Global transcriptional responses of denitrifying bacteria to functionalized single-walled carbon nanotubes revealed by weighted gene-coexpression network analysis

Functionalized single-walled carbon nanotubes (f-SWNTs) are widely used in many fields due to the unique structure and the excellent properties. Although these nanomaterials have been reported to enable to cause negative effects on denitrifying bacteria once they enter the environment, the toxic behaviors and regulatory mechanisms of f-SWNTs to denitrification remain unclear. In this study, the denitrification performance of a model denitrifier exposed to pristine and functionalized SWNTs was investigated, and the global transcriptional responses were comprehensively explored by RNA-seq and weighted gene-coexpression network analysis (WGCNA). Although both hydroxyl SWNTs (SWNTs-OH) and carboxyl SWNTs (SWNTs-COOH) showed inhibitory effects on bacterial denitrification, the former more severely inhibited denitrification than the latter. Transcriptional profiles showed that compared with SWNTs-COOH, SWNTs-OH much more strongly influenced the expressions of the key genes related to signal transduction, substance transport, electron transfer and transcriptional regulation. Functional analysis further indicated that the genes associated with substrate transport, carbon source metabolism and electron transfer underwent dramatic down-regulation. Using WGCNA, 12 gene modules were established corresponding to various types of carbon nanotubes, and eigengene adjacency analysis revealed the key gene modules related to denitrification performance under different conditions. Hub gene network analysis revealed the key regulatory factors of bacterial denitrification induced by f-SWNTs. The results suggested that f-SWNTs modulated the key genes responsible for the glycerolipid/free fatty acid (GL/FFA) cycle, and thus disturb processes associated with denitrification, including signaling process, energy homeostasis, intracellular redox balance and transportation.