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Abstract: A 56‐day experiment was conducted to investigate the effect of no carbohydrate addition applied to control water quality in water‐reusing biofloc systems for tilapia (GIFT Oreochromis niloticus) cultivation. Reusing water‐contained flocs was initially inoculated into six 300 L indoor tanks. Thirty fish (average individual weight 99.62 ± 7.34 g) were stocked in each tank. Glucose was extra added into three tanks (GLU‐tanks) according to biofloc technology, while other tanks were no carbohydrate added (NCA‐tanks). Concentrations of total ammonia nitrogen in GLU‐tanks and NCA‐tanks were fairly consistent and below 4.74 ± 0.35 mg/L. Nitrite concentrations in NCA‐tanks were significantly lower than GLU‐tanks, which were below 0.59 ± 0.10 mg/L during the later culture period. NCA‐tanks achieved a low relative abundance of denitrifiers and high concentrations of nitrate. Soluble reactive phosphorous in NCA‐tanks was consistently increased, which was decreased to a low level in GLU‐tanks. However, growth parameters in NCA‐tanks were similar to GLU‐tanks (p > .05) and reach a high finial density of 24.32 ± 1.04 kg/m3. Cetobacterium sp. was the first‐dominant bacterial genus in all tanks, which was a commonly indigenous bacterium in the intestinal tract of freshwater fish. The results demonstrate the feasibility of no carbohydrate addition in water‐reusing biofloc systems for tilapia. [ABSTRACT FROM AUTHOR]

[Display omitted] • Novel single stage oxic process for rapid nutrient recovery via bioflocs technology. • Achieved up to 5.5 mgP/gTSS/d phosphate removal efficiency in high nitrate conditions. • Amplified genes linked to phosphate transport and aerobic assimilation. • More frequent carbon usage shortens phosphorus removal times and enhances rates. • Converting waste into feed supports the circular economy in aquaculture. Intensive aquaculture systems often contain high concentrations of nitrate and phosphate, leading to environmental and economic burdens. Bioflocs technology, a novel approach in aquaculture, addresses these issues by removing these nutrients and producing protein rich microbial biomass. This study explored a novel single stage oxic process for simultaneous rapid recovery of phosphate and nitrate using different carbon addition strategies in bioflocs growth reactors. Our findings revealed that bioflocs rapidly and effectively assimilated nutrients from high concentration environments. The phosphate removal mechanism involved biomass formation, achieving simultaneous removal efficiencies of 5.5 ± 0.2 mgP/gTSS/d for phosphate and 41.8 ± 2.0 mgN/gTSS/d for nitrate, harvesting biofloc concentrations of 3599 ± 33 mg/L. The crude protein content of the bioflocs exceeded 50 %, with essential amino acid indices over 0.9, indicating potential for high quality aquafeed. Actinobacteriota and Bacteroidota were dominant during the phosphorus removal process, with significant proliferation of Nakamurella. Additionally, gene amplification related to assimilation, aerobic denitrification and inorganic phosphate transport was observed, suggesting biofloc technology is a promising method for efficient phosphate and nitrate removal. This research promotes the circular economy by recovering nutrients, reducing reliance on traditional feed sources, and minimizing environmental contamination. [ABSTRACT FROM AUTHOR]

