1. Introduction
From the economic and nutritional point of view, shrimp is the most important aquaculture market, generating 4 million tons annually [
1]. Shrimp hyperintensive culture is a culture system carried out in open-air ponds or ponds covered with a plastic membrane ranging from 0.5 to 1 ha and is characterized by stocking densities exceeding 100 organisms m
-3. Therefore, it is necessary to add oxygen to the water through aeration equipment, allowing to improve culture conditions and optimize feeding [
2]. Due to the high stocking density, excess nitrogenous waste (ammonium and nitrites) is generated, which deteriorates the water quality and makes it toxic to shrimp. The main ammonium sources are shrimp excretions and sediment derived from mineralization of organic matter and molecular diffusion of reduced sediment [3, 4].
Nitrification is the general biochemical process of oxidation of ammonium (NH
4+) to nitrite (NO
2-) and, finally, to nitrate (NO
3-) [
5]. Bacterial nitrification is one of the commonly used methods for ammonium removal from aquaculture systems without water exchange [
6]. The oxidation of ammonium to nitrite is generally carried out by bacteria that possess the ammonia-oxygenase enzyme such as
Nitrosomonas,
Nitrosococcus,
Nitrosospira,
Nitrosolobus, and
Nitrosovibrio; whereas oxidation of nitrite to nitrate can be produced by the genera
Nitrospina,
Nitrococcus and
Nitrobacter, using the nitrite-oxidase enzyme. However, these genera of anaerobic bacteria are more susceptible to changes in the environment and have a slower metabolism [7, 8]. Therefore, there are other bacterial groups used for bioremediation in aquaculture, which include species such as
Bacillus licheniformis,
Bacillus subtilis,
Bacillus cereus,
Pseudomonas, and
Paracoccus [
9,
10,
11]. These bacteria are also recognized as probiotics in aquaculture because they provide protection against bacterial and viral invasions in shrimp by stimulating the immune response (cellular and humoral reactions) [12, 13].
The invertebrate immune system is made up of cellular (hemocytes) and humoral effectors. Hemocytes, which are the first line of defense, participate in phagocytosis, capsule and nodule formation, cell adhesion, cytotoxicity, and coagulation [
14]. Humoral effectors found in plasma or bound to hemocytes include antimicrobial peptides, lysozyme, lysosomal hydrolytic enzymes, lectins, the prophenoloxidase system, α2-macroglobulin, and transglutaminase [
15,
16,
17,
18].
Some bacterial species are culturable in the bacteriological media used in aquaculture; however, they could not offer an accurate picture as other species of bacteria could be present but hardly or even not culturable (Intriago et al. 2018). Therefore, metagenomics can be used to know the structure and function of the microorganisms of a specific sample [
19]). The modulation of the intestinal microbiota in cultured shrimp has been carried out with the addition of probiotic bacteria [
20]. The study of the microbial communities in the digestive tract of aquatic animals aims to highlight the benefit provided by the microbe-host relationship, which influences the health of the organism and its protection against pathogens [
21].
Therefore, in this work, the effect of nitrifying-probiotic bacteria on water quality, growth, intestinal microbiota, immune response, and survival of Litopenaeus vannamei cultured with zero water exchange was determined.
4. Discussion
The high animal densities that are managed in aquaculture systems deteriorate the water quality, which generates stress and susceptibility to diseases [39, 40]. Ammonia and nitrites at high concentrations are toxic to aquaculture animals and their toxicity depends on temperature, salinity, pH, or the developmental stage of the organism [
41,
42,
43,
44]. Furthermore, aquatic nitrogenous wastes, such as nitrites, can directly affect the nitrite content in shrimp intestine as this organ is exposed to the water environment [
45]. Therefore, reducing nitrogenous waste in culture systems is very important for producing cultured animals and preventing deterioration of the surrounding environment [
46].
The bacterial consortia, composed of several probiotic strains, usually have synergy leading to a bioremediation effect to improve water quality [
47]. Some bacterial strains contain enzymes that oxidize ammonia and nitrites, contributing to the nitrification process of nitrogenous waste in culture systems [
48]. In this work, only bacilli oxidize TAN and nitrites and reduce nitrates
in vitro since nitrogen bubbles were produced, indicating a nitrification and denitrification process. The addition of bacilli and LAB to the water decreased TAN and nitrite concentrations, showing a nitrifying effect, so a denitrifying effect was not observed as occurred
in vitro with bacilli. Also, it is important to mention that in the control there was also a nitrification process as TAN and nitrites were not high (0.3 and 0.4 mg L
-1) and nitrates were 2.47 mg L
-1, which could be caused by the natural bacteria community from the shrimp and initial filtered water. The nitrification process was reported in the culture of
L. vannamei [49, 50], when
Bacillus subtilis L10 and G1 were inoculated into the water. Denitrification process was observed in
Bacillus sp. SC16 in an intensive fishery aquaculture pond [
51]. Finally, there are no reports indicating
Pediococcus and
Leuconostoc as nitrifying bacteria.
