Effects of Different Light Spectra on Oxidative Stress and Nutritional Quality of the Fish Plectropomus leopardus

Abstract

This study investigated the impacts of light spectra on oxidative stress and nutrient quality of the fish Plectropomus leopardus in indoor recirculating aquaculture systems. The fish (100 g ± 0.45 g [wet weight]) were cultures in five different light spectra (full-spectrum (400–800 nm), blue (450 nm), green (530 nm), red (630 nm), and dark) for 60 days. After experimentation, blood and muscle tissue were collected and analyzed for biochemical variables and nutritional quality. We demonstrated that the total cholesterol, triglycerides activities of P. leopardus in the dark groups were substantially elevated, relative to other groups (p < 0.05). Glutathione and glutathione peroxidase activities were elevated in the green light group versus other red groups, and cortisol was drastically reduced in the red group relative to other groups (p < 0.05). The crude ash concentration in the blue and full-spectrum group was substantially more elevated than in other groups (p < 0.05). Thr, Glu, Cys, Val, Met, Ile, Leu, Phe, Lys, His, Arg were markedly higher in the blue light versus the red light group (p < 0.05). The muscle of P. leopardus was rich in lysine and its essential AA index was in the order of blue light, full-spectrum, green light, dark and red group. The content of total saturated fatty acids in the blue light group was drastically lower relative to the dark, green and red groups (p < 0.05), and the total polyunsaturated fatty acids and DHA + EPA contents in the blue light group were substantially elevated relative to the other groups (p < 0.05). These results revealed that different light environments had certain effects on blood biochemical, antioxidant capacity, nutrient composition and proportion of P. leopardus. A comprehensive evaluation found that the blue light environment had more positive effects on the physiological, biochemical and nutritional quality of P. leopardus. This result provides a theoretical reference for the lighting strategy of an indoor recirculating aquaculture system.

1. Introduction

Plectropomus leopardus is a high value-added fish species raised in mariculture in China. It is valued by consumers for its superior meat quality, high protein, low fat and cholesterol, and bright color [1]. The annual production of Epinephelus in China was 240,000 tons in 2023, and the annual production of P. leopardus was about 20,000 tons, four times more than the same period in 2023 [2]. The recirculating aquaculture system is a major culture strategy for P. leopardus, and it is characterized by enhanced density, good regulation and independence from geographical and climatic factors [3,4]. However, the factory buildings of this recirculating aquaculture system also hinder the sunlight to a certain extent [5]. After sunlight passes through the water surface, a series of physical phenomena will occur, including refraction, scattering and absorption. Different spectral components of light will decay and change to different degrees, and eventually lead to changes in the spectral environment of fish survival. Therefore, different fish may prefer different spectral compositions. It was found that female Acrossocheilus fasciatus preferred blue light, while male A. fasciatus preferred red light [6]. Red light significantly increases dopamine activity in the brain of Sparus aurata [7]. P. leopardus lives near coral reefs in water depths of 3 to 100 m. After being cultured, its water depth is mostly concentrated within 1 m, so to explore P. leopardus, a suitable spectral environment is conducive to artificially supplementing the light source in the breeding environment to make its living environment more similar to the original natural environment, which may be more conducive to its survival.

The light spectrum environment directly or indirectly affects aquaculture animals’ nutritional growth [8] metabolism [9], immune stress [10], vision [11], biological rhythms [12] and behavior [13]. Different aquaculture animals respond differently to the light environment, and the study found that red light is more beneficial to Cyprinus carpiovar. For the low-density culture of C. carpiovar and the high-density culture of C. carpiovar blue light is more beneficial [14]. Studies have found that exposure to blue light has a positive effect on the physiological function of Sparus aurata [7]. Blue light increases the antioxidant capacity of fish by promoting antioxidant oxidase activity in Oncorhynchus mykiss [15]. While different species may have different adaptations to light and color, studies have found that the red spectrum induces oxidative stress in Amphprion Clarkii, thereby reducing oxidative stress in fish [16]. Green and blue wavelengths inhibit oxidative stress and apoptosis of Paralichthys olivaceus [17]. These results indicate that the appropriate spectral environment can promote the antioxidant stress and other physiological functions of aquaculture organisms. Therefore, in the industrial culture environment, providing a suitable spectral environment for aquaculture organisms will promote the physiological and biochemical ability of aquaculture organisms.

The nutritional quality of fish is one of the important factors that determine the market price of fish, which is usually affected by genetic factors, feed, growth environment, and so on. With the in-depth understanding of the light environment, exploration of the light environment-mediated regulation of the nutritional quality of cultured fish has gradually increased. Currently, there are some investigations examining the influences of the light environment on the nutritional quality of fish: Dicentrarchus labrax has higher collagen content and muscle quality under 8L:16D and 12L:12D photoperiod [18] and Maccullochella peelii has the highest crude ash and crude fat content in muscle at a light intensity of 1500 lx and a photoperiod of 18L:16D [19]. Regarding the light spectra, it was found that green light increased muscle protein content, improved muscle quality, nutritional value and economic benefits of D. labrax [20]. Hence, the nutritional quality of different fish is affected differently by the spectrum. Suitable light environment can promote the nutritional quality of fish muscle to a certain extent.

