In the aquaculture system, the microbial community plays a significant role in providing additional feed, enhancing nutrient utilization efficiency, reducing anoxic conditions, and minimizing environmental impacts. Several forms of Silicon (Si) are commonly presented in natural water as monosilicic acid and polysilicic acid. This research aimed to evaluate the concentrations of those acids in shrimp ponds and to determine the effect on microbial growth in both pond water and probiotic solution.
Shrimp farming production is one of the fastest-growing segments of world agriculture. Its management should be based on a systems approach including the application of different materials to provide high productivity and quality and minimize the negative impact on the environment.
To develop an efficient management strategy, an understanding of the factors that maintain and control shrimp production is required.
“The microbial community plays a significant role in providing additional feed, enhancing nutrient utilization efficiency, reducing anoxic conditions, and minimizing environmental impacts in the aquaculture system.”
Over the last decade, Silicon (Si) has been recognized as an essential trace element in the metabolism of higher plants and animals. In-plant biology and agriculture, Si has been declared a beneficial element being particularly important for the immune system induction in response to abiotic and biotic stresses.
Improved Si nutrition has a multi-effect on plant growth and cell functioning. Also, Silicon-mediated impacts like sugar formation, DNA stability, and transport regulation have been reported as a result of Si addition in some studies.
“Si is commonly presented in natural water in several forms such as monosilicic acid, polysilicic acid, and organo-silicon compounds. However, plants uptake Si only in the form of monosilicic acid.”
In the aquatic system, Si is recognized as a key nutrient for diatoms and some sponges. The predominance of diatoms in shrimp ponds is highly desired because their nutritional properties. In the presence of diatoms, the biochemical composition of shrimp is characterized by higher proteins, lipids, essential amino acids, and unsaturated fatty acids.
In addition, Silicon fertilization is a common approach to encourage diatom growth. Among the serious problems faced by shrimp farming are infectious diseases and environmental deterioration. Probiotics are successfully used to overcome these challenges.
“Probiotics improve growth performance, stimulate immune responses, enhance the disease resistance of shrimp, inhibit the growth of pathogens as well as improve water quality parameters.”
Probiotics usually include different bacteria, bacteriophages, microalgae, and yeast species. However, information about the effect of Si on the metabolism of microorganisms other than diatoms and their impact on their growth is extremely scarce.
Knowledge about the relationship between Si application rates and monosilicic and polysilicic acid concentrations in pond water, how fast Si disappears from water as well as the relationship between mono- and polysilicic acid concentrations and microbial growth is critical to manage the microbial community.
Therefore, this study aimed to evaluate the concentrations of monosilicic and polysilicic acids in shrimp ponds and to determine the effect of monosilicic acid on microbial growth in pond water and a probiotic solution.
Materials and Methods Field tests
Water samples were collected on 3 shrimp farms located in Jiangsu Province, China. The first farm used green-house-enclosed raceways for intensive shrimp production. Fresh groundwater was directly pumped into separated ponds under a greenhouse, each pond was 6×50 m in size. The shrimp’s age was 2 months.
Water was sampled from a well, 6 ponds, and a small creek filled with effluents from ponds. The second shrimp farm had a system of open ponds (100 x 50 m each), filled with unsalted water from a local canal. The shrimps were 2 months old.
Water was sampled from the canal and 6 ponds. The third shrimp farm took water directly from the Yellow Sea. Open ponds were 250 x 60 m each. Water was collected from 6 ponds, and a canal supplied water to the ponds. The shrimps were 1 month old.
“Samples were collected in triplicate in 100 mL plastic bottles early in the morning and were immediately transported to a laboratory to determine monosilicic acid, polysilicic acid, pH, and microbial cell abundance.”
The concentration of monosilicic acid was determined using the modified molybdenum blue method with a spectrophotometer V5800 XZBELEC (China). This method tests Si only in the form of monosilicic acid, without interference from phosphorus.
