The proximate compositions of the feed ingredients, experimental feeds, and shrimp samples were analyzed following the methodology outlined by the Association of Official Analytical Chemists (AOAC, 2005). The values are the means of two replicates. The specific methods employed in this study were based on our previous publication (Zhang et al., 2023). Moisture content was determined by drying minced samples in a 105°C oven until a consistent weight was achieved. After acid digestion with an Auto-Digester (KjelFlex K-360, BUCHI, Switzerland), crude protein content was calculated using the Kjeldahl technique and calculated by multiplying nitrogen by 6.25. Crude lipids were extracted using a Soxtec System HT (Soxtec 2055, FOSS Tecator) through ether extraction. The samples were burned in a muffle furnace at 550°C for 8 h to assess the amount of ash content. The analysis of amino acid content in the samples was conducted according to our previous publication (Zhang et al., 2023). The determination of phosphorus content in the samples was carried out using the molybdenum blue colorimetric method (Tan et al., 2022a).

Yarrowia lipolytica GTLB P42 was obtained and cultured with Jitilabo (Beijing) Biotechnology Development Co., Ltd., Beijing, China. Yarrowia lipolytica was produced using the method described by Hu et al. (2016), and the general process is presented in Figure 1 . In the first stage, Moorella thermoacetica produced acetic acid by anaerobic fermentation using syngas (a mixture of CO, H_2, and CO₂) as the sole carbon source, after which the fermentation broth was purified and filtered. In the second stage, Yarrowia lipolytica uses the above acetic acid product as a substrate to produce yeast proteins by aerobic fermentation. The Yarrowia lipolytica strains used in the trial were analyzed by the methods described below and contained (dry matter) 57.66% crude protein, 0.22% crude lipid, 7.49% ash, and 1.56% phosphorus. The amino acid compositions of Yarrowia lipolytica are shown in Supplementary Table S1 .

Seven equal protein content (40.00% crude protein), equal lipid content (8.00% crude lipid), and equal energetic (16.33 MJ/kg) diets were formulated by supplementing YL at levels of 0%, 1.50%, 3.00%, 4.50%, 6.00%, 9.00%, and 12.00% to a basal diet. According to previous studies, DL-methionine, L-lysine, and L-arginine were added to maintain amino acids balance to support animal growth and health (Cheng et al., 2021; Ji et al., 2021). To determine the apparent digestibility, yttrium oxide (Y₂O₃) was supplemented at a concentration of 0.1%. The formulation and approximate compositions of the experimental diets are shown in Table 1 . Feed pellets with a diameter of 1.20 mm were prepared and steam-sterilized according to the specific production process described in previous study (Xu et al., 2021).

Litopenaeus vannamei used in the experiment were obtained from the Xixuan Fishery Science and Technology Island Farm in Zhoushan City, Zhejiang Province, China. Commercial diets were fed to the shrimp for 2 weeks for acclimation. After domestication for 2 weeks, 2,100 shrimp (initial body weight of 0.32 ± 0.02 g) were transferred to a flowing water system and randomly divided into 42 tanks (six replicates for each treatment) with 50 individuals per tank. During the feeding period, the water was subjected to daily exchanges, and continuous aeration was ensured through the use of air stones. The water temperature was carefully regulated within the range of 25.0 ± 3.0°C while maintaining a salinity level of 28.0 ± 1.0 g/L. Additionally, the dissolved oxygen content was maintained above 6.0 mg/L, the ununionized ammonia nitrogen concentration was kept below 0.05 mg/L, the nitrite concentration was kept below 0.1 mg/L, and the pH was monitored within the range of 7.7 ± 0.1. Shrimp were fed the respective experimental diets to apparent satiation three times daily at 07:00, 12:00, and 18:00 for 56 days. After 3 h of feeding, the remaining pellets were collected using a siphon, dried, and weighed. Additionally, feces were thoroughly removed.

Starting from the 7th week of the formal trial, the feces were drained every night after feeding for 3 h. The feces were collected and dried by siphoning at 6:00 am on the following day. The yttrium content was determined by inductively coupled plasma atomic emission spectrometry (Thermo Electron, U.S.A.) after digestion.