In this study, we selected five different LED spectral environments (full-spectrum, blue, green, red and dark) to evaluate the effects of the spectral environment on the antioxidant capacity and nutritional quality of P. leopardus. The results of this study can provide parameters for selecting a suitable light environment for industrial cultured P. leopardus, improve the culture effect of indoor P. leopardus, and lay a theoretical foundation for the establishment of P. leopardus indoor industrial culture lighting technology.

2. Materials and Methods

2.1. Experimental Fish

P. leopardus were acquired from Hainan province for the experiment. The fish with no external injuries weighed 100 ± 0.45 g and were 20.8 ± 0.50 cm long. In all, 3330 fish were taken from Hainan to Mingbo Aquaculture Co., Ltd. in Laizhou, Shandong Province. They were kept in circular polypropylene (PP) tanks (2 m diameter, 1.1 m height, water depth 1 m) with recirculating water for 7 days prior to the experiment.

2.2. Experimental Design

The experiment was performed at the Mingbo Aquaculture Co. Ltd.’s small recirculating water workshop in Laizhou, Shandong Province. It made use of a recirculating aquaculture system and lasted for 60 days. We introduced 185 fish per tank to achieve a density of 6.6 kg/m². Feedings were administered twice daily at 8:00 and 16:00 (Santong Bio-engineering Co., Ltd., Weifang, China). Feed at 2% of body weight, when excess bait sedimented to the bottom of tank, we stopped feeding, and removed 10 min later. The tank was maintained with 24 number/d recirculating water at 25 ± 0.2 °C, 27 ± 0.25 salinity, 7.6 ± 0.1 pH, and ≥8.0 mg/L dissolved oxygen content. Total ammonium nitrogen (TAN) and nitrite levels were monitored in all tanks every 2 days and both compounds were regulated/maintained at TAN < 0.3 mg/L and nitrite < 0.1 mg/L.

The tanks described above were constructed with lamps from Foshan Electrical and Lighting Co., Ltd. (FSL, Hainan, China) connected to the company software, which allowed us to modify the light cycle and intensity. In total, we established 5 distinct light spectrum compositions: full-spectrum (400 to 800 nm), blue (450 nm), green (530 nm), red (630 nm) lights, and darkness (Figure 1). We covered each tank with blackout cloths to minimize contamination risk from other light sources. In addition, we conducted routine (weekly) PLA-30 calibration (Hangzhou Faraway Optoelectronic Information Co., Ltd., Hangzhou, China) to maintain a stable 2 W/m² irradiance above the water surface center in tanks. The lighting cycle was 16/8 h light/dark, gradually increasing or decreasing the brightness to reach the set light intensity to avoid stress to P. leopardus.

2.3. Sample Collection

After 60 days of experimentation, and after 20 min of anesthesia for MS-222 (80 mg/L) whole tanks, we randomly selected 3 fish from individual tanks for dissection in the dissecting room, with a total of 9 fish in each light environment. A blood sample was obtained from the caudal vein of the fish using a disposable syringe. The collected blood samples were centrifuged at 4000 rpm and 4 °C for 10 min to obtain the supernatant, which was then stored at −20 °C until further analysis. An appropriate amount of P. leopardus back muscle was placed in a 15 mL cryopreservation tube and stored at −80 °C in a freezer for determination of nutrient content, fatty acid (FA) content and amino acid (AA) content. The sampling time of each individual optical environment treatment group did not exceed 60 min.

2.4. Analysis of Enzyme Activity

Total cholesterol (TC), triglycerides (TG), Plasma glucose (Glu), Albumin, Maltose, lactose glutathione (GSH), glutathione peroxidase (GPx), Cortisol and NO activities were measured with diagnostic reagent kits from Nanjing Jiancheng Bioengineering Institute (Nanjing, China). Circulating protein contents were quantified via the Bradford protocol [21], with bovine serum albumin as the standard. GSH content was determined via colorimetric analysis using protocol by [22] and is presented as microgram per milligram protein. One unit of GPx activity was described as 1 mmol NADPH oxidized per minute. All enzymatic activities are presented as units per mg of soluble protein.

2.5. Routine Nutritional Components

Moisture was quantified by employing the 105 °C direct-drying protocol (GB 5009.3-2016). The ash concentration was computed via the muffle furnace volatilization constant weight protocol (GB 5009.4-2016). The crude protein levels were measured via the micro-Kjeldahl protocol (GB 5009.5-2016). The crude fat content was quantified via the Soxhlet extraction protocol (GB 5009.6-2016).

