In recent years, China's aquaculture industry has developed rapidly, the intensification of aquaculture has been continuously improved, the aquaculture environment and the water quality of aquaculture waters are deteriorating, which restricts the healthy and sustainable development of China's aquaculture industry, and causes a series of environmental and social problems. To this end, concerned fishery is in charge of a department to begin to popularize aquatic product to breed technology healthily. The so-called healthy breeding means to maintain the best growth and development of breeding organisms, reduce the occurrence of breeding diseases and improve the quality of breeding products by introducing non-pestilence seedlings, adopting reasonable breeding mode, strengthening the management of input products and controlling the breeding environment and other technical measures.
The use of microecological agents is one of the important technical means to implement healthy aquaculture. Microecological preparations are also called microecological mediators, probiotics, probiotics, proliferators, bacteriostatic agents, etc. Under the guidance of microecological theories, they are living bacteria preparations made by special techniques using beneficial microorganisms isolated from cultured animals or their living environment. It has the characteristics of non-toxic side effect, no pollution, no residue and low cost. It can inhibit the growth of pathogenic microorganisms, improve the immunity of the breeding objects, and maintain the ecological balance of breeding.
This paper introduces the application of several major microecological preparations in aquaculture in recent years, and puts forward some personal views on some problems that need to be solved urgently in the production of microecological preparations in order to provide reference for the rational use of microecological preparations in aquaculture.
1 The types of microecological preparations and their application in healthy aquaculture
According to their USES, microecological preparations can be divided into two categories: one is the microecological improvement agent in vivo, which is added to the feed to improve the composition of the microbial community in the culture object. Lactobacillus, bacillus, yeast and EM bacteria are commonly used. The other is microecological water quality improver, which is put into the culture water environment to improve the substrate or water quality, mainly including photosynthetic bacteria, bacillus, nitrifying bacteria, denitrifying bacteria, EM bacteria, etc.
1.1 photosynthetic bacteria
Photosynthetic bacteria are a group of bacteria that can carry out photosynthesis. Its bacteria are rich in protein, various vitamins, biotin, carotenoid, coenzyme Q and other physiological active substances. The bacteria can absorb harmful substances such as ammonia nitrogen, nitroso nitrogen, hydrogen sulfide and organic acid in water and inhibit the growth of pathogenic bacteria. Wang Yiping (1999) et al. used the fixed photosynthetic bacteria in the artificial breeding of Mitten crabs, and when the mass fraction was (1.5 ~ 2.5)×109 /mL, the removal rates of ammonia nitrogen and nitroso nitrogen were around 90%, the chemical oxygen demand in the water decreased significantly, and the survival rate of crab seedlings was also 15.7% higher than the control group. The author has done an experiment, in the breeding of Chinese mitten crabs in the pond, applied photosynthetic bacteria, every 1000 square meters, 750 crabs, every 15 ~ 20 days applied photosynthetic bacteria, the whole breeding cycle was not applied any fish medicine, the pond kept fresh water, good bottom quality, and no disease.
1.2 bacillus
Bacillus is a gram-positive bacterium, which is a kind of aerobic bacteria. The bacteria can exist in the intestinal tract of aquatic animals in the form of endospores and secrete highly active protease, lipase and amylase, which can effectively improve the utilization rate of feed and promote the growth of aquatic animals. It can also improve water quality by eliminating or reducing pathogenic bacteria; Bacillus can also decompose and absorb proteins, starches, fats and other organic substances in water and sediment to improve water quality and sediment. Liu Kelin, He Mingqing et al. tested the growth and immune function of carp with 1×107 feeds per g of bacillus. The results showed that the weight gain of the experimental group was 11.8% higher than that of the control group, and the feed coefficient decreased by 0.24. In the experimental group, the immune organs of carp, thymus and spleen, grew rapidly, the number of T and B lymphocytes grew rapidly, the number of antibodies produced increased, the immune function was enhanced, and the intestinal mucosa also showed physiological changes conducive to weight gain, disease prevention and treatment. Li Zhuojia, Zhang Qing et al. carried out the decomposition test of organic sludge in fish ponds by using microbial composite preparations with bacillus as the leading bacteria, and the mass fraction of beneficial bacteria was (1.5 ~ 4.5)×106 /L. After 1 month, the original 3 ~ 125px thick organic sludge at the bottom of the pool was decomposed, and the growth of fish was significantly accelerated.
1.3 nitrifying bacteria
Nitrifying bacteria is a general term for nitrifying bacteria and nitrifying bacteria, belonging to a class of self-supporting bacteria. Nitrifying bacteria convert ammonia nitrogen in water to nitrite, which oxidizes nitrite into nitrite that is harmless to aquatic animals. The nitrifying bacteria were mainly made together with other bacteria into complex microecological preparations. The author once tried nitrifying bacteria and denitrifying bacteria to treat the aquaculture wastewater of loach. After 24 h, the chemical oxygen demand decreased by 80.6%, the removal rate of nitroso nitrogen and ammonia nitrogen reached 90.2% and 98.5% respectively.
1.4 denitrifying bacteria
Denitrifying bacteria are composed of denitrifying microorganisms. They are mainly used to change nitrate or nitrite into nitrogen and release them. They are often used in the treatment of bottom sediments. When the dissolved oxygen at the bottom of the tank is less than 0.5 mg/L and the pH value is 8 ~ 9, denitrifying bacteria can use the sediment in the organic matter as carbon source to convert the nitrate in the sediment into harmless nitrogen and discharge it into the atmosphere. Denitrification process can consume a large amount of fermentation products and organic matter deposited in the bottom of the pond, rapidly reduce the content of organic matter and nitrate in the bottom sludge, and effectively prevent the abrupt change of water quality caused by climate change.
1.5 Lactic Acid Bacteria
Lactobacillus is a kind of