmentation or microbe-assisted phytoremediation strategies. The present study aimed to assess whether HCH contaminated soil can act as a bacterial inoculum in post-excavation bioaugmentation strategies and whether such an inoculum can improve the performance and tolerance to HCH of the early colonizer legume Lotus tenuis. In addition, effects of HCH on the potential nitrogen fixation of free-living and symbiotic diazotrophs was assessed. HCH was the main shaping factor of the microbial communities in long-term contaminated bulk soil. We identified possible HCH tolerating genera such as Sphingomonas and Altererythrobacter. However, the excavation process caused a shift in the HCH tolerant microbial community towards a higher abundance of Burkholderiaceae and Xanthomonadaceae (Lysobacter) to the detriment of sphingomonads. The L. tenuis rhizosphere microbial community composition was influenced by both HCH contamination and the plant developmental stage. Sphingobium (positive responder to HCH), Sphingomonas and Altererythrobacter were the bacterial genera characteristic for the contaminated rhizosphere of the old plants. The early succession stages in the rhizosphere of the young plants growing on the excavated HCH hotspot were characterized by Massilia (positive responder) and Lysobacter. Lotus tenuis growth and development was affected by long-term HCH contamination, the root-associated bacterial community composition being mainly driven by the plant age, whereas the HCH effect was negligible. Nevertheless, L. tenuis was able to acquire possible HCH tolerant bacteria which could at the same time offer plant growth promoting (PGP) benefits for the host, such as Sphingomonas in the old plants or Massilia or Pantoea in the HCH contaminated young roots. Furthermore, we identified an inoculum with possibly HCH tolerant PGP bacteria transferred from the contaminated bulk soil to L. tenuis roots through the rhizosphere compartment consisting of Mesorhizobium loti, Neorhizobium galegae, Novosphingobium lindaniclasticum, Pantoea agglomerans and Lysobacter bugurensis. The potential nitrogen fixation was not affected by HCH contamination in bulk soil, old rhizosphere and plant roots. The lower nifH gene count in the young contaminated rhizosphere is most probably a short-term HCH effect on the microbial growth in the early bacterial succession in rhizosphere.