Sthenoteuthis oualaniensis constitutes a significant fishery resource in the South China Sea. Previous studies have indicated that intestinal microbiota varies with sex and sexual maturity and is closely associated with growth traits and feeding patterns. Among them, the intestinal microbiota in female sexual maturity of medium-form populations of S. oualaniensis differed significantly from those of the micro-form population and individuals of different sexes. To investigate the intestinal microbial community structure and metabolic functions of the cultivable microbiota in female sexual maturity of the medium-form populations of S. oualaniensis, we collected the specimens from the offshore northern South China Sea and the northern Nansha fishing ground, and comparatively analyzed the intestinal microbial community structure, and the metabolic profiles of culturable isolates on carbon and nitrogen substrates. Mycoplasma dominated the intestinal microbiota in both regions, accounting for 82.95% and 93.37% of relative abundance, respectively. There were no significant differences in the α-diversity indices (Shannon, Simpson, Ace and Chao1) (p>0.05). However, six non-dominant bacterial genera, including Enhydrobacter of the phylum Proteobacteria, exhibited significantly higher abundances in the offshore northern South China Sea than in the northern Nansha fishing ground (p<0.05). NMDS ordination revealed a clear spatial separation between the two regions, indicating that environmental heterogeneity drove differentiation among non-dominant taxa. Functional predictions indicated highly similar between regions, particularly in major COG categories such as translation and ribosomal structure and amino-acid transport and metabolism. The average well color development (AWCD) values of the culturable microbial communities peaked at 168^th hour (1.48) and 120^th hour (1.46), respectively. Carbohydrate utilization was highest at 24^th hour, with absorbance values of 10.97 and 11.84. The culturable gut microbiota showed high utilization (Rs>1.0) for 13 and 12 carbon and nitrogen sources in the two regions, with no significant difference in diversity indices (p>0.05). Overall, the intestinal microbiota of sexually mature, medium-form female S. oualaniensis exhibited conserved core taxa, α-diversity, functional repertoire, and metabolic characteristics across the two regions, whereas divergence occurred only among non-dominant taxa diverged in response to environmental heterogeneity.

To facilitate the industrial application of the algicidal bacterium Bacillus cereus CZBC1, this study employed 2 L and 50 L fermenter systems to scale up and validate the optimal fermentation process previously established in shake flask experiments, and assessed process reproducibility by comparing the dynamic changes in cell density, morphological evolution and process parameters under the two scales, and systematically evaluated the algicidal efficacy of different fermentation products and their dosages against Microcystis aeruginosa. The optimal fermentation parameters for CZBC1 in the 50 L fermenter were determined as follows: wheat bran and molasses (combined) at 6.84 g·L⁻¹, yeast extract 3.36 g·L⁻¹, NaCl 1 g·L⁻¹, KH₂PO₄ 0.2 g·L⁻¹, MgSO₄ 0.2 g·L⁻¹, FeCl₃ 0.1 g·L⁻¹, 3.08% (φ) MnSO₄ solution 1 mL·L⁻¹, and CaCO₃ 0.1 g·L⁻¹. Fermentation was carried out with a filling volume of 70% (φ), an inoculum size of 1% (φ), a temperature of 30 °C, an agitation speed of 500 r·min⁻¹, an aeration rate of 0.6 (V·V⁻¹·min), a fermenter pressure of 0.03–0.05 MPa, and a controlled pH of 6.5. After 24 h of fermentation, the sporulation rate exceeded 90%. In the 2 L fermenter, the cell concentration reached a peak of 4.12×10⁹ CFU·mL⁻¹ at 24 h, with a sporulation rate exceeding 90% at 36 h; in the 50 L fermenter, the peak cell concentration of 3.72×10⁹ CFU·mL⁻¹ was achieved at 16 h, and the sporulation rate exceeded 90% by 24 h, indicating that scale-up significantly accelerated both cell proliferation and sporulation. Algicidal assays revealed significant differences in the efficacy of the various fermentation products: the crude fermentation broth and non-activated spore powder exhibited no algicidal activity, whereas the supernatant, activated spore powder, and vegetative cells all showed potent algicidal effects, achieving 10-day algicidal rates of 97.45%, 86.73%, and 97.10%, respectively, suggesting that the algicidal activity is mainly dependent on extracellular metabolites and the metabolically active state of the cells. Furthermore, dose-response analysis demonstrated that, in an experimental system with an initial M. aeruginosa concentration of 8.31×10⁵ cell·mL⁻¹, the effective threshold concentration of activated spore powder was no less than 10⁵ CFU·mL⁻¹; when the concentration increased to 10⁶ CFU·mL⁻¹, the algicidal rate reached 99.01%, highlighting its excellent potential for cyanobacterial bloom control. In summary, these findings confirm the promising application prospects of the CZBC1 strain and its fermentation process in the biological control of cyanobacterial blooms.

