Effects of Nepeta cataria and Lactuca sativa on Microbial Community Structure and Nitrogen–Phosphorus Biogeochemical Cycling in Aquaculture Water

As the scale of aquaculture continues to expand, aquaculture waters are under increasing environmental pressure, particularly regarding total nitrogen (TN) and total phosphorus (TP). To address this challenge, this study evaluated the efficacy of Nepeta cataria and Lactuca sativa in removing nitrogen and phosphorus from aquaculture water and investigated the underlying microbial mechanisms. The experiment consisted of three groups: the Nepeta cataria group (NC) with Nepeta cataria planted, the Lactuca sativa group (HL) with Lactuca sativa planted, and the control group (CT) without any plants. The results indicated that, compared with the control group, both the Nepeta cataria group and the Lactuca sativa group significantly reduced the concentrations of TAN, nitrite, nitrate, TN, and TP in the aquaculture water (p < 0.01). Plant species exerted a significant regulatory effect on both planktonic and rhizosphere microorganisms. The phyla Proteobacteria, Actinobacteriota, and Bacteroidota were the core dominant phyla across all groups; hgcI_clade, Rhodoluna, and Sediminibacterium were the signature dominant genera in the plant groups’ planktonic water. The microbial community is highly coupled with nitrogen and phosphorus nutrients. Nepeta cataria and Lactuca sativa enhanced the nitrification–denitrification coupling process by enriching microbial communities with nitrogen and phosphorus transformation functions, thereby forming a synergistic purification model of “plant absorption–microbial transformation.” This study elucidated the regulatory mechanisms by which Nepeta cataria and Lactuca sativa influence microbial communities and nitrogen and phosphorus cycles in aquaculture water, providing a theoretical basis and technical reference for plant selection in aquaponics systems and the ecological purification of aquaculture water.