Metagenomics indicates abundance of biofilm related genes and horizontal transfer of multidrug resistant genes among bacterial communities in nano zinc oxide polluted soil.

The unsafe and reckless disposal of metal oxide nanoparticles like ZnO (nZnO) into the soil could seriously impact bacterial behavioural responses and functions. Under such stress, biofilm formation is considered to be a robust mechanism for bacterial survival in soil. We examined the response of bacterial metagenomes in soils exposed to varying levels of Zn (50, 200, 500 and 1000 mg kg-1) as nano Zn oxide (nZnO) in terms of biofilm genesis and regulation and their co-occurrences with multidrug resistance genes (MDRGs) and mobile genetic elements (MGEs). The size-specific effects of nZnO were verified using its bulk counterpart (bZnO). Both nZnO and bZnO facilitated profusion of biofilm related genes (BGs) especially at higher Zn levels (500 and 1000 mg kg-1 Zn), though maximum abundance was registered at a comparatively lower level under nZnO. In general, nZnO favoured an enhancement of genes involved in exopolysaccharide biosynthesis and attachment, while bZnO favoured genes related to capsule formation, chemotaxis and biofilm dispersion. Co-occurrence network analysis revealed significant positive correlations between abundances of BGs, MDRGs and MGEs, indicating an enhanced probability for horizontal gene transfer of MDRGs in nZnO polluted soils.

Soil polluted with nano ZnO reveals unstable bacterial communities and decoupling of taxonomic and functional diversities.

Due to relentless production and disposal of nano zinc oxide (nZnO), it has become critical to comprehend the serious risks large-scale accumulation of nZnO pose to bacterial communities in soil. The primary objective was to evaluate the changes in bacterial community structure and associated functional pathways through predictive metagenomic profiling and subsequent validation through Quantitative Realtime PCR in soil spiked with nZnO (0, 50, 200, 500 and 1000 mg Zn kg-1) and similar levels of bulk ZnO (bZnO). The results revealed that soil microbial biomass-C, -N, -P, soil respiration and enzyme activities decreased markedly at higher ZnO levels. The alpha diversity decreased with increasing ZnO level, with more impact under nZnO, while beta diversity analyses indicated a distinct dose- dependent separation of bacterial communities. The dominant taxa including Proteobacteria, Bacterioidetes, Acidobacteria and Planctomycetes significantly increased in abundance, while Firmicutes, Actinobacteria and Chloroflexi decreased in abundance with elevated nZnO and bZnO levels. Redundancy analysis indicated that changes in bacterial community structure instilled a greater dose- rather than size- specific response on key microbial parameters. Predicted key functions did not show a dose- specific response, and at 1000 mg Zn kg-1, methane metabolism as well as starch and sucrose metabolism were attenuated, while functions involving two component systems and bacterial secretion systems were enhanced under bZnO indicating better stress avoidance mechanism than under nZnO. Realtime PCR and microbial endpoint assays confirmed the metagenome derived taxonomic and functional data, respectively. Taxa and functions that varied substantially under stress were established as bioindicators to predict nZnO toxicity in soils. Taxon-function decoupling indicated that the soil bacterial communities deployed adaptive mechanisms under high ZnO, with lesser buffering capacity and resilience of communities under nZnO.