In the aquatic environment, bacterial components in the sediment and water affect the bacterial communities in the shrimp intestine [
52,
53,
54] mention that 35.32% of bacteria in the intestine of the white shrimp come from water and 54.58% from sediments. The intestinal microbiota of organisms affects their digestion, absorption, growth, and immune response [
55]. Regarding the effect of bacteria and nitrogenous waste on growth, the best shrimp growth was observed in the treatments with bacilli or the mixture of bacilli and LAB in the water. It is known that bacterial species, such as
Bacillus, inoculated into the water, improve the growth of
L. vannamei [49, 56, 57] thanks to the increase in the production of enzymes that digest feed nutrients [
58] and short-chain fatty acids [
59]. On the other hand, elevated nitrites affected the growth of
L. vannamei [60, 61] and reduced food consumption in
Farfantepenaeus brasiliensis [
62]. Similarly, [
63] mention that ammonia and nitrites reduce the growth in weight and length of
L. vannamei cultured in the laboratory.
The optimal concentration of nitrogen in water in the form of ammonium (NH
4+), ammonia (NH
3), nitrites (NO
2-), and nitrates (NO
3-) is 0.2-2.0, 0.09-0.11, < 0.23, and 0.2-10 mg·L
-1, respectively [
64,
65,
66] mention that crustaceans excrete ammonia and cannot convert it to the less toxic compounds, so the high concentration of these compounds is toxic to crustaceans through the gill absorption. In the present work, TAN’s concentration was below the toxic concentration. However, the stress caused by ammonia reduces the survival of
L. vannamei grown in the laboratory [63, 65]. Nitrites are toxic to crustaceans because they convert hemocyanin into meta-hemocyanin, which is incapable of transporting oxygen [
68]. In our work, nitrites were in the range of 0.40 and 0.46 mg·L
-1, above the optimal concentration in all treatments. Therefore, it is possible that mortality was caused, in part, due to the stress induced by nitrites. The nitrification driven by bacteria (bacilli and LAB) could thus protect
L. vannamei against the negative effect of nitrites. In the work of [
60], a high mortality of
L. vannamei was observed due to nitrite poisoning (5.0-40.0 mg L
-1) and they mention that during molting there is a high consumption of oxygen so nitrites cause hypoxia or metabolic anoxia. In contrast, [
61] did not observe mortalities in
L. vannamei exposed to high concentrations of nitrites (2.0, 6.67, and 20 mg L
-1).
Invertebrates possess a natural, non-specific immune response, which plays an important role in resistance to microbial diseases [
68] as they do not produce antibodies like vertebrates [
17]. In our work, no significant change was observed in the number of hemocytes in shrimp treated with bacteria in the water and the accumulation of nitrogenous waste. [
69] found that
L. vannamei treated with 400 and 600 mg·L
-1 of
Gracilaria tenuistipitata extract for 3 h and then exposed to 5 mg·L
-1 of ammonia, increased hemocytes compared to the control group. Regarding the superoxide anion, it increased in the hemocytes of the organisms treated with LAB in the water compared to the control and the other treatments. In this treatment, nitrites were above the optimal range (0.09-0.11 mg L
-1). [
70] found that the concentration of nitrites in the hemolymph of
L. vannamei was similar to that of the culture water and that this exposure increased the generation of ROS (measured indirectly by the high activity of catalase in the hepatopancreas). Similaly, [
69] found that in the hemolymph of shrimp treated with
G. tenuistipitata extract and then exposed to ammonia, the activity of the enzyme superoxide dismutase increased, which has the superoxide anion radical as a substrate. In contrast, [
61] found no changes in SOD activity in the hepatopancreas of
L. vannamei exposed to high nitrite concentrations, and [
71] found similar results in
L. vannamei pretreated with dietary inositol and exposed to ammonia during 24 h. Regarding phenoloxidase, its activity decreased in the treatments with LAB alone and the mixture of bacteria in the water. The decrease in phenoloxidase activity could indicate an optimal physiological state with less biotic stress. Converseley, data reported by [