2.6. Determination of Muscular AA and FA Composition

The muscle samples were acidified with hydrochloric acid, and the AA concentration was quantified via Hitachi L-8900 AA analyzer using GB 5009.124-2016 method. After freeze-drying the muscle samples to constant weight, FA composition was determined by gas chromatograph (7890A, Agilent) using the GB 5009.168-2016 method. Compared with the residence time of standard FAs, the relative content of individual FA were computed by area normalization.

2.7. Muscle Nutritional Quality Evaluation

Based on current nutritional quality assessment methods, the AA score (AAS), chemical score (CS), and essential AA index (EAAI) for muscle nutritional quality evaluation were based on the standardized pattern of AAS per gram proposed by the Food and Agriculture Organization of the United Nations/World Health Organization (FAO/WHO) in 1973, and the standardized pattern of AAS per gram recommended by the Institute of Nutrition and Food Hygiene of the Chinese Academy of Preventive Medical Sciences (CAPMS) in 1991. The AA pattern of whole egg protein recommended by the Institute of Nutrition and Food Hygiene of the Chinese Academy of Preventive Medical Sciences in 1991 was compared.

aa = (Mass fraction of this AA in sample/Mass fraction of crude protein in the sample) × 6.25 × 1000.

AAS = aa/AA (FAO/WHO)

CS = aa/AA (egg)

EAAI = (100 A/AE × 100 B/BE × 100 C/CE × … × 100 H/HE) ^1/n

where aa represents certain AA content in the test sample (mg/g), AA (FAO/WHO) represents the AA standard content (mg/g), AA (egg) is the same AA content in the protein of whole egg (mg/g), n represents the comparative EAas content, and A, B, C, …, H represent a certain EAA content in fish muscle protein (mg/g); AE, BE, CE, …, IE represent each type of EAA content in the protein of whole egg (mg/g). A, B, C, …, H represent certain EAas content in fish muscle protein (mg/g), and AE, BE, CE, …, IE represent various EAAs content in whole egg protein (mg/g).

2.8. Statistical Analysis

All data are expressed as mean ± standard deviation (Mean ± SD). Inter-group differences were assessed using one-way analysis of variance (ANOVA) in SPSS 22.0. Multi-group differences were assessed via the Duncan approach. p < 0.05 indicated significance. Visual representation of data outcomes was carried out in Origin 2021.

3. Results

3.1. Influence of Light Spectral on Blood Biochemical Parameters

We observed that the TC and TG concentration in the black group was significantly higher than in the full-spectrum and red groups (p < 0.05), while the TG concentration of the blue and green groups were significantly lower than the dark group (p < 0.05) (Figure 2A,B). The albumin, maltose and lactose concentration in the blue and green groups were significantly higher than that in the full-spectrum or dark groups (p < 0.05) (Figure 2C,E,F).

3.2. Influence of Light Spectral on Levels of Antioxidant Enzymes

We found the GSH activities in the green and blue light groups were substantially elevated relative to other groups (p < 0.05) (Figure 3A). The GSH-PX activities in the green and blue light groups were substantially enhanced relative to the other groups (p < 0.05) (Figure 3B). The cortisol levels in the red group were significantly higher than the other groups (p < 0.05) (Figure 3C). These figures revealed the same pattern from elevated to reduced: green light, blue light, dark, full-spectrum and red light (p < 0.05).

3.3. Influence of Light Spectral on P. leopardus Body Composition

As summarized in

Table 1. Effects of different Light Spectral on the nutrient composition of P. leopardus g/100 g.

Parameters	Groups
	Full Spectrum Light	Dark Light	Blue Light	Green Light	Red Light
Moisture	75.60 ± 0.40 b	75.50 ± 0.41 b	75.70 ± 0.32 ab	76.30 ± 0.24 a	75.40 ± 0.27 b
Ash	0.80 ± 0.08 a	0.50 ± 0.09 b	0.80 ± 0.15 a	0.50 ± 0.03 b	0.40 ± 0.07 b
Crude lipid	1.20 ± 0.19	1.10 ± 0.19	1.20 ± 0.09	1.20 ± 0.03	1.20 ± 0.33
Crude protein	21.50 ± 0.70	21.30 ± 0.78	22.00 ± 0.41	21.00 ± 0.62	21.90 ± 0.09

Notes: The values are expressed as the mean ± SD, n = 3. Different letters indicate significant differences (p < 0.05) among groups.

, the moisture levels of P. leopardus muscle in the green group was substantially elevated relative to the full-spectrum, dark and red groups (p < 0.05). The ash content of P. leopardus muscle in the full-spectrum and blue groups were substantially elevated relative to the other groups (p < 0.05).