To investigate the effects of exogenous functional microorganisms on microbial community structure, carbon metabolism characteristics, and organic carbon content in aquaculture water, we designed a blank control group, a feed control group, and three bacterial supplementation groups treated with Bacillus licheniformis, Lactobacillus plantarum, and Rhodopseudomonas palustris, respectively. We measured the total organic carbon (TOC), dissolved organic carbon (DOC), total bacterial count, microbial community structure, and carbon metabolism function to analyze their dynamic changes. The results showed that the addition of three beneficial bacteria significantly enhanced microbial metabolic activity and community diversity. On Day 7, the Shannon index in the bacterial supplementation groups reached 3.17–3.18, which was significantly higher than that in the control groups (p<0.05). The exogenous probiotics exerted limited influence on overall bacterial abundance and community succession. In the later stages, Pseudomonadaceae and Rhodobacteraceae became the dominant families. The degradation rates of TOC and DOC in the bacterial supplementation groups reached 59.0%–59.5% and 57.1%–57.7%, respectively, which were significantly higher than those in the blank control group (p<0.05). Among the three strains, R. palustris exhibited the strongest carbon source utilization capacity, B. licheniformis showed a clear advantage in degrading complex organic carbon, and L. plantarum exerted a mild and stable effect. This study indicates that the addition of Bacillus, photosynthetic bacteria, and lactic acid bacteria to aquaculture water can improve microbial community diversity and metabolic activity and accelerate organic carbon degradation.

To screen high-efficiency purification systems for aquaculture effluent from Litopenaeus vannamei high-position ponds, we utilized four biological filter media were utilized including sponge (HM), modified high-density polyethylene (K5), ceramic stone (TC), and fiber ball (XW) to immobilize Spirulina platensis and Chlorella vulgaris, respectively. The purification performance of different combinations on aquaculture effluent was systematically compared. Results showed that fiber balls yielded the highest cell immobilization density for both microalgae, reaching 1.78×10⁸ cells·g⁻¹ for S. platensis and 4.79×10⁸ cells·g⁻¹ for C. vulgaris, respectively. Moreover, C. vulgaris exhibited a significantly higher attachment density than S. platensis across all tested media (p<0.05). For a given microalga, the filter medium type significantly influenced the removal rates of all water quality parameters, with XW demonstrating the best performance. Under the same filter medium, C. vulgaris showed significantly higher removal rates of total nitrogen (TN), total phosphorus (TP) and chemical oxygen demand (COD) than S. platensis, whereas S. platensis exhibited a significantly higher removal rate of total suspended solids (TSS) (p<0.05). Two-way ANOVA revealed significant interactive effects between microalgal species and filter medium type on the removal rates of total inorganic nitrogen (TIN), orthophosphate ( ${\mathrm{P}\mathrm{O}}_4^{3-}\text{-}\mathrm{P}$), TN, TP, TSS, and COD (p<0.01). The fiber ball-C. vulgaris system achieved the highest removal rates for TIN (95.29%), ${\mathrm{P}\mathrm{O}}_4^{3-}\text{-}\mathrm{P}$ (96.25%), TN (55.78%), and COD (75.09%); the fiber ball-S. platensis system achieved the highest removal rate for TSS (85.65%). Therefore, targeted purification of aquaculture effluent can be realized by precisely selecting microalga-filter material combinations according to the target pollutants. This coupled system not only offers a key technological solution for resource utilization and discharge standard compliance of high-pond aquaculture effluent, but also corresponds to the current strategic needs for green, low-carbon, and circular development in aquaculture. It shows great potential for engineering applications in various scenarios such as ecological purification ponds.