Before the start of the experiment, 30 initial shrimp were randomly selected and stored in a −20°C freezer for whole-body proximate composition analysis. At the end of the 8-week feeding trial, a fasting period of 12 h was imposed on the shrimp. Subsequently, the shrimp were sedated using a concentration of 60 mg/L tricaine methanesulfonate (MS-222, Sigma−Aldrich, USA) to minimize any potential problems. The shrimp were then carefully captured using a net and promptly transferred to an ice plate, where they were weighed tail by tail after they had remained stationary. Ten shrimp were selected from each tank for whole shrimp analysis, while the remaining shrimp were subjected to heart blood sampling. After a 2 h standing period, the samples were centrifuged at 8,000 ×g for 15 min. The serum samples were carefully collected and stored at −80 °C until analysis. The shrimp visceral mass was carefully extracted and dissected on ice to sequentially isolate the hepatopancreas, stomach, and intestines. The hepatopancreas was weighed and collected to measure digestive, antioxidant, and immune parameters. Muscle samples were analyzed for muscle composition. All the samples were stored at −80 °C until analysis. The hepatopancreas and midgut of six shrimp were randomly taken from each tank and divided into two small portions, which were stored in 4% paraformaldehyde (PFA) solution and 2.5% glutaraldehyde solution (4 °C). These samples were subjected to histological staining with hematoxylin and eosin (H & E) and electron microscopy.

The stomach, hepatopancreas, and midgut tissues were accurately weighed, and 9 volumes of homogenization medium (cold PBS, 0.1 M, pH 7.0, 4°C) were added at a weight (g): volume (mL) ratio = 1:9. The tissues were mechanically homogenized in an ice-water bath. The homogenate was centrifuged at 2,500 ×g for 10 min at 4°C. The supernatant was carefully collected and stored at −80 °C, after which the digestive enzyme (trypsin, lipase, and amylase) activity was determined. The serum samples were used to analyze the total protein (TP), albumin (ALB), glucose (GLU), blood urea nitrogen (BUN), triglyceride (TG), total cholesterol (TC), total bilirubin (TBIL), and creatinine (Cr) contents as well as alanine transaminase (ALT), aspartate aminotransferase (AST), and polyphenol oxidase (PPO) activities. Furthermore, the serum and hepatopancreatic malondialdehyde (MDA), catalase (CAT), total superoxide dismutase (T-SOD), total antioxidant capacity (T-AOC), lysozyme (LZM), glutathione peroxidase (GSH-Px), alkaline phosphatase (AKP), and acid phosphatase (ACP) activities were also determined. The biochemical parameters of the serum and tissues were determined using assay kits provided by Nanjing Jiancheng Bioengineering Institute (China), and the measurements were conducted according to the manufacturer’s instructions.

In accordance with the methods of Xu et al. (2021), tissue samples were subjected to H & E staining and analyzed via histological sectioning. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analyses of tissue samples were performed as described previously by Tan et al. (2022b). Eighteen images were selected for each group, and Imagine-Pro Plus 6.0 (Media Cybernetics, USA) software was used for further analysis of the microvillus height in the transmission electron microscopy sections (Penn et al., 2011).

The following variables were calculated:

Survival rate (SR, %) = N_t × 100/N₀;

Weight gain rate (WGR, %) = 100 × (W_t − W₀)/W₀;

Specific growth rate (SGR, %/day) = 100 × (Ln W_t – Ln W₀)/t;

Hepatosomatic index (HSI, %) = 100 × (liver weight/W_t);

Feed intake (FI, %/day) = 100 × dry feed intake/[(initial body weight + final body weight)/2]/t;

Feed conversion ratio (FCR) = dry feed fed/wet weight gained;

Protein efficiency ratio (PER) = 100 × (W_t − W₀)/(feed intake × feed protein content);

Protein productive value (PPV, %) = 100 × protein gain/total protein intake;