3.4. Influence of Light Spectral on AA Levels

Seventeen AAs, including seven essential AAs (EAA), five semi-essential AAs (HEAA), and ten non-essential AAs (NEAA), were detected in the muscle of the P. leopardus at five different light spectra (

Table 2. Effects of different light spectra on the amino acid composition of P. leopardus g/100 g.

Amino Acids	Groups
	Full Spectrum Light	Dark Light	Blue Light	Green Light	Red Light
Aspartic acid (Asp) #	1.62 ± 0.09	1.57 ± 0.31	1.84 ± 0.04	1.74 ± 0.04	1.56 ± 0.02
Threonine (Thr) *	0.57 ± 0.04 ab	0.54 ± 0.18 ab	0.70 ± 0.00 a	0.63 ± 0.02 ab	0.50 ± 0.01 b
Serine (Ser) #,&	0.63 ± 0.03	0.61 ± 0.12	0.71 ± 0.01	0.67 ± 0.02	0.60 ± 0.01
Glutamic acid (Glu) #	2.17 ± 0.10 ab	2.11 ± 0.51 ab	2.57 ± 0.04 a	2.39 ± 0.05 ab	2.07 ± 0.02 b
Glycine (Gly) #,&	0.79 ± 0.01	0.82 ± 0.09	0.85 ± 0.02	0.82 ± 0.03	0.81 ± 0.02
Alanine (Ala) #	0.99 ± 0.05	0.97 ± 0.13	1.06 ± 0.00	1.03 ± 0.01	0.98 ± 0.02
Cystine (Cys) #,&	0.09 ± 0.01 ab	0.08 ± 0.05 b	0.13 ± 0.01 a	0.11 ± 0.01 ab	0.09 ± 0.01 ab
Valine (Val) *	0.40 ± 0.03 ab	0.38 ± 0.15 b	0.51 ± 0.01 a	0.44 ± 0.02 ab	0.33 ± 0.00 b
Methionine (Met) #	0.45 ± 0.02 ab	0.42 ± 0.06 b	0.50 ± 0.03 a	0.48 ± 0.00 ab	0.43 ± 0.00 b
Isoleucine (IIe) *	0.36 ± 0.03 ab	0.34 ± 0.13 ab	0.46 ± 0.00 a	0.39 ± 0.01 ab	0.30 ± 0.01 b
Leucin e(Leu) *	1.13 ± 0.05 ab	1.09 ± 0.21 b	1.29 ± 0.01 a	1.20 ± 0.03 ab	1.06 ± 0.01 b
Tyrosine (Tyr) #,&	0.54 ± 0.01	0.54 ± 0.04	0.57 ± 0.01	0.56 ± 0.00	0.55 ± 0.00
Phenylalanine (Phe) *	0.56 ± 0.04 ab	0.53 ± 0.15 ab	0.66 ± 0.01 a	0.60 ± 0.01 ab	0.50 ± 0.01 b
Lysine (Lys) *	1.23 ± 0.08 ab	1.17 ± 0.31 ab	1.46 ± 0.01 a	1.31 ± 0.04 ab	1.11 ± 0.02 b
Histidine (His) *	0.29 ± 0.02 ab	0.27 ± 0.07 ab	0.33 ± 0.00 a	0.31 ± 0.01 ab	0.26 ± 0.01 b
Arginine (Arg) #,&	0.81 ± 0.04 ab	0.78 ± 0.17 b	0.95 ± 0.00 a	0.87 ± 0.04 ab	0.76 ± 0.01 b
Proline (Pro) #	0.43 ± 0.03	0.43 ± 0.08	0.48 ± 0.00	0.47 ± 0.01	0.43 ± 0.01
Total amino acids (∑TAA)	13.04 ± 0.66 ab	12.64 ± 2.77 ab	15.07 ± 0.14 a	14.01 ± 0.35 ab	12.33 ± 0.13 b
Total essential amino acids (∑EAA)	4.53 ± 0.29 ab	4.33 ± 1.21 ab	5.42 ± 0.03 a	4.87 ± 0.14 ab	4.05 ± 0.06 b
Total non-essential amino acids (∑NEAA)	8.51 ± 0.37	8.32 ± 1.56	9.66 ± 0.11	9.14 ± 0.21	8.28 ± 0.07
Total semi-essential amino acids (∑HEAA)	2.86 ± 0.09	2.82 ± 0.47	3.21 ± 0.01	3.03 ± 0.09	2.81 ± 0.02
EAA/TAA	0.35 ± 0.00 ab	0.34 ± 0.02 bc	0.36 ± 0.00 a	0.35 ± 0.00 ab	0.33 ± 0.00 c
EAA/NEAA	0.53 ± 0.01 b	0.52 ± 0.04 b	0.56 ± 0.00 a	0.53 ± 0.00 b	0.49 ± 0.01 b

Note: ^* semi-essential amino acid; ^#: non-essential amino acid; ^&: essential amino acid. The values are expressed as the mean ± SD, n = 3. Different letters indicate significant differences (p < 0.05) among groups.