The management of biofloc quality and quantity is critical in biofloc-based aquaculture systems. This research explores the effects of the selective removal of settleable and unsettled bioflocs on biofloc quality, fish growth, nitrogen transformation and bacterial community dynamics in these systems. Nile tilapia (Oreochromis niloticus) was reared at a density of 2.7 kg/m3 in nine 250-L conical breeding tanks over a 47-day period. Systems where unsettled bioflocs were removed outperformed others, achieving a 26 % higher weight gain in tilapia and maintaining more stable and lower levels of total ammonia (0.45 mg-N/L) and nitrite (0.35 mg-N/L). Additionally, these systems maintained a healthier bacterial environment, evidenced by a 27 % increase in beneficial bacterial populations and a 30 % reduction in harmful bacteria. Conversely, tanks removing settleable bioflocs showed a 19 % higher microbial diversity but experienced fluctuations in nitrogen levels. Notably, the intestinal bacterial community in tilapia from the unsettled biofloc group closely mirrored (87 % similarity) that of their corresponding bioflocs, with significant pathogenic symptoms observed (p < 0.05). These findings suggest the importance of implementing precise biofloc management strategies, highlighting that the selective removal of unsettled bioflocs enhances fish growth and improves overall system health. Future research should explore the mechanistic relationships between settleable and unsettled bioflocs to refine management strategies that bolster sustainability and economic benefits. • Effects of settleable and unsettled biofloc removal in biofloc-based tilapia systems. • Unsettled biofloc removal boosts tilapia growth with stable TAN and nitrite levels. • Settleable biofloc removal increased microbial diversity by 19 % but gut lesions. • Different biofloc removal strategies impact microbial community dynamics. • Improving aquaculture efficiency via precise biofloc removal management. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carbon sources affected single stage oxic process for nutrient removal by bioflocs. • High removal rates of 1.74 mg-N/gTSS/h and 0.23 mg-P/gTSS/h using glucose. • Glucose enhanced phosphorus removal by 109% primarily through assimilation process. • Acetate and propionate reduce phosphorus removal by favoring denitrification. • Glucose is practicable for simultaneous nitrogen and phosphorus removal by bioflocs. Bioflocs can efficiently achieve simultaneous nitrate and phosphate removal through a single-stage aerobic process, provided they are continuously supplemented with an organic carbon source. This study investigated the effects of different carbon sources on this process. Results revealed that phosphate removal rate in the glucose group was 0.61 ± 0.02 mg/L/h, significantly higher than those in the acetate (0.28 ± 0.01 mg/L/h) and propionate (0.29 ± 0.03 mg/L/h) groups (p < 0.05). However, the three groups observed no significant differences in nitrate removal rates (p > 0.05). The superior performance of the glucose group in simultaneous nitrogen and phosphorus removal is likely due to the higher biomass synthesis. In contrast, nitrate removal in the acetate and propionate groups was primarily driven by denitrification, resulting in lower sludge production and reduced phosphate uptake. For practical application of bioflocs in simultaneous nitrogen and phosphorus removal, glucose is recommended as the optimal carbon source. [ABSTRACT FROM AUTHOR]

In the current study, two nitrifying suspended growth bioreactor processes (denoted as R1 and R2) was investigated based on biofloc technology (BFT) in recirculating aquaculture systems (RAS). The conversion of nitrifying R1 to denitrifying biofilter via adding carbohydrates was studied. It was observed that the dominant functional microbes of nitrifying BFT biofilters were ammonia-oxidizing bacteria (AOB), denitrifying bacteria (DNB), phosphorus/glycogen accumulating organisms (PAOs/GAOs), and nitrite-oxidizing bacteria (NOB). The removal efficiencies of the total ammonium nitrogen and nitrite were prominent, while the treatment efficiencies respected to nitrate and soluble reactive phosphate (SRP) were insignificant. More than 16 h after the addition of carbohydrates to R1, the regular dynamics of dissolved oxygen (DO), dissolved organic carbon, and C/N ratio became conducive to the notable removal of nitrate and SRP. Furthermore, the relative abundances of AOB and NOB decreased, and that of PAOs, GAOs, and DNB increased. Redundancy analysis indicated that, among the environmental factors and functional microbes involved in converting to a denitrifying biofilter, DO was the most important. Also, Spearman correlation heatmap elucidated Flavobacterium sp. and Norank_Nitrosomonadaceae were the dominants bacteria affected by environmental factors. This study found a high abundance of functionally active microbes participated in nitrogen and phosphorus removal in the BFT biofilters. Thus, the water-treatment efficiency of the BFT biofilter could be flexibly and controlled by adding carbohydrates. • RAS with zero water discharge using nitrifying BFT biofilters can be developed. • Nitrifying BFT biofilter can be converted to a denitrifying role via C addition. • The RAS can be applied to control N and P simultaneously. • Convertible functions of BFT biofilters helped to simplify the RAS setup. [ABSTRACT FROM AUTHOR]