Apparent digestibility coefficient (ADC, %) of dry matter = (1 − dietary Y₂O₃/fecal Y₂O₃) × 100;

Apparent digestibility coefficients of a certain nutrient in feed (%) = [1 − (dietary Y₂O₃/fecal Y₂O₃) × (certain nutrient content in feces/certain nutrient content in feed)] × 100;

Apparent digestibility coefficients of a certain nutrient in the test ingredient (%) = [(ADC _{test diet} × nutrient _{test diet}) – (0.7 × ADC _{REF diet} × nutrient _{REF diet})]/(0.3 × nutrient _ingredient).

where W_t and W₀ represent the final and initial shrimp weights, respectively; N_t and N₀ are the final and initial shrimp quantities, respectively, in each tank; t is the trial period in days; and REF diet is the reference diet.

All the data were analyzed using IBM SPSS Statistics 26.0 (SPSS, Inc., USA) and are presented as the mean ± standard error of the mean (S. E. M.). Levene’s test was used to confirm the normality and homogeneity of the variance. The Kruskal-Wallis test was performed when the data were not homogeneous. Otherwise, one-way ANOVA with Duncan’s multiple range test for multiple data groups was used to assess significant differences among groups. Differences were considered significant at P < 0.05. If statistical significance was detected, the data were further subjected to orthogonal polynomial comparisons, which were used to assess the significance of linear, quadratic, and cubic regression models to fit the best model. If the cubic regression was found to be significant, the quadratic and linear regressions were disregarded; otherwise, the significance of the quadratic regression was examined; if the quadratic regression was significant, the linear regression was not considered; otherwise, the significance of the linear regression was tested (Yossa and Verdegem, 2015). Origin 2021 (OriginLab, Inc., USA) was used to fit the regression model; correlation analyses were performed using the R package (4.1.2) ggplot2, linkET, dplyr, and FactoMineR. BioRender.com was used to create YL production workflows.

Table 2 shows that the relationships between dietary YL levels and shrimp growth performance (SR, WGR, and SGR), as well as feed utilization (FR, FCR, PER, and PPV), were best explained using a cubic model. Compared to that in the control group (Y0), the SR of the shrimp in the Y1.5, Y4.5, Y6.0, and Y12.0 groups significantly increased (P < 0.05). As the YL level in the feed increased, the WGR and SGR initially increased and then decreased. A cubic regression model based on the SGR indicated that the optimal YL level in shrimp diets was 2.68% (R² = 0.90658, Figure 2A ), which accounted for 10.54% of the dietary FM content. When the FCR increased, the PER and PPV decreased when dietary YL inclusion exceeded 3.0% (P < 0.05). Moreover, increases in dietary YL inclusion resulted in increased FI and FCR (P < 0.05, Figures 2B, C ), while PER showed a decreasing trend ( Figure 2D ). The responses of all three variables to the increase in dietary YL inclusion followed a cubic model (R² = 0.95365, 0.95067, and 0.94102, respectively). A growth-promoting effect of YL inclusion in the range of 0-3.0% was observed. Furthermore, growth performance did not significantly decrease when YL inclusion exceeded 6.0%, which is equivalent to replacing over 23.6% of the FM, but increases in FI and FCR were noted.

Tables 3 , 4 present the whole-body and muscle proximate compositions at different YL levels. Cubic responses of whole-body moisture, crude protein, and crude lipid, as well as muscle moisture and phosphorus contents, were observed. Dietary YL inclusion at 6.0%, 9.0%, and 12.0% significantly increased whole-body moisture content (P < 0.05), while no significant differences in whole-body ash and phosphorus content were found among the groups (P > 0.05). The Y12.0 group had significantly lower crude protein and crude lipid contents in the whole body and greater muscle moisture content compared to the Y0 group (P < 0.05). The shrimp muscle crude protein content was significantly lower in the Y9.0 and Y12.0 groups than in the Y0 and Y3.0 groups (P < 0.05). Muscle phosphorus content was significantly lower in the Y3.0, Y6.0, and Y9.0 groups compared to the Y0 and Y1.5 groups (P < 0.05). The inclusion of 0-12.0% YL had no significant effect on the muscle crude lipid and ash contents (P > 0.05).