69] showed an increase in the activity of the enzyme in shrimp treated with
G. tenuistipitata and ammonia.
In the shrimp intestine, the microbiota is complex and variable, mainly affected by diet, developmental phase, immune response, metabolism, and the environment surrounding the animals [72, 73]. At the phylum level, Proteobacteria and Bacteroidota predominated; however, no significant differences were observed in bacterial treatments. In the work of [
61], at the concentration of 2 mg L
-1 of nitrites, the highest relative abundance corresponded to Bacteroidota, followed by Proteobacteria and Actinobacteria. At the highest nitrite concentration (20 mg L
-1), the highest abundance corresponded to Proteobacteria followed by Bacteroidota and Actinobacteria. All the above demonstrated that Proteobacteria and Bacteroidota are the most important components in shrimp intestine under this condition. Proteobacteria is a core member of shrimp gut microbiota [
74]. According to [
75] and [
76], the abundance of this phylum indicates efficient colonization of the shrimp intestinal epithelium, and possible degradation of agar and cellulose and nitrogen fixation in the shrimp rectum [
77]. Regarding the phylum Bacteroidota, it increases when the amount of dietary protein and fat increases [78, 79] and has a very important role due to its ability to utilize nitrogenous waste, biotransform steroids, and ferment carbohydrates [71, 72, 79, 80].
Within the native microbiota that predominates in marine species, the genus
Vibrio constitutes the greatest abundance [
81]. In our study, the highest relative abundance was presented by the
Vibrio genus, with the highest abundance in the bacilli treatment followed by the treatment with LAB in the water. [
82] and [
83] reported the
Vibrio genus as abundant in white shrimp intestines in treatments supplementing
Bacillus species to the culture water, which coincides with the report of [
84], where the mentioned genus predominated in most treatments supplemented with different
Bacillus mixtures. The control of
Vibrio in shrimp culture is very important as it could affect the shrimp health [
85]. However, some
Vibrio strains are beneficial to shrimp health such as and
V. hepatarius and
V. diabolicus that protect
Penaeus vannamei larvae against
V. parahaemolyticus [
86] or
V. parahaemolyticus,
V. diazotrophicus,
V. natriegens, and
V. campbellii strains that utilize several organic carbon sources (unused organic matter) and can fix nitrogen [87, 88]. The second most abundant genera was
Pseudoalteromonas, which is a probiotic bacteria that is antagonistic against
V. parahaemolyticus,
V. harveyi, and
V. nigripulchritudo [
89,
90,
91]. Bioencapsulated
Pseualteromonas in
Artemia sp. increases the immune response of shrimp and their resistance to infections caused by
V. harveyi [92, 93].
Functional redundancy provided by high microbial diversity allows an ecosystem to be more stable and resistant to stress [94, 95]. The total bacterial species richness in a sample can be determined with the Chao1 and ACE alpha indices [96, 97]. On the other hand, Shannon and Simpson alpha indices consider the richness of the microbial community and the evenness (relative abundance of different species) [
100,
101,
102,
103]. The treatment with bacilli in the water showed lower species richness (alpha diversity) compared to the control and the other treatments. [
54] found that the intestine of
L. vannamei cultured in a nutrient-rich environment (shrimp feces and organic waste) showed higher bacterial diversity. It is possible that the treatment with bacilli in the water had a lower nutrient load and, although it presented a lower richness of bacteria, it did show greater survival. Beta diversity is the degree of change or replacement in species composition between different communities [
104]. Regarding the analysis of diversity among communities (NMDS), this showed a grouping in each treatment; however, the bacterial community of the control was significantly different from the community of the treatments with bacteria in the water, where it was similar. In contrast, [
105] found that the bacterial community in intestines of shrimp cultured in an indoor-culture system showed no variations. Similarly, [
106] found that the intestine of
L. vannamei fed with synbiotics and postbiotics of bacilli and vibrio showed similar bacterial communities.
The potential function of shrimp gut bacterial microbiota can be predicted using the KEGG database [
107]. The functional profile has a fundamental role in the ecological balance of intestinal microbiota; however, it is important to mention that the predominant function is metabolism [106, 107]. [
108] mentioned that overrepresented bacterial metabolism may be related to energy consumption to satisfy the physiological activities of the host (shrimp). In this work, the bacterial functional profile did not change significantly by the treatments. However, linoleic acid metabolism (fatty acid) was positively correlated with final weight. Fatty acids play a fundamental role in cell structure and cell homeostasis [
109] and are a source of energy stored in triacylglycerols [
110]. It is known that fatty acids modulate the immune response thanks to their influence on the structure, function, metabolism, surface proteins, and intracellular receptors of cells [
109]. Linoleic acid stimulates the production of ROS by activating the NADPH oxidase enzyme in rat fibroblasts [
111], which agrees with the positive correlation of the immune response as revealed by the detection of superoxide anion in the present study.
