In the present study, a Gram-positive bacterium was isolated from the intestine of healthy crucian carp Carassius auratus and named strain R8. It was initially identified as Enterococcus faecium according to its morphological, physiological and biochemical characteristics. Further identification by using 16S rRNA gene sequence analysis confirmed the R8 strain (Genbank accession no. MF928076) as E. faecium. Challenge and hemolysis experiments showed that the E. faecium R8 strain had no toxicity to the crucian carp. Bacteriostatic experiment showed that this isolate obviously inhibited the growth of Aeromonas veronii and Staphylococcus haemolyticus. The proliferation of E. faecium R8 strain occurred after exposure to various growth conditions such as at pH values from 2.0 to 4.0 for 8 h, bile concentrations from 0.2 to 1.2% and high temperature of 80 °C. This bacterial strain grew best under the condition of 37 °C, pH 7.0 and salinity 30 ppt, and its growth curve exhibited four distinct phases. These results showed that the E. faecium R8 strain had potential probiotic characteristics and could be used as a candidate strain for aquatic probiotics.

A modified genomic DNA extraction method named the combination of lysozyme and ultrasonic lysis (CLU) method was used to analyze the fish intestinal microflora. In this method, the physical disruption and chemical lysis steps were combined, and some parameters in the key steps were adjusted. In addition, the results obtained by this method were compared with the results obtained by the Zirmil-beating cell disruption method and the QIAamp Fast DNA Stool Mini Kit. The OD260/OD280 ratio and concentration of the DNA extracted using the CLU method were 2.02 and 282.8 µg/µL, respectively; when the incubation temperatures for lysozyme and RNase were adjusted to 37 °C, those values were 2.08 and 309.8 µg/µL, respectively. On the agarose gel, a major high-intensity, discrete band of more than 10 kb was found for the CLU method. However, the smearing intensity of degraded DNA was lower when the incubation temperatures were 60 °C for lysozyme and 30 °C for RNase than when incubation temperatures of 37 °C for lysozyme and 37 °C for RNase were used. The V3 variable region of the prokaryotic 16S rDNA was amplified, and an approximately 600-bp fragment was observed when the DNA extracted using the CLU method was used as a template. The CLU method is simple and cost effective, and it yields high-quality, unsheared, high-molecular-weight DNA, which is comparable to that obtained with a commercially available kit. The extracted DNA has potential for applications in critical molecular biology techniques.

Shrimp aquaculture ponds are dynamic ecosystems in which water quality and microbial interactions play a central role in animal health. This study aimed to investigate the relationship between the intestinal microbiota of Penaeus monodon and the microbial community of polyculture pond water. Shrimp and water samples were collected from polyculture ponds at four time points during the rearing period. Water-quality parameters were measured, and microbial community structures were analyzed by high-throughput 16S rRNA sequencing. Statistical analyses, including one-way ANOVA, revealed significant variations in water-quality parameters and microbial diversity among sampling stages (p < 0.05). Water quality indicators showed progressive changes from July to September, with pH decreasing from 8.1 to 7.5 but remaining within a suitable range. Nitrogen compounds, including ammonia, nitrite, and nitrate, increased steadily, with total nitrogen rising from 0.71 to 1.86 mg·L⁻¹, while phosphate and total phosphorus reached 0.31 and 0.36 mg·L⁻¹, respectively, exceeding thresholds commonly associated with algal bloom risk. Microbial community profiling using Illumina MiSeq sequencing revealed 166 OTUs shared between shrimp intestine and pond water, while both habitats contained more than 350 OTUs overall. Alpha diversity analysis showed higher microbial richness in water than in shrimp intestines, dominated by unclassified taxa, whereas shrimp guts were enriched in specific genera such as Vibrio. Cluster analysis indicated partial overlap but distinct grouping of gut and water microbiota, with the PMB intestinal community diverging most strongly. These findings highlight a close link between water quality and microbial composition, emphasizing the importance of pond management for maintaining ecological stability and shrimp health.

The intensification of mariculture has increased nitrogen and phosphorus pollution in aquaculture wastewater. Conventional treatments often prove inefficient under saline conditions. To address this challenge, we developed an innovative aerobic denitrification reactor using Pseudomonas chloritidismutans K14, specifically tailored for seawater systems. Our study examined the effects of hydraulic retention time (HRT) and carbon-to-nitrogen (C/N) ratio on pollutant removal and critically assessed the physiological benefits for hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂). At an optimal HRT of 8 h and a C/N ratio of 8, the system achieved high removal efficiencies: 95.5% for NH₄⁺-N, 92.4% for NO₂⁻-N, 93.8% for NO₃⁻-N, 92.8% for total nitrogen (TN), and 84.8% for chemical oxygen demand (COD), along with a 47.3% reduction in total phosphorus (TP). More importantly, the system significantly enhanced grouper health by improving growth performance, digestive enzyme activities, and antioxidant capacity, along with a trend toward improved nonspecific immune indicators. The feed conversion ratio was optimized to 2.28, with a weight gain rate of 117.07%. Our findings establish this optimized aerobic denitrification system as an integrated strategy that effectively combines water purification with indirect health benefits, offering a sustainable solution for mariculture.