Table 5 shows the muscle amino acid composition at different dietary YL levels. Glycine and proline contents were cubically affected by the YL levels. Compared to the Y3.0 group, the muscle glycine content significantly reduced in the Y9.0 group, while the proline content significantly increased in the Y6.0, Y9.0, and Y12.0 groups (P < 0.05). The glycine content in the Y12.0 group was significantly higher than in the Y1.5, Y4.5, and Y9.0 groups, and the proline content in the Y9.0 group was significantly higher than in the Y0, Y1.5, Y3.0, and Y4.5 groups (P < 0.05). Dietary inclusion of 0-12.0% YL had no significant effect on the other 14 amino acids (P > 0.05).

As depicted in Table 6 , the serum TG and TC contents showed a decreasing trend and were cubically affected by dietary YL levels. The serum TG content was significantly lower in the Y4.5, Y9.0, and Y12.0 groups compared to the Y0, Y1.5, and Y3.0 groups (P < 0.05). There was a significant decrease in the serum TC content when dietary YL level exceeded 3.0% compared to the Y0 group (P < 0.05). However, no significant differences were noted among the treatments for serum TP, ALB, GLU, BUN, TBIL, Cr contents, or ALT and AST activities (P > 0.05).

Table 7 reveals that the serum T-AOC content exhibited a cubic increasing trend with increasing levels of YL replacement. The serum T-AOC content was significantly higher in the Y6.0, Y9.0, and Y12.0 groups than in the Y0 group (P < 0.05). However, no significant differences were observed among the groups for other serum antioxidant and immune parameters, including the MDA content and CAT, T-SOD, LZM, GSH-Px, AKP, ACP, and PPO activities (P > 0.05).

Orthogonal polynomial contrast analysis, as presented in Table 8 , indicated that the hepatopancreatic MDA content decreased, while the CAT and T-AOC activities increased, all of which responded cubically to dietary YL levels. The MDA content was significantly lower in the Y3.0, Y6.0, Y9.0, and Y12.0 groups, while the T-AOC activity was significantly higher in the Y4.5, Y9.0, and Y12.0 groups than in the Y0 group (P < 0.05). CAT activity was significantly elevated in the Y12.0 group relative to the Y0, Y3.0, Y4.5, and Y6.0 groups (P < 0.05). However, including dietary YL at levels ranging from 0% to 12.0% showed no significant effect on hepatopancreatic T-SOD, LZM, GSH-Px, AKP, and ACP activities (P > 0.05).

Digestive enzyme activities in the stomach, hepatopancreas, and midgut, detailed in Table 9 , showed that hepatopancreatic trypsin activity exhibited a cubic increase with increasing dietary YL inclusion. Specifically, trypsin activity was significantly higher in the Y3.0, Y9.0, and Y12.0 groups than in the Y0 group (P < 0.05). However, no significant differences were observed in stomach and midgut trypsin, lipase, and amylase activities or hepatopancreatic lipase and amylase activities among the different groups (P > 0.05).

The histological structures of midgut tissue from the Y0, Y3.0, and Y12.0 groups, displayed in Figure 3 , showed no significant differences or damage to the intestinal mucosa. SEM and TEM revealed no significant differences or damage in the surface morphology of intestinal microvilli among the groups, with the intestinal microvilli neatly arranged and closely packed. A statistical analysis of intestinal TEM microvillus height revealed no significant difference between the Y0 (46.38 ± 2.55 μm), Y3.0 (45.66 ± 3.37 μm), and Y12.0 (42.61 ± 2.69 μm) groups, demonstrating that dietary YL inclusion at levels of 0-12.0% did not significantly affect shrimp intestinal microvillus height (P > 0.05).

The histological structure of the shrimp hepatopancreas, shown in Figure 4 , indicated that all groups had visible and neatly organized structures, suggesting that dietary YL inclusion at levels of 0-12.0% did not led to significant damage to the histological structure of the shrimp hepatopancreas.