Quorum sensing (QS) is a cell-to-cell communication system that regulates biofilm formation and the expression of virulence genes [112, 113]. In this work, QS was not significantly different among treatments. However, a positive correlation with shrimp survival was observed, which suggests a benefit to its health. Similarly, QS had a health benefit on farmed
L. vannamei shrimp [
113] and the farmed turbot (
Scophthalmus maximus) [
112].
Figure 1.
Concentration of TAN, nitrites, and nitrates in the shrimp culture system without water exchange and treated with bacteria. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Data are mean ± SD. Different letters indicate significant differences.
Figure 1.
Concentration of TAN, nitrites, and nitrates in the shrimp culture system without water exchange and treated with bacteria. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Data are mean ± SD. Different letters indicate significant differences.
Figure 2.
Survival of shrimp cultured without water exchange and treated with bacteria. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Data are mean ± SD. Different letters indicate significant differences.
Figure 2.
Survival of shrimp cultured without water exchange and treated with bacteria. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Data are mean ± SD. Different letters indicate significant differences.
Figure 3.
Total hemocyte count in L. vannamei cultured without water exchange and treated with bacteria. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Data are mean ± SD.
Figure 3.
Total hemocyte count in L. vannamei cultured without water exchange and treated with bacteria. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Data are mean ± SD.
Figure 4.
Superoxide anion in hemolymph of L. vannamei cultured without water exchange and treated with bacteria. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Data are mean ± SD. Different letters indicate significant differences.
Figure 4.
Superoxide anion in hemolymph of L. vannamei cultured without water exchange and treated with bacteria. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Data are mean ± SD. Different letters indicate significant differences.
Figure 5.
Phenoloxidase activity in hemolymph of L. vannamei cultured without water exchange and treated with bacteria. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Data are mean ± SD. Different letters indicate significant differences.
Figure 5.
Phenoloxidase activity in hemolymph of L. vannamei cultured without water exchange and treated with bacteria. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Data are mean ± SD. Different letters indicate significant differences.
Figure 6.
Venn analysis of the bacterial communities in the intestine of shrimp at the OTUs level. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water.
Figure 6.
Venn analysis of the bacterial communities in the intestine of shrimp at the OTUs level. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water.
Figure 7.
Most abundant bacterial phyla (%) in the intestines of shrimp. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Patescibacteria phylum (*no significant differences [P > 0.05], **significant differences [P < 0.05]). The analysis was done with Shaman.
Figure 7.
Most abundant bacterial phyla (%) in the intestines of shrimp. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Patescibacteria phylum (*no significant differences [P > 0.05], **significant differences [P < 0.05]). The analysis was done with Shaman.
Figure 8.
Most abundant bacterial genera (%) in the intestines of shrimp. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Different letters indicate significant differences (P < 0.05). The analysis was done with Shaman.
Figure 8.
Most abundant bacterial genera (%) in the intestines of shrimp. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. Different letters indicate significant differences (P < 0.05). The analysis was done with Shaman.
Figure 9.
Beta diversity of intestinal microbiota of L. vannamei at the genus level using non-metric multi-dimensional scaling based on Jaccard distances in MicrobiomeAnalyst. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. ANOSIM test, P < 0.008.
Figure 9.
Beta diversity of intestinal microbiota of L. vannamei at the genus level using non-metric multi-dimensional scaling based on Jaccard distances in MicrobiomeAnalyst. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. ANOSIM test, P < 0.008.
Figure 10.
Correlation among linoleic acid metabolism of intestinal bacteria of L. vannamei and immune and productive variables. Spearman correlation analysis.
Figure 10.
Correlation among linoleic acid metabolism of intestinal bacteria of L. vannamei and immune and productive variables. Spearman correlation analysis.
Figure 11.