To achieve high water-treatment efficiencies and simplify the setup of recirculating aquaculture systems (RAS), this study examined the use of suspended growth reactors (R1 and R2) based on biofloc technology (BFT) as water-treatment biofilters. Moreover, the conversion of the heterotrophic R1 biofilter to a nitrifying role was investigated. During RAS operation using heterotrophic BFT biofilters, R1 and R2 simultaneously controlled total ammonium nitrogen, nitrite (NO 2 −-N), nitrate (NO 3 −-N), soluble reactive phosphate (SRP), and alkalinity, with relevant functional microbes including denitrifying bacteria (DNB), phosphorus accumulating organisms (PAOs), denitrifying PAOs (DNPAOs), glycogen accumulating organisms, ammonia oxidizing bacteria, and nitrite oxidizing bacteria. To achieve low concentrations of nitrogen, phosphorus, and save carbon sources, we were able to quickly convert R1 into a nitrifying BFT biofilter by stopping carbohydrate addition. Although there were dominant relative abundances of DNB, PAOs, and DNPAOs in the converted R1, the lack of carbon sources resulted in continuous rise of NO 3 −-N in the effluent, stable NO 2 −-N removal efficiency, and absence of SRP removal after 40 h. However, R2 retained the previous NO 3 −-N and SRP removal efficiencies with carbohydrate addition. This indicated that this novel RAS using BFT biofilters achieved simultaneous nitrogen and phosphate removal, and that the convertible water-treatment efficiencies of BFT biofilters could be controlled by carbohydrate addition. This approach could simplify the RAS setup and meet real-time water quality demands. Unlabelled Image • Recirculating aquaculture systems were associated with biofloc technology. • RAS with heterotrophic BFT biofilters simultaneously controlled N and P. • Stopping carbohydrate addition converted the BFT biofilter into a nitrifying one. • The system can be applied to save C resources when NO 3 −-N and SRP are very low. • Convertible functions of BFT biofilters helped to simplify the RAS setup. [ABSTRACT FROM AUTHOR]

Biofloc technology (BFT) is a promising technique, but there has been ongoing debate between biofloc's properties and particle size. This study investigated the relationship between long-term particle size variation and the production performance of BFT aquaculture systems. External shearing force introduction could keep bioflocs at a smaller size level chronically, and 97% of smaller bioflocs were below 181.97 μm which was obviously lower than the normal size group (Control). No significant difference in the daily water treatment capacity for bioflocs within different particle size was detected (p > 0.05), but the activity of Soil-Dehydrogenase (S-DHA), Ammonia Monooxygenase (AMO), Hydroxylamine Oxidase (HAO) and Nitrite Oxidase (NOR) were all higher in smaller bioflocs. Smaller bioflocs were better enriched heavy metal elements and less damage to the gill lamellae was also detected in smaller size group. The survival rate and final mean weight of tilapia (Oreochromis niloticus) cultured in the BFT system based on smaller size bioflocs were significantly higher than the control (p < 0.05). The microbial compositions of varying particle sizes bioflocs demonstrated remarkable distinctions, and higher abundance of particle-attached bacteria (e.g., Flavobacteriaceae, Rhodobacteraceae) was detected in normal size bioflocs. Overall, this study comprehensively evaluated the long-term application of various particle size bioflocs and provided a practical basis for the sustainable BFT development. • External shearing force introduction can maintain bioflocs in the smaller particle size. • The activity of S-DHA, AMO, HAO and NOR are higher in bioflocs with smaller size. • Smaller bioflocs provide relatively better growth performance of tilapia (Oreochromis niloticus). • The microbial composition of smaller-sized bioflocs becomes more diverse as time progresses. [ABSTRACT FROM AUTHOR]

Aquaponic systems consisting of recirculating aquaculture systems and hydroponic beds. Raft-floating aquaponics integrated with a moving-bed biofilm reactor (MBBR) was established. Humic acid (HA) was added to the aquaponic systems to promote nutrient recovery. An elementary analyzer was used to determine the nitrogen content of fish, plants, and sediments to calculate the NUE. High-throughput sequencing and quantitative polymerase chain reaction (qPCR) were conducted to reveal the influence of HA on microbial communities and nitrification genes. During an 80 day experimental period, the effects of water quality parameters, nitrogen utilization efficiency (NUE), and microbial communities in variable concentrations of HA-based aquaponic systems were compared. Among the HA-based systems, the one with an optimal concentration of 15 ppm indicated the best performance. In HA-based aquaponic systems with 15 ppm concentration, the maximum concentrations of total ammonia nitrogen and NO 2 −–N were 6.02 mg/L and 12.693 mg/L, which were significantly lower than those in aquaponic systems with 0 ppm and 30 ppm. However, the accumulation of nitrate in the HA-based aquaponic systems with 15 ppm was greater than that in the others. The NUE of the 15 ppm HA-based aquaponic system was 58.34%, which was significantly higher than that of the 0 ppm (50.83%) and the 30 ppm (47.25%) systems. Consequently, the threshold concentration of HA was established as 15 ppm. The microbial communities of different HA-based systems revealed the presence of denitrifying and nitrifying microbial populations for nitrogen recovery in moving bed biofilm reactors (MBBRs); higher levels of these microbial populations thrived in the 15 ppm HA system than in the other systems. However, there was no significant difference in microbial communities from root samples. The highest copies of amoA and nxrB were found at 15 ppm for both MBBRs and roots, indicating that the addition of HA to the aquaponic systems yielded more products and improved water quality and NUE by changing the composition of microbial communities based on the threshold for HA input concentration. This study provided that HA show enormous potential for an interesting research and the development of green technologies for biological wastewater treatment and bioremediation. • The performance, water quality and NUE in the 15 ppm HA concentration were greater than with 0 ppm and 30 ppm HA. • The higher levels of microbial population were observed in the 15 ppm than in the other HA concentration-based systems. • The highest number copies of amoA and nxrB genes were detected in 15 ppm HA of both MBBRs and roots. [ABSTRACT FROM AUTHOR]