). ANOVA revealed five EAAs (threonine (Thr), isoleucine (IIe), phenylalanine (Phe), lysine (Lys), histidine (His)), and one NEAA (glutamic acid (Glu)) in the blue group of the muscle in P. leopardus were substantially elevated relative to the red group, along with the total AAs (∑TAA) and total EAAs (∑EAA) (p < 0.05). There were two EAAs (valine (Val) and leucine (Leu)), and two NEAAs (methionine (Met) and arginine (Arg)) in the blue group of the muscle in P. leopardus were substantially elevated relative to the green and red groups (p < 0.05). Cysteine (Cys) content was markedly raised in fish grown with blue rather than green light (p < 0.05). EAA/TAA in the blue group was substantially elevated relative to the green group (p < 0.05), strongly upregulated relative to the red group (p < 0.05). According to the AA muscle composition, Glu was the largest among 17 AAs in the muscle of P. leopardus under five different light spectra, then Asp, Lys and Leu, Cys content was the lowest.

3.5. Influence of Light Spectral on the Muscle Nutritional Quality

The muscle EAA content of P. leopardus at different light spectra varied in different groups in

Table 3. Comparison of essential amino acid content in muscle species of P. leopardus with FAO/WHO and egg protein models at different light spectra.

EAA	FAO/WHO Amino Acid Pattern	Whole Egg Protein Amino Acid Pattern	Groups
			Full Spectrum Light	Dark Light	Blue Light	Green Light	Red Light
Ile	250	331	193.94	193.68	216.51	193.25	172.80
Leu	440	534	614.46	593.61	607.60	612.23	605.40
Lys	340	441	668.19	651.05	688.24	659.89	632.54
Thr	250	292	311.72	310.94	331.23	303.26	283.18
Val	310	411	217.80	215.73	240.74	211.34	185.13
Met + Cys	220	386	296.08	292.94	295.38	282.83	292.89
Phe + Tyr	380	565	599.37	576.37	582.21	600.43	594.93
Total	2190	2960	2901.57	2834.32	2961.90	2863.24	2766.87

. The highest value was 2961.90 in the blue light group, the dark group followed with 2834.32, and the red group was the lowest at 2766.87. All groups were elevated relative to the FAO/WHO AA pattern (2190 mg/g N, without Trp), while only the blue light group was higher than the Whole egg protein AA pattern. Leu, Lys, Thr, Met + Cys and Phe + Tyr content exceeded the FAO/WHO AA pattern and Whole egg protein AA pattern (2960 mg/g N, without Trp). Ile and Val were lower than the FAO/WHO AA profile and Whole egg protein AA pattern in all groups. It can be seen that P. leopardus has higher nutritional value, and the nutritional quality of P. leopardus in the blue light and full-spectrum group is higher.

As can be seen from

Table 4. Comparison of AAS, CS and EAAI of muscle essential amino acids of P. leopardus at different light spectra.

Essential Amino Acid	AAS					CS
	Full Spectrum Light	Dark Light	Blue Light	Green Light	Red Light	Full Spectrum Light	Dark Light	Blue Light	Green Light	Red Light
Ile	0.78	0.77	0.87	0.77	0.69	0.59	0.59	0.65	0.58	0.52
Leu	1.40	1.35	1.38	1.39	1.38	1.15	1.11	1.14	1.15	1.13
Lys	1.97	1.91	2.02	1.94	1.86	1.52	1.48	1.56	1.50	1.43
Thr	1.25	1.24	1.32	1.21	1.13	1.07	1.06	1.13	1.04	0.97
Val	0.70	0.70	0.78	0.68	0.60	0.53	0.52	0.59	0.51	0.45
Met + Cys	1.35	1.33	1.34	1.29	1.33	0.77	0.76	0.77	0.73	0.76
Phe + Tyr	1.58	1.52	1.53	1.58	1.57	1.06	1.02	1.03	1.06	1.05
Essential amino acid index (EAAI)	89.78	88.21	93.15	88.24	84.05

, except Ile and Val, the AAS of P. leopardus muscle EAAs in all treatment groups are >1, the CS are >0.5, and the AAS and CS of P. leopardus EAAs in all treatment groups basically conform to the rule that the blue light group is the highest, then the full-spectrum group, and the red group is the lowest. This indicates that the EAAs composition in P. leopardus muscle under blue light is more balanced and nutritionally rich than that in red light. Based on these results for AAS and CS, the first limiting AA in the muscles of P. leopardus under the five light conditions was Val, and the second limiting AA was Ile. The EAAI index of P. leopardus under different light conditions was blue (93.15) > full-spectrum (89.78) > green light (88.24) > dark (88.21) > red light (84.05).