To analyze polysilicic acid, 2 g of NaOH was added to 20 mL of centrifuged water and kept in a refrigerator at + 4°C for 2 weeks. During this time all polymers of silicic acid are transformed into monomers. After that monosilicic acid was determined as described above.
The concentration of polysilicic acid was calculated. Microbial abundance (cells mL− 1) was estimated on the day of sampling by binocular biological microscope according to the method proposed by Newell and Newell (2006). The same method was used to calculate the cell density in the laboratory experiment.
A laboratory experiment was conducted with collected farm water summarized samples and 3 commercial probiotics used in shrimp farming: dry probiotics “Ecopro” and “Ecopro Cold” (Ecomicrobial Co, USA) and liquid probiotic “HeJunMei” (Jiangsu Aijiafuru Soil Remediation Co, China). In dry and liquid probiotics, the amounts of bacteria and yeast spores were no less than 1×1012 and 1x1010cells kg- 1, respectively.
To activate dry probiotics, 1 g of probiotic was mixed with 1 L of sterilized distilled water (DW) and kept at + 24°C for 24 h. The liquid probiotic was diluted 1:10. 1 L of the nutrient solution was prepared with K2 HPO4 3.125 g; KH2 PO4 3.125 g; (NH4 )2HPO4 3.125 g; MgSO4 .7H2 O 0.25 g; FeSO4 .7H2 O 0.0125 g; MnSO4 .7H2 O 0.00875 g and sucrose 12.5 g. 80 mL of this nutrient solution was added to each flask. 10 mL of pond water collected on the day of sampling, probiotic solution, or DW was added to the flasks.
“Considering that Farm 3 used seawater, NaCl (35 g L-1) was added to the flasks with pond water from Farm 3. Then 10 mL of DW or monosilicic acid solution at 10 and 20 mM Si was added to reach the Si concentrations of 0, 1 and, 2 mM.”
Monosilicic acid solutions were prepared from concentrated monosilicic acid. The pH in each flask was adjusted to 7 by adding 0.1 M HCl or 0.1 M NaCl. Flasks were kept in a climatic chamber at 24 ± 1°C. The light/night regime was 12/12 hours with a light intensity of 600 µmol photons M-2 s-1.
Flasks were aerated twice a day for 1 h (morning and evening). After 3 days, the concentration of monosilicic acid and the density of microorganisms were determined using the method described above. Each treatment and each analysis were conducted in triplicate.
The pH, concentration of mono- and poly-silicic acids, and density of microorganisms in tested solutions are presented in Table 1.
Monosilicic acid in water supplied to shrimp ponds differed greatly among farms, from 49.3 to 517.0 µM Si, with a higher value in fresh underground water (Farm 1) and a minimum value in coastal seawater (Farm 3).
Although the maximum polysilicic acid also was in the fresh underground water, its proportion increased: Farm 1 < Farm 2 < Farm 3 and accounted for 3.1; 10.0; and 16.6 %, respectively.
Water was pumped to the shrimp pond daily on all farms. In ponds, the concentrations of monosilicic acid were remarkably lower as compared with incoming water: by 26.3 times (517.0 vs 19.6 µM Si), 6.4 times (100.0 vs 15.6 µM Si), and 2.8 times (49.3 vs 17.3 µM Si), respectively on Farm 1, Farm 2, and Farm 3.
“The concentrations of polysilicic acid decreased as well, but not as significantly. The cell abundance in ponds of Farm 1 was higher than in others, probably due to a more intensive farming system. However, the microbial densities differed, sometimes significantly, between ponds of each farm.”
For example, on Farm 1, the cell numbers ranged between 2.4 ± 0.1 and 3.9 ± 0.2 x10 5 mL-1, while on Farm 2 and Farm 3 the cell numbers ranged from 1.1 ± 0.1 to 1.6 ± 0.2 x10 5 mL-1 and from 1.4 ± 0.2 to 2.0 ± 0.2 x10 5 mL-1, respectively.
