Ammonia is the main pollution factor of the aquatic environment in marine shrimp culture systems. In order to demonstrate the feasibility of the combination of biofloc technology and nitrifying biofilter for the ammonia removal, a 70-day production trial was conducted in a simplified pilot-scale hybrid biofloc-based recirculating aquaculture system (biofloc-RAS) with the intensive culture of Litopenaeus vannamei. Nitrogen dynamics and nitrifying microbial communities were investigated in three replicated systems simultaneously under the conditions of high feed loading and zero water exchange. Along with biofloc development in the culture tank and biofilm formation in the nitrifying biofilter during the trial, nitrification could be fastly and effectively established in the system, which was indicated by the dynamics of total ammonia nitrogen (TAN), NO₂^–-N, NO₃^–-N, and total nitrogen (TN) concentrations. Meanwhile, similar nitrifying microorganisms could be found between biofloc and biofilm, despite some differences in abundance, diversity, and composition of ammonia-oxidizing archaea and bacteria and nitrite-oxidizing bacteria. High TAN removal rate could be achieved and was significantly and positively correlated with abundances of these nitrifying microbial communities in both biofloc and biofilm, further indicating that both biofloc and biofilm could contribute highly to nitrification performance of the biofloc-RAS. The results of this study indicate a potential application of the biofloc-RAS in coastal intensive aquaculture.

The potential effects of Astragalus polysaccharides (APS) were evaluated in coral trout (Plectropomus leopardus). Five APS levels (0%, 0.05%, 0.10%, 0.15%, and 0.20%) were added to the diet of coral trout, and a 56-day growth trial (initial weight 18.62 ± 0.05 g) was conducted. Dietary APS enhanced growth performance, with the highest improvement observed in fish fed the 0.15% APS diet. This concentration also enhanced the antioxidant capacity and immunomodulation of the fish by regulating the expression of genes associated with antioxidant enzymes and immune responses. Intestinal microbiota analysis revealed that APS supplementation significantly increased the Chao1 index and relative abundance of beneficial bacteria (Firmicutes and Bacillus). A high level of APS (0.20%) did not provide additional benefits for growth and health compared to a moderate level (0.15%). These findings indicate that an optimal APS dose promotes growth, enhances antioxidant activity, supports immune function, and improves intestinal microbiota in coral trout. Based on a cubic regression analysis of the specific growth rate, the optimal APS level for the maximal growth of coral trout was determined to be 0.1455%.

Given the widespread use of commercial probiotics in aquaculture, it is important to evaluate the quality and environmental effects of these additives. Here, the effects of a Bacillus probiotic compound, BG4, on Penaeus vannamei survival rate, water quality factors, and microbial communities were assessed. An analysis of the BG4 powder confirmed the content of probiotic bacteria: 50.2% Bacillus licheniformis, 48.4% Bacillus subtilis, and 1.4% Bacillus amyloliquefaciens, consistent with the information on the product packaging. The effective bacterial quantity (1.1 × 10⁹ CFU/g) was higher than that indicated on the product label (2 × 10⁸ CFU/g). BG4 was added to a sterilized aquaculture water system, according to the indicated bacterial quantity; after 7 days, the maximum ammonia nitrogen, phosphate, and chemical oxygen demand degradation rates were 36.3%, 28.9%, and 15.2%, respectively. In the shrimp culture experiment, the survival rate of P. vannamei and water quality did not differ significantly between the BG4 and control groups. The bacterial quantity and high-throughput sequencing results indicated that Bacillus content in BG4 group decreased continuously from 3.5 × 10⁴ CFU/mL to 6.9 × 10² CFU/mL. To ensure the desired effect of probiotics in commercial aquaculture applications, additional quality evaluations and scientific assessments are needed.

Biologically controlling Microcystis aeruginosa blooms in saline–alkaline environments remains a major challenge in aquatic ecosystem management. Here, the algicidal performance of an indigenous algicidal bacterium, Bacillus cereus strain PT1 isolated from a sulfate-type saline–alkaline pond, against M. aeruginosa was evaluated, and the underlying metabolic mechanisms were elucidated using non-targeted metabolomics. PT1 exhibited pronounced, stable algicidal activity under saline–alkaline conditions, decreasing the algal cell density from 2 × 10⁶ to 1.25 ± 0.5 × 10⁵ cells mL⁻¹ within 4 days at a rate of 93.75 ± 2.5% (p < 0.05). The above results demonstrate that strain PT1 has a significant lytic effect on M. aeruginosa. Non-targeted liquid chromatography–mass spectrometry analysis identified 298 PT1-induced accumulated metabolic features, and the top 30 candidates comprised organic acids and aromatic compounds, including benzoic acid, coumarin, malonic acid, and signaling-related molecules, including indoleacetaldehyde and nitroprusside. These differential metabolites were associated with algicidal-related pathways, including quorum sensing, two-component systems, ABC transporters, and tryptophan metabolism, outlining a coordinated “regulation–transport–metabolic remodeling” framework. Our findings demonstrate the potential of the indigenous algicidal strain PT1 from saline–alkali ponds to control M. aeruginosa blooms. They also provide an important theoretical basis and data foundation for further elucidating the molecular characteristics of algae solubilizing activity under saline–alkali conditions and developing microbial agents for preventing and controlling Microcystis blooms in saline–alkali ponds.