Combined toxicity of nanoplastics and microcystin-LR to sulfate-reducing bacteria and the underlying mechanisms

Sulfate-reducing bacteria (SRB) play important roles in the global sulfur cycle and profoundly affect the transformation of toxic sulfide in aquaculture environments. Nanoplastics (NPs) and microcystin-LR (MC-LR), as new pollutants that are widespread and harmful in freshwater aquaculture environments, have attracted considerable attention. However, the effects of combined pollution with NPs and MC-LR on sulfur cycling processes and SRB in aquaculture systems remain unclear, and the corresponding ecological risks induced by this cocontamination scenario remain inadequately assessed. Therefore, in the present work, PENPs and MC-LR, two contaminants frequently detected throughout China's freshwater aquaculture environments, were chosen to create a microcosm model imitating freshwater aquaculture ponds. Based on this model, we performed MC-LR exposure experiments mediated by PENPs, aiming to elucidate the effects of the combination of PENP and MC-LR pollution on sulfur cycling and SRB microbial communities in aquaculture systems, while simultaneously assessing the potential ecological hazards of PENPs to freshwater aquaculture systems. The findings demonstrated that PENPs exhibited a comparatively strong adsorptive capacity for MC-LR. Single and combined exposure to these two pollutants disrupted the steady state of the sulfur cycle in the aquaculture water, leading to increased sulfide accumulation in the water body; specifically, the MC-LR group exhibited the most significant increase in sulfide accumulation. Furthermore, compared with single-pollutant exposure, coexposure to MC-LR and PENPs had a more pronounced effect on the composition and structure of aquatic microbial communities. Analysis of the integrated biomarker response (IBR) index calculated through sulfur cycling genes indicated that compared to PENPs, MC-LR poses a higher pollution risk to aquaculture ecosystems, necessitating urgent attention and targeted risk mitigation strategies.