The ADC for the test ingredient Yarrowia lipolytica, as displayed in Supplementary Tables S2–S4 , showed that, compared to those in the Y0 group, there were no significant differences in the ADC of dry matter, crude protein, crude lipid, phosphorus, and amino acids in the YADC group (P > 0.05), although a decreasing trend was observed.

The PCA biplot ( Figure 5 ) clearly differentiated the Y12.0 group from the Y0, Y1.5, Y3.0, and Y4.5 groups. The first principal component (PCA1) accounted for 39.67% of the dataset’s variance and was influenced predominantly by FCR, FI, PER, and PPV. The second principal component (PCA2), which explained 11.02% of the variance, primarily reflected variations in the whole-body ash content of the shrimp. The PCA results revealed a positive correlation between FI and FCR, and between PER and PPV, with these two sets of variables showing negative correlations with each other. This pattern aligned with the findings from the multidimensional correlation analysis.

As illustrated in Figure 6 , the multidimensional correlation analysis revealed that the SGR was significantly negatively correlated with FI, FCR, whole-body moisture, and muscle moisture content (P < 0.05). Conversely, the SGR was significantly positively correlated with the PER, PPV, whole-body crude lipid, and both whole-body and muscle crude protein contents (P < 0.05). Additionally, the FI and FCR were negatively correlated with whole-body crude protein, crude lipid, and muscle crude protein contents, while the PER and PPV exhibited significant positive correlations with these parameters (P < 0.05). FI and FCR were also positively correlated with whole-body and muscle moisture contents, whereas PER and PPV were negatively correlated with these moisture contents (P < 0.05). Significant negative correlations were observed between whole-body and muscle moisture contents, whole-body crude protein and crude lipid contents, while muscle phosphorus content was positively correlated with whole-body ash and phosphorus contents (P < 0.05). Moreover, physiological and biochemical parameters were significantly correlated with whole-body crude lipid content (Mantel’s r ≥ 0.2, P < 0.01), and digestive enzyme activities were significantly related to FCR, PER, and PPV (Mantel’s r < 0.2, P < 0.01), as well as with whole-body moisture and crude protein contents (Mantel’s r < 0.2, P < 0.05).

Optimal YL levels promoted growth performance, echoing findings in turkeys (Meleagris gallopavo) (Czech et al., 2020), piglets (Sus scrofa) (Czech et al., 2016, 2018), Atlantic salmon (Berge et al., 2013), Nile tilapia (Neuls et al., 2021), and Largemouth Bass (Micropterus salmoides) (Fei et al., 2022). The nutrient-rich composition of yeast, which contains β-glucan, mannan oligosaccharides (MOS), amino acids, and nucleotides, likely contributed to this enhancement in growth (Gamboa-Delgado and Márquez-Reyes, 2018; Jin et al., 2018; Patsios et al., 2020). However, higher YL inclusions above 6.0% resulted in decreased feed utilization due to the indigestible nature of some yeast nutrients, leading to a decrease in the protein efficiency ratio (PER) (Patsios et al., 2020). A cubic regression model pinpointed 2.68% as the optimal YL substitution level, which is equivalent to replacing 10.54% of the FM, aligning with Hertrampf and Piedad-Pascual’s (Hertrampf and Piedad-Pascual, 2003) suggested range for yeast inclusion in shrimp diets.

In addition, bacterial proteins produced using C1 gases (e.g. FeedKind^® and RichMore) have been evaluated in aquatic animals. These studies illustrate their potential in replacing certain levels of FM in various species’ diets without negatively impacting growth performance (Biswas et al., 2020; Xu et al., 2021; Zhang et al., 2022). It is worth noting that dietary high RichMore levels can have adverse effects on their growth and immune function in Pacific white shrimp (Yao et al., 2022). This may be attributed to the insufficient supply of essential amino acids, fatty acids, and other functional substances, as well as the negative impact of elevated RichMore levels on protein synthesis and nutrient utilization (Jiang et al., 2021). Similar results were found in this study, elevated YL levels were found to reduce protein and lipid deposition in shrimp, especially at a 12.0% substitution level. Principal component analysis (PCA) revealed that the Y12.0 group was distinct from the other groups in terms of growth performance, feed utilization, and nutritional composition.

