Acute high carbonate alkalinity alters ion regulatory functions, chitin turnover, and intestinal microbial...

Carbonate alkalinity is an important environmental stressor in saline-alkali aquaculture. However, its comprehensive effects on the physiological regulation, transcriptional responses, and intestinal microbial homeostasis of Penaeus monodon remain unclear. In this study, shrimp were exposed to high carbonate alkalinity stress for 96 h. Key physiological indicators were evaluated, and gill transcriptome analysis and intestinal 16S rRNA gene sequencing were performed. The results showed that high alkalinity reduced the survival rate of P. monodon, increased hemolymph osmotic pressure and carbonic anhydrase (CA) activity, and simultaneously decreased Na⁺/K⁺-ATPase (NKA) activity and hemolymph ammonia content. Transcriptomic analysis revealed that upregulated genes were mainly enriched in functions related to calcium-ion binding, chitosan degradation, and chitosanase activity and that lysosomal and glycolytic pathways were also activated. In contrast, downregulated genes were associated with chloride ion transport, voltage gated chloride channel activity, and mineral absorption. Intestinal microbial analysis showed that microbial diversity increased significantly and that the relative abundances of short chain fatty acid producing genera, such as Bacteroides and Roseburia, and stress related taxa, such as Selenomonas, increased. In contrast, the relative abundances of opportunistic pathogens, such as Vibrio and Photobacterium, decreased, and the predicted microbial phenotype shifted toward aerobic metabolism. These results indicate that high carbonate alkalinity stress may affect shrimp survival by disrupting gill ion homeostasis, inducing chitin turnover and cuticle remodeling responses, and altering the intestinal microbial community. Collectively, these findings provide a mechanistic basis for determining shrimp tolerance thresholds in saline alkali aquaculture and for developing breeding strategies.