System-dependent divergence of microbial community and resistome in two anaerobic niches under sulfamethoxazole...

The prevalence of sulfamethoxazole (SMX) in high-strength wastewater poses a significant threat to the stability and efficiency of anaerobic biological treatment systems, particularly when deployed as initial treatment units. However, the complex interactions arising from SMX biodegradation and their resultant effects on typical anaerobic digestion (AD) and sulfate-reducing (SR) systems are not thoroughly understood. This study revealed that SMX exposure stimulated methanogenesis in the AD system and sulfate reduction in the SR system, driven primarily by enriched key functional taxa (e.g., methanogens, sulfate-reducing bacteria). Organic matter removal efficiency increased significantly in the AD system under SMX stress, attributed to the enrichment of fermentative bacteria. Notably, the enriched class Actinomycetes was capable of SMX biodegradation, thereby likely mitigating SMX stress for other microorganisms. In contrast, the SR system exhibited significantly diminished organic matter removal despite developing a more functionally specialized community under SMX exposure. This community harbored fewer SMX degraders, perpetuating selective pressure on the microbiota. Increasing SMX concentrations failed to induce significant shifts in overall community structure in either system, while significantly promoted the proliferation of antibiotic resistance genes (ARGs), particularly pronounced in the SR system exhibiting high SMX accumulation. Moreover, mobile genetic elements mediated the horizontal transfer of the sulfonamide resistance gene sul1 and other co-occurring ARGs located on plasmids. This study provides novel insights into the convergent and divergent microbial responses in the AD and SR systems under SMX exposure, highlighting the dual effects (both stimulatory and inhibitory) of SMX on the functionality of these anaerobic systems.