The numbers of microbial cells and soluble forms of Si in the laboratory test are presented in Table 2.
Supplementation of monosilicic acid significantly increased the microbial density, up to 60 and 33% in pond water and probiotic solution, respectively. Over 3 days, the concentration of monosilicic acid decreased in all microorganism-containing solutions in comparison with the corresponding sterile solutions.
“Remarkable reductions in monosilicic acid were detected in all pond water samples, whereas probiotic solutions demonstrated much smaller changes.”
The process of polymerization was more intense in pond water and especially in probiotic solutions. The polysilicic acid concentration reached up to 230 ± 21 mg L-1 Si in liquid probiotic as compared to 10.5 ±0.3 mg L-1 Si in the corresponding sterile solution.
It was observed very fast reductions in pond water Si, despite daily water exchange. The monosilicic acid concentration decreased more than the polysilicic acid (Table 3).
It is well known that higher plants take up Si only in the form of monosilicic acid. Perhaps algae, being phototrophic organisms like higher plants, have the same mode of Si uptake.
With decreasing monosilicic acid, equilibrium between soluble forms of Si shifts, resulting in acceleration of depolymerization, which is typical for the systems with low concentrations of monosilicic acid, in turn leading to decreasing polysilicic acid.
The correlation coefficients between soluble forms of Si and cell abundance evidence that the number of microorganisms in ponds correlates positively with monosilicic acid (R = 0.80–0.84) (Table 4).
There was no correlation between cell abundance and polysilicic acid. Therefore, unlike polysilicic acid, monosilicic acid is an essential factor in the regulation of microbial growth in shrimp ponds. The laboratory experiment has shown that monosilicic acid affected beneficially microbial populations in pond water and probiotics solution (Table 5).
In pond water, Si may be consumed mainly by different algae species, including diatoms. The tested probiotics contained only bacteria having less need for Si, though additional Si benefitted their growth as well. The increase in polysilicic acid with the addition of monosilicic acid could be the result of polymerization (Table 5).
The formation of polymers was higher in probiotic solutions. Although the significance of this process in shrimp cultivation is unknown, some researchers concluded that Si polymers generally possess high adsorption properties to organic and inorganic molecules.
“Thus, new-formed silicagel may adsorb organic compounds and nutrients promoting the attraction of microorganisms and floc formation. This hypothesis requires further confirmation.”
With decreasing the Si concentration, other phytoplanktonic algae that do not need so much Si can replace diatoms (Boyd, 2014). Among undesirable algae species, blue-green algae are of particular concern. Blooms of blue-green algae cause a lack of dissolved oxygen, off-flavors problems, and toxin formation, thus deteriorating water quality and declining shrimp productivity (Jescovitch et al., 2018).
The abundance of silicic acid is an essential requirement to achieve desirable diatom domination in algal communities. However, no systematic studies have been conducted showing the Si limitation and influence of its addition on shrimp production.
The obtained data have demonstrated that all tested shrimp ponds were characterized by extremely low concentrations of monosilicic acid, while the supply waters originally were high in DSi. Monosilicic acid applied to shrimp pond water or probiotic solution significantly increased the microbial cell abundance.
It is important to distinguish between monomeric and polymeric forms of DSi because these substances affect the microbial population in aquaculture in different ways.
Also, this research demonstrates the importance of systematic studies related to the functions of Si in shrimp aquaculture.
This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “MICROBIAL GROWTH IN SHRIMP PONDS AS INFLUENCED BY MONOSILICIC AND POLYSILICIC ACIDS” developed by RUIPING ZHANG–Beijing Plum Agrochemicals; ELENA BOCHARNIKOVA – Institute Basic Biological Problems RAS; VLADIMIR MATICHENKOV – Institute Basic Biological Problems RAS.
The original article was published in RESEARCH SQUARE, in September 2021.
The full version can be accessed freely online through this link: https://doi.org/10.21203/rs.3.rs-908767/v1<span style=”color: #00ccff”>