Moreover, YL did not adversely impact the activity of digestive enzymes or digestive tissue morphology. Increased hepatopancreatic trypsin activity was noted, potentially leading to enhanced intestinal trypsin activity, possibly due to yeast organic acids reducing digestive tract pH and thus boosting proteolytic enzyme activity (Lückstädt, 2008; Zhao et al., 2017). Additionally, YL maintains the integrity of the intestinal mucosa and hepatic structure, likely attributable to nucleotides and oligosaccharides in yeast facilitating the growth of intestinal cells and the synthesis of cell membranes (Uauy, 1994; Torrecillas et al., 2011a; Torrecillas et al., 2011b, 2013, 2014; Hossain et al., 2020). However, the limited digestibility of YL poses a challenge. The yeast cell wall restricts enzyme access to cellular contents, thereby limiting nutrient utilization (Yamada and Sgarbieri, 2005; Overland and Skrede, 2017). Disrupting the yeast cell wall, as shown to increase protein digestibility in rainbow trout (Rumsey et al., 1991), improving the ADC in shrimp diets (Zhao et al., 2017), and other processing methods (Hansen et al., 2021), are essential for enhancing the bioavailability of YL.

The inclusion of high dietary levels of YL improved lipid metabolism in shrimp, as evidenced by the significant reductions in serum triglyceride (TG) and total cholesterol (TC) contents. The serum biochemical parameters, known indicators of nutritional status and physiological conditions (Parrino et al., 2018; Burgos-Aceves et al., 2019), markedly decreased with increasing YL. Comparable effects on lipid profiles in turkeys and piglets were observed with 3.0% to 6.0% YL inclusion, decreasing TG, TC, and LDL-C levels. The production of bile salt hydrolase (BSH), which catalyzes bile salt hydrolysis, increases the excretion of endogenous cholesterol, thus regulating serum and hepatic cholesterol levels (Reis et al., 2017). Multivariate correlation analysis revealed a significant association between serum physiological and biochemical indices and whole-body crude lipid content, indicating enhanced blood lipid metabolism due to dietary YL.

Dietary YL inclusion positively impacted nonspecific immunity by enhancing antioxidant capacity. Antioxidant enzymes such as catalase (CAT) and superoxide dismutase (SOD) play crucial roles in balancing oxidative and antioxidant status by eliminating excess reactive oxygen species (ROS) (Gao et al., 2014; Xu et al., 2018). Malondialdehyde (MDA), an indicator of lipid peroxidation, was used to assess oxidative damage (Del Rio et al., 2005). Dietary YL inclusion increased the total antioxidant capacity (T-AOC) in the serum, liver, and pancreas while reducing the MDA content. Dietary YL inclusion has been shown to promote antioxidant capacity and increase immune-related gene expression in Largemouth bass (Fei et al., 2022), Pacific red snapper (Lutjanus peru) (Alamillo et al., 2017; Reyes-Becerril et al., 2021), and shrimp (Licona-Jain et al., 2020; Liu et al., 2022). β-glucan in YL enhances the organism’s antioxidant capacity by scavenging free radicals (Kofuji et al., 2012; Nakashima et al., 2018), and MOS in YL binds pathogens in the gastrointestinal tract, restricting colonization, enhancing intestinal mucosa integrity, stimulating the body’s immune response, and contributing to the antioxidant process (Torrecillas et al., 2014; Spring et al., 2015). In addition, yeast is rich in components such as peptides, organic acids, and vitamins, all of which have been shown to enhance the antioxidative capacity (Suchner et al., 2000; Ringø et al., 2012). Therefore, dietary YL inclusion effectively improves antioxidant performance in shrimp by increasing their antioxidant capacity.