3.6. Influences of Varying Light Spectra on the FAs of P. leopardus

Gas chromatography-mass spectrometry was used to detect the FA composition of P. leopardus muscle under different light environments by the area normalization method, and the results are summarized in

Table 5. Composition and content of fatty acids in different light spectra of P. leopardus.

Fatty Acids (%)	Groups
	Full Spectrum Light	Dark Light	Blue Light	Green Light	Red Light
C14:0	4.28 ± 2.10	6.19 ± 0.76	4.89 ± 0.49	4.41 ± 1.04	4.74 ± 0.28
C15:0	0.43 ± 0.12	0.57 ± 0.10	0.50 ± 0.04	0.49 ± 0.10	0.52 ± 0.07
C16:0	25.81 ± 3.40 bc	32.43 ± 3.40 a	23.92 ± 1.92 c	30.91 ± 5.11 ab	30.74 ± 1.77 abc
C17:0	0.50 ± 0.02	0.62 ± 0.10	0.50 ± 0.01	0.66 ± 0.10	0.66 ± 0.13
C18:0	8.75 ± 1.40	10.22 ± 1.66	10.92 ± 2.31	11.58 ± 1.43	11.73 ± 2.32
C20:0	0.36 ± 0.02 b	0.48 ± 0.07 ab	0.38 ± 0.02 ab	0.46 ± 0.06 ab	0.50 ± 0.10 a
C21:0	0.07 ± 0.01	0.09 ± 0.02	0.07 ± 0.00	0.08 ± 0.02	0.09 ± 0.02
C22:0	0.21 ± 0.04 b	0.27 ± 0.04 ab	0.20 ± 0.02 b	0.26 ± 0.03 ab	0.32 ± 0.06 a
C23:0	0.10 ± 0.01	0.13 ± 0.01	0.10 ± 0.01	0.12 ± 0.03	0.13 ± 0.03
C24:0	0.50 ± 0.07	0.57 ± 0.03	0.60 ± 0.13	0.58 ± 0.06	0.62 ± 0.13
C14:1	0.12 ± 0.09 ab	0.18 ± 0.05 a	0.08 ± 0.05 ab	0.06 ± 0.02 b	0.13 ± 0.01 ab
C16:1	5.43 ± 1.69 ab	6.73 ± 0.32 a	3.96 ± 1.57 b	5.77 ± 0.92 ab	5.94 ± 0.32 ab
C18:1n9c	17.00 ± 1.16 bc	19.13 ± 0.54 a	15.48 ± 1.47 c	18.25 ± 1.01 ab	18.61 ± 0.94 ab
C20:1	3.22 ± 0.39 ab	3.64 ± 0.30 a	2.77 ± 0.47 b	3.44 ± 0.35 ab	3.68 ± 0.33 a
C22:1n9	0.66 ± 0.13	0.76 ± 0.08	0.61 ± 0.08	0.72 ± 0.08	0.80 ± 0.12
C24:1	2.66 ± 0.38	2.75 ± 0.26	3.27 ± 0.73	2.99 ± 0.06	3.26 ± 0.28
C18:2n6c	4.58 ± 0.11	3.59 ± 1.06	4.46 ± 0.31	3.61 ± 0.71	3.35 ± 1.20
C18:3n3	0.09 ± 0.01 a	0.05 ± 0.02 b	0.08 ± 0.01 ab	0.07 ± 0.02 ab	0.06 ± 0.02 ab
C18:3n6	0.59 ± 0.06	0.38 ± 0.18	0.47 ± 0.16	0.38 ± 0.14	0.34 ± 0.19
C20:2	0.46 ± 0.03	0.44 ± 0.13	0.47 ± 0.01	0.54 ± 0.12	0.50 ± 0.07
C20:3n6	0.16 ± 0.04 ab	0.08 ± 0.05 b	0.18 ± 0.03 a	0.12 ± 0.07 ab	0.11 ± 0.05 ab
C20:3n3	0.17 ± 0.04	0.18 ± 0.06	0.21 ± 0.06	0.20 ± 0.07	0.21 ± 0.02
C22:2	0.07 ± 0.01	0.05 ± 0.02	0.08 ± 0.02	0.08 ± 0.01	0.07 ± 0.01
C20:4n6 (ARA)	4.81 ± 0.62	4.64 ± 0.31	4.12 ± 0.87	4.38 ± 0.55	4.82 ± 0.53
C20:5n3 (EPA)	4.53 ± 1.02 ab	1.79 ± 1.42 c	4.86 ± 0.04 a	2.51 ± 1.94 abc	1.96 ± 1.70 bc
C22:6n3 (DHA)	14.37 ± 6.96 a	3.96 ± 3.64 b	16.30 ± 3.05 a	7.25 ± 7.00 ab	6.69 ± 3.34 ab
∑SFA	41.00 ± 4.29 bc	51.56 ± 5.41 a	42.07 ± 0.83 c	49.56 ± 7.89 ab	49.38 ± 4.32 ab
∑MUFA	29.09 ± 3.05 ab	33.19 ± 0.83 a	26.18 ± 2.88 b	31.23 ± 2.11 a	32.42 ± 1.85 a
∑PUFA	29.85 ± 7.36 ab	15.15 ± 6.20 c	31.24 ± 1.86 a	19.13 ± 9.99 bc	18.11 ± 5.77 bc
EPA + DHA	18.91 ± 7.97 ab	5.74 ± 5.06 c	21.16 ± 3.09 a	9.76 ± 8.94 bc	8.66 ± 4.74 bc

