Joint effects of rice straw-derived biochar and microcystin-LR on splenic histopathological injuries, inflammation, and innate immune responses in male zebrafish

Microcystin-LR (MC-LR), a toxin produced by cyanobacterial blooms, poses a significant threat to aquatic animals and humans. Biochar (BC), known for its high adsorption capacity, is increasingly being utilized to remove hazardous pollutants from aquatic environment. In the present study, we conducted a full factorial experiment to investigate the potential immunotoxicity in male zebrafish (Danio rerio) exposed to environmentally relevant levels of MC-LR (0, 1, 5, 25 μg/L) and rice straw-derived BC (0, 100 μg/L) for 30 d. The findings revealed that subchronic MC-LR exposure caused concentration-dependent splenic histopathological injuries, characterized by an augmentation of melano-macrophage centers, edematous mitochondria, and vacuolation. While the presence of BC mitigated the inflammatory response and mitochondrial damage in the combined groups. Furthermore, in contrast to the group solely exposed to 25 μg/L MC-LR, decreased levels of interleukin 1β (IL1β), tumor necrosis factor α (TNFα), and interleukin 6 (IL6) as well as significant downregulation of inflammation and immune-related genes (tlr4a, myd88, p38a, tnfα) were noticed in the corresponding co-exposure group, which confirmed that BC can reduce MC-LR-induced inflammatory response. Concurrently, a significant increase in complement C3 (C3) content, along with higher splenic c3b expression levels, was observed in the MC-LR-co-BC group in relative to the group exposed solely to MC-LR, suggesting that BC alleviated MC-LR-induced innate immune inhibition. Our results also demonstrated that BC can decrease MC-LR contents in both water and spleen, thereby alleviating MC-LR-induced inflammation and innate immune inhibition via the MyD88-dependent toll-like receptor (TLR/MyD88) signaling pathway in male zebrafish. Our results underscore the potential of BC to mitigate the deleterious impacts of MC-LR on aquatic organisms in blooms-contaminated water.