Evidence for high-risk pollutants and emerging microbial contaminants at two major bathing ghats of the river Ganga using high-resolution mass spectrometry and metagenomics

An efficient wastewater treatment plant is imperative to limit the entry of emerging pollutants (EPs) and emerging microbial contaminants (EMCs) in the river ecosystem. The detection of emerging EPs in aquatic environments is challenging due to complex sample preparation methods, and the need for sophisticated accurate analytical tools. In Varanasi (India), the river Ganga holds immense significance as a holy river but is consistently polluted with municipal (MWW) and hospital wastewater (HWW). We developed an efficient method for untargeted detection of EPs in the water samples using High-resolution mass spectrometry (HRMS), and identified 577 and 670 chemicals (or chemical components) in the water samples from two major bathing ghats, Assi Ghat (AG) and Dashashwamedh Ghat (DG), respectively. The presence of EPs of different categories viz chemicals from research labs, diagnostic labs, lifestyle and industrial chemicals, toxins, flavor and food additives indicated the unsafe disposal of MWW and HWW or inefficient wastewater treatment plants (WWTPs). Besides, shotgun metagenomic analysis depicted the presence of bacteria associated with MWW viz Cloacibacterium normanse, Sphaerotilus natans (sewage fungi), E. coli, and Prevotella. Also, the presence of human pathogens Arcobacter, Polynucleobacter, Pseudomonas, Klebsiella, Aeromonas, Acinetobacter, Vibrio, and Campylobacter suggests the discharge of HWW. EPs are linked to the development, and transmission of antimicrobial resistance (AMR). Occurrence of antibiotic resistance genes (ARGs), plasmid-borne β-lactamases, aminoglycoside transferases, and ARGs associated with integrons, transposons and plasmids viz mcr-3 gene that confer resistance to colistin, the last resort of antibiotics confirmed the presence of emerging microbial contaminants. Subsequent genome reconstruction studies showed the presence of uncultivable ARB and transmission of ARGs through horizontal gene transfer. This study can be used to monitor the health of aquatic bodies as well as the efficiency of WWTPs and raise an urgent need for efficient WWTPs to safeguard the river, Ganga.