Notes: ∑SFA is the total amount of saturated fatty acids; ∑MUFA is the total amount of monounsaturated fatty acids; ∑PUFA is the total amount of polyunsaturated fatty acids. The values are expressed as the mean ± SD, n = 3. Different letters indicate significant differences (p < 0.05) among groups.

. The FA composition of P. leopardus muscle under different light conditions was similar, and 26 FAs were identified, namely 10 saturated FAs (SFA), six monounsaturated FAs (MUFA), and 10 polyunsaturated FAs (PUFA).

There were 11 FAs with a content higher than 1%, and for their average content, the top two were C16:0 and C18:1n9c. In SFA and MUFA, the red and dark groups showed higher FA content, while the blue group was significantly or not drastically diminished relative to the other groups (p < 0.05), such as C20:0, C22:0, and C20:1. However, PUFA levels were markedly greater in the blue and full-spectrum groups relative to the dark group (p < 0.05), such as C18:3n6, C20:3n6, C20:5n3 (EPA) and C22:6n3 (DHA). SFA in P. leopardus muscle under different light conditions were substantially augmented in dark versus other groups (p < 0.05), while drastically diminished in the blue group (p < 0.05). MUFA were strongly decreased in blue light versus other groups (p < 0.05). Alternately, the PUFA content in P. leopardus muscle was strongly elevated in blue light versus other groups (p < 0.05), while significantly lower in the dark group, and the EPA + DHA content also showed the same trend (p < 0.05).

4. Discussion

Albumin is an important plasma protein that is responsible for maintaining the osmotic pressure of plasma and for modulating fat-soluble substance transport [23]. The impact of discreet light intensity on the blood glucose of Megalobrama amblycephala increased with the increase of light intensity, but the effect on albumin was not significant [24]. Maltose and lactose are sugars, and the change of their content can indicate the intake of glycogen in food [25]. Herein, the albumin concentration of P. leopardus in the blue light group was significantly higher relative to other groups, indicating that the blue light environment promoted the increase of fat-soluble substance content, which may be caused by the increase of feeding by blue light [5]. P. leopardus had more significant feeding behavior and food intake in blue and green light environments, which resulted in significantly higher maltose and lactose content in these two light environments than in other groups.

TG is mainly used for energy storage and TC is very important for cell membrane structure and hormone synthesis, its content being strongly associated with the body’s lipid metabolism [26]. GSH is an important antioxidant that helps scavenge free radicals, and GSH-PX uses GSH to protect the body’s cells from oxidative damage. Generally, organisms protect themselves against ROS-mediated toxicity by upregulating antioxidant enzyme expressions [27]. Previous studies have shown that augmented intensity light can cause lipid metabolism impairment in Labeo rohita [28]. The GSH activity of M. amblycephala increases first and then decreases with the increase of light intensity, while the GSH-PX activity continues to increase [24]. A blue light environment can significantly increase the GSH-PX activity of shrimp [27]. With the increase of light intensity, the cortisol level of Clarias gariepinus gradually increased [29]. Trachinotus blochii larvae under green light exhibited markedly augmented cortisol concentrations at 4dph and 12dph relative to the other light [30]; yellow light decreases stress-triggered cortisol responses of Pearl gourami fish, Trichopodus leerii [31]. In this study, serum glucose concentration remained unchanged in the five different light conditions, but TG and TC content in the dark group was substantially elevated relative to the other groups, which may be caused by excess lipid metabolism caused by stress. The content of TG and TC in the red light and the full-spectrum group was strongly reduced relative to the other groups, suggesting that these two light environments caused the damage of lipid metabolism to a certain extent. Cortisol levels drastically reduced in the red light versus other groups and low levels of GSH-Px and GSH activity were produced in the full-spectrum, dark and red light groups, which indicates that the physiological antioxidant system may not effectively eliminate or neutralize excessive ROS when P. leopardus are exposed to dark and red light for a long time, and even severe oxidative damage occurs, which eventually decreases content or even degrades antioxidant enzymes [32]. Taken together, we demonstrated that red light and dark environments exposure caused considerable stress to P. leopardus, while blue and green light environments are more conducive to P. leopardus to clear free radicals and protect cells from oxidative damage.

Fish body composition is important for fish culture, indicating the allocation of energy by the body and affecting growth, food efficiency and survival [33]. Prior investigations revealed that light-colored light environments have an impact on the body composition of juvenile and adult fish [7,34]. For instance, red and white for Oncorhynchus mykiss and blue and white for Sparus aurata did not alter body composition [7]. Herein, we observed no marked change in crude fat and crude protein among all groups, and the lipid levels of P. leopardus in green light and dark groups were lower. This suggests that P. leopardus in these light environments use energy to cope with stressful situations, as evidenced by their augmented cortisol content, and the same results were found in [30] study. Crude ash is the mineral portion of the material that remains after high temperature incineration and can be used to estimate the organic content. In our study, crude ash level in the blue light and full-spectrum groups was substantially elevated relative to other groups, and the water content in the green light and blue light groups was strongly augmented relative to other groups. Combining the above results, blue light and full-spectrum environments may be more conducive to energy and organic matter accumulation in P. leopardus.

Protein in feed is the primary source of nutrients for fish and is composed of a variety of AAs. The freshness of animal protein is correlated with its umami AAs content, among which, Asp and Glu are the most important umami AAs, and aromatic AAs mainly consist of Tyr and Phe [35]. Herein, the Glu and Phe content in the red group was drastically reduced relative to the other groups, while the content in the blue light and green light groups was higher, indicating that blue and green light were more conducive to the related enzyme synthesis in P. leopardus muscle cells, thus promoting the occurrence of TCA and glycolysis, while the red group was not conducive to the umami and fragrance presentation of P. leopardus. Studies have shown that a green light environment promotes fiber production, and the umami AAs, sweet AAs and flavor AAs contents are higher than in other light environments [36], which corroborates the findings of this study. Thr and Lys are EAAs that cannot be synthesized by the human body and are critical indicators to evaluate the nutritional value of dietary proteins. Herein, the Thr and Lys concentrations of P. leopardus in the blue and green groups were augmented relative to the other groups. According to the model assessment of FAO/WHO, the AA concentration of the EAA/TAA composition of the protein with better quality should be about 40%, and the EAA/NEAA content should be about 60%. In this study, ∑TAA and ∑EAA in the red group were drastically reduced relative to other groups, and the blue light group and green light group were the highest. In EAA/TAA and EAA/NEAA, the blue light group was elevated relative to other groups (36% and 56%, respectively), then the green light group, and the red group was significantly lower at only 33% and 46%, respectively. EAAI is a frequently applied index for nutritional value evaluation of protein. The higher the content of EAAI, the more balanced the AA composition and enhanced protein quality. Notably, the AAs in muscle under blue, green, and full-spectrum light environments meet the quality protein standard, and the balance effect is good, and has a higher EAAI index. This further indicates that P. leopardus in blue and green light environments have better protein quality.

Lipids are key components of energy storage in fish, maintaining the integrity of cells and sub cell membranes [37], and regulating lipid accumulation mainly through lipogenesis and lipolysis [38]. Studies have shown that SFAs play an important decomposition function after the body is stimulated by the outside world, among which C14:0, C16:0 and C18:0 are the main ones, and C16:0 is preferentially used for energy consumption [39]. The high ∑PUFA content in fish muscle can substantially increase the aroma after heating, which reflects the juiciness of muscle to a certain extent. Herein, the ∑PUFA content in the muscle in the blue light group and full-spectrum group was substantially elevated relative to other groups, especially EPA and DHA. DHA and EPA are essential FAs of marine fish, and critically modulate cell membrane structure. Herein, the DHA and EPA contents in the muscle in the blue light group were 9.64% and 3.07% elevated relative to the dark group, respectively, indicating that blue and full-spectrum light environments were more favorable to the synthesis and accumulation of EPA and DHA in P. leopardus. Furthermore, muscle EPA +DHA content was successively from high to low in blue, complete spectrum light, green light, red light and dark groups. It can be seen that P. leopardus muscle fat has high health care and nutritional value under blue light and full-spectrum environments.

5. Conclusions

This study found that the dark and full-spectrum environments had a negative impact on the blood biochemistry, and that red light led to an increase in environmental stress, while blue and green light were more conducive to improvements in the antioxidant capacity of P. leopardus. In terms of nutritional quality and evaluation, the blue and green light increased some muscle AAs, EAAs PUFA and EPA + DHA contents in P. leopardus to different degrees and the AAS and CS showed higher values under blue light. Therefore, the blue light is more conducive to improving the antioxidant capacity and nutritional quality of P. leopardus, which may indicate that blue light is more conducive to the survival and life of P. leopardus. The results provided a theoretical basis and reference for the selection of light environment parameters of P. leopardus in indoor recirculating aquaculture.