Single-cell proteins are attracting growing attention as viable alternatives for fishmeal (FM) in aquatic feed. Methanotroph (Methylococcus capsulatus, Bath) bacteria meal FeedKind® (FK) is a type of single cell protein with high protein content (75.14%) and desirable amino acids profile, produced by Methylococcus capsulatus (Bath) living on methane consumption. The present study evaluated the potential of replacing FM with FK in the diet of black sea bream (Acanthopagrus schlegelii). Five iso-energetic and iso-nitrogenous diets were designed with FK replacing 0, 4.13, 8.27, 16.53, and 24.80% FM protein in the basal diet (40% FM content), respectively. All the diets were fed to three replicates of fish (initial weight 6.56 ± 0.02 g) for 70 days. After the feeding trial, replacing dietary 8.27% FM protein with FK significantly improved the weight gain and specific growth rate of fish (P < 0.05), while other groups showed no significant difference in the growth performance (P > 0.05). The fish fed diets with 8.27 and 16.53% replacement levels exhibited significantly increased feeding rates. The 8.27% FK diet significantly increased the whole-body and muscle crude protein contents, apparent digestibility of crude lipid, foregut, and midgut amylase activities. The microvillus density in the midgut of fish fed the 24.80% FK diet significantly increased. The diet with 8.27% FK increased the serum triglyceride content of the fish, while the 24.80% FK diet reduced the serum triglyceride, total cholesterol, and low-density lipoprotein cholesterol contents of the fish. In conclusion, the results indicated that replacing dietary FM protein with up to 24.80% FK had no adverse effects on the growth of black sea bream, whilst replacing 8.27% FM protein with FK enhanced its growth performance and feed utilization.

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The interactive effect of dietary inactivated Lactobacillus plantarum and berberine on black sea bream, Acanthopagrus schlegelii, was investigated with three diets designated as D1 (Con: basal diet), D2 (LP: basal diet + 400 mg/kg L. plantarum), and D3 (LPBB: basal diet + 400 mg/kg L. plantarum + 50 mg/kg berberine) and fed to juvenile black sea bream ( 5.67 ± 0.05  g) for 56 days. The growth performance and feed utilization parameters, as well as intestinal trypsin activity, were significantly improved in the LP and LPBB groups ( P < 0.05 ). Fish fed the LPBB diet showed better serum and hepatic antioxidant capacity, whilst the LP group had better hepatic antioxidant capacity, than the control fish ( P < 0.05 ). Intestinal IgM and C3 levels significantly increased in the LPBB fish than the rest of the groups ( P < 0.05 ). NF-κB was significantly upregulated in the LP group ( P < 0.05 ). Nrf2 and IL-10 were significantly upregulated, whilst Keap1b and NF-κB were significantly downregulated in the LPBB group ( P < 0.05 ). These findings show that a combination of inactivated L. plantarum and berberine in the diet of black sea bream can improve the immune response and antioxidant capacity than a diet with only the inactivated probiotic, whilst both diets can equally improve growth performance.

An 8-week feeding trial was conducted to evaluate the effects of L-methionine and methionine hydroxy analogue calcium (MHA-Ca) supplements in low-fishmeal diet on growth performance, hepatopancreas morphology, protein metabolism, anti-oxidative capacity, and immunity of Pacific white shrimp (Litopenaeus vannamei). Four isonitrogenous and isoenergetic diets were designed: PC (203.3 g/kg fishmeal), NC (100 g/kg fishmeal), MET (100 g/kg fishmeal +3 g/kg L-methionine) and MHA-Ca (100 g/kg fishmeal +3 g/kg MHA-Ca). White shrimp (initial body weight 0.23 ± 0.00 g, 50 shrimp per tank) were allocated to 12 tanks and divided among 4 treatments in triplicates. In response to L-methionine and MHA-Ca supplementations, the shrimp exhibited higher weight gain rate (WGR), specific growth rate (SGR), condition factor (CF), and lower hepatosomatic index (HSI) compared to those fed the NC diet (p &lt; 0.05). The WGR and SGR of shrimp fed L-methionine and MHA-Ca showed no difference with those in the PC diet (p &gt; 0.05). Both of L-methionine and MHA-Ca supplementary diets significantly decreased the malondialdehyde (MDA) levels of shrimp when compared with the NC diet (p &lt; 0.05). L-methionine supplementation improved the lysozyme (LZM) activity and total antioxidant capacity (T-AOC) of shrimp, while the MHA-Ca addition elevated the reduced glutathione (GSH) levels in comparison with those fed the NC diet (p &lt; 0.05). Hypertrophied blister cells in hepatocytes were observed in shrimp fed the NC diet, and alleviated with L-methionine and MHA-Ca supplementations. Shrimp fed the MET and MHA-Ca diets had higher mRNA expression levels of target of rapamycin (tor) than those fed the NC diet (p &lt; 0.05). Compared to the NC group, dietary MHA-Ca supplementation upregulated the expression level of cysteine dioxygenase (cdo) (p &lt; 0.05), while L-methionine supplementation had no significant impact (p &gt; 0.05). The expression levels of superoxide dismutase (sod) and glutathione peroxidase (gpx) were significantly upregulated by L-methionine supplemented diet in comparison with those in the NC group (p &lt; 0.05). Overall, the addition of both L-methionine and MHA-Ca elevated the growth performance, facilitated protein synthesis, and ameliorated hepatopancreatic damage induced by plant-protein enriched diet in L. vannamei. L-methionine and MHA-Ca supplements enhanced anti-oxidants differently.

Litopenaeus vannamei was divided into seven groups (defined as diets A0–A6) and fed with diets respectively containing 0, 0.5, 1, 2, 4 and 8 g/kg Yarrowia lipolytica (astaxanthin content: 1.5%) and 3 g/kg Haematococcus pluvialis (astaxanthin content: 2%). After an eight-week feeding trial, the results reflected that different levels of Y. lipolytica and H. pluvialis could significantly increase the weight gain rate of L. vannamei (p < 0.05). The condition factor and weight gain rate of group A4 were significantly higher than those of the other groups (p < 0.05); the HSI significantly decreased with the increase of Y. lipolytica (p < 0.05). The addition of Y. lipolytica to the diet had significant effects on total protein (TP), albumin (ALB), glutathione peroxidase (GSH-Px), malonaldehyde (MDA) and total antioxidant capacity (T-AOC) (p < 0.05). The total protein and albumin of the A5 and A6 groups were significantly higher than those of the other groups (p < 0.05). The GSH-Px activity of the A5 group was the highest and the T-AOC of the A0 group was the lowest. Inducible nitric oxide synthase (I-NOS) increased with the addition of Y. lipolytica (p < 0.05). Y. lipolytica inclusion had no negative effect on physiological and biochemical parameters and some serum immune and antioxidant indexes (p > 0.05). Astaxanthin in Y. lipolytica had an obvious effect on body color. After cooking, the body color of the shrimp deepened with increasing Y. lipolytica content. The red body color of L. vannamei was significantly improved by adding yeasts hydrolysate 2~8 g/kg to the diet. According to the regression analysis between the level of Y. lipolytica added to the diets and the weight gain rates, the optimal level of Y. lipolytica is 4.64 g/kg.