Recirculating aquaculture systems (RAS) using biofloc biofilters can address the high accumulation of nitrate (NO 3 −-N) and total suspended solids (TSS) in in-situ biofloc technology (BFT) aquaculture systems. A RAS was divided into the following four zones: reaction zones of the BFT biofilters, settling zones of the BFT biofilters, culture tanks, and a buffer tank. During the 95-day experimental period, the water quality parameters and microbial communities in different zones of the RAS were compared. The NO 3 −-N, nitrite (NO 2 −-N), total ammonia nitrogen (TAN), and TSS in the culture tanks were continuously controlled at average concentrations of 9.84 ± 5.15, 0.28 ± 0.15, 0.10 ± 0.18, and 96.92 ± 33.82 mg·L−1, respectively. Because denitrification produced alkalinity in the BFT biofilters, the alkalinity of the culture tanks reached an appropriate concentration of 109.40 ± 10.57 mg CaCO 3 ·L−1 without intentional adjustment. The concentrations of NO 3 −-N, NO 2 −-N and TAN in the reaction zones were significantly lower than those in the influent (buffer tank) and culture tanks. However, there was no significant difference found for major water quality parameters between the reaction zones and the settling zones. There was also some removal of soluble reactive phosphate by the BFT biofilters, although there was no significant difference between the biofilters and influent. Microbial communities in the reaction zones were more diverse than those in the culture tanks, buffer tank and settling zones. Denitrifiers, nitrifying bacteria and important functional microbial populations for phosphorus removal were all present in the reaction zones, and at higher levels than those in other zones. Nitrogen and phosphorus removal were mainly achieved in the reaction zones of the BFT biofilters, and the microbial communities in the other three zones were similar. Besides, Scortum barcoo grew from an individual weight of 347.7 ± 73.0 to 472.1 ± 106.2 g, achieving a final stocking density of 32.81 kg·m−3 at the end of the experiment. • Characteristics between the reaction zones and other three zones were different. • BFT reactors continuously treated NO 3 −-N, NO 2 −-N, TAN and TSS, which in culture tanks were controlled in low concentrations. • BFT biofilters could provide alkalinity to the consumption of the current RAS. [ABSTRACT FROM AUTHOR]

• Particle size did not influence the TAN removal rate or syndromes of ammonia, nitrite and nitrate. • Bacterial community compositions did not differ significantly among the different floc sizes. • The C/N ratio of bioflocs was not influenced by biofloc size. The total ammonia nitrogen (TAN) removal efficiency and bacterial community composition of bioflocs with <50-μm particle size, > 50-μm particle size and un-sieved bioflocs were investigated in the current study. The initial ratio of dissolved organic carbon to TAN (DOC/TAN) in the three groups were about 14:1. No significant difference was found in the removal rate of TAN, average concentrations of TAN and nitrite nitrogen among the three groups (P > 0.05). The C/N (w/w) ratio of the > 50-μm bioflocs was significantly higher than those of the other groups. No significant differences were found in the crude protein content in the bioflocs among the three groups. The development of the bacterial community compositions of the bioflocs was analyzed by Illumina MiSeq sequencing analyses. Most OTUs were shared among the three groups at all the sampled time points. With the increase in the relative abundance of phylum Firmicutes, that of phylum Proteobacteria, Chorolexi, and Bacteroidetes decreased in all the three groups. The phylum Firmicutes and genus Bacillus were predominant in all the sampled time points. At the end of the experiment, genus Bacillus accounted for 81% in the < 50-μm group, 82% in the > 50-μm group, and 75% in the un-sieved group. [ABSTRACT FROM AUTHOR]