Correlation among quorum sensing of intestinal bacteria of L. vannamei and immune and productive variables. Spearman correlation analysis.
Figure 11.
Correlation among quorum sensing of intestinal bacteria of L. vannamei and immune and productive variables. Spearman correlation analysis.
Table 1.
Physicochemical parameters in shrimp culture system without water exchange and treated with nitrifying bacteria. Treatments: I) Control; II) Bacilli in the water); III) LAB in the water; IV) Bacilli + LAB in the water. Values = mean ± SD.
Table 1.
Physicochemical parameters in shrimp culture system without water exchange and treated with nitrifying bacteria. Treatments: I) Control; II) Bacilli in the water); III) LAB in the water; IV) Bacilli + LAB in the water. Values = mean ± SD.
Treatment |
DO (mg mL-1) |
pH |
T (°C) |
S (PSU) |
Control (I) |
5.2±0.05 |
8.2±0.05 |
29.8±0.3 |
30±0.03 |
II |
5.2±0.01 |
8.2±0.02 |
30.0±0.4 |
30±0.02 |
III |
5.2±0.04 |
8.2±0.04 |
29.8±0.5 |
30±0.04 |
IV |
5.3±0.08 |
8.2±0.06 |
30.0±0.3 |
30±0.03 |
Optimal range |
4 to 10 |
8.1 to 9 |
23 to 30 |
15 to 35 |
Table 2.
Growth of shrimp cultured without water exchange and treated with bacteria. Control and with bacteria in the water: I) Control without bacteria; II) bacilli); III) LAB; IV) bacilli + LAB. Data are mean ± SD. Different letters indicate significant differences.
Table 2.
Growth of shrimp cultured without water exchange and treated with bacteria. Control and with bacteria in the water: I) Control without bacteria; II) bacilli); III) LAB; IV) bacilli + LAB. Data are mean ± SD. Different letters indicate significant differences.
Shrimp growth |
I |
II |
III |
IV |
Initial weigth (g) |
0.7±0.05 |
0.79±0.06 |
0.73±0.05 |
0.77 0.07 |
Final weigth (g) |
3.09±0.27b
|
4.18±0.53a
|
3.87±0.23ab
|
4.10±0.09a
|
SGR (%d-1) |
4.16±0.32 0.3271b
|
4.75±0.25 25.2522 |
4.76±0.33 |
4.78±0.35 |
Table 3.
Alpha diversity indices in the treatment group and the control derived from MicrobiomeAnalyst. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. The mean ± SD are indicated. Different letters indicate significant differences.
Table 3.
Alpha diversity indices in the treatment group and the control derived from MicrobiomeAnalyst. Treatments: I) Control; II) Bacilli in the water; III) LAB in the water; IV) Bacilli + LAB in the water. The mean ± SD are indicated. Different letters indicate significant differences.
Indices |
I |
II |
III |
IV |
Shannon |
1.71±0.62 |
1.22±0.18 |
1.19±0.53 |
1.47±0.35 |
Simpson |
0.60±0.16 |
0.53±0.13 |
0.40±0.21 |
0.47±0.13 |
Chao1 |
149.74±27.71a
|
93.95±25.25b 25.2522 |
137.03±10.27a
|
137.10±14.54a
|
ACE |
146.34±27.87a 27.87.64 |
88.95±28.97b 2282828.9728.9705 |
132.22±15.70ab
|
115.50±45.48ab
|
Table 4.
The KEGG functional categories at level 1 (iVikodak) of microbiota found in the shrimp gut.
Table 4.
The KEGG functional categories at level 1 (iVikodak) of microbiota found in the shrimp gut.
Treatment |
Metabolism (%) |
GIP (%) |
HD (%) |
EIP (%) |
CP (%) |
OS (%) |
Control |
61.05±1.22 |
11.01±0.15 |
11.43±0.63 |
1.39±0.23 |
6.90±0.58 |
8.19±0.11 |
Bacilli in water |
60.03±0.14 |
10.92±0.05 |
11.94±0.04 |
1.59±0.01 |
7.38±0.04 |
8.10±0.01 |
LAB in water |
60.19±0.44 |
10.88±0.13 |
11.87±0.17 |
1.55±0.10 |
7.22±0.35 |
8.27±0.32 |
Bacilli and LAB in water |
60.16±0.39 |
11.00±0.05 |
11.85±0.17 |
1.49±0.14 |
7.21±0.22 |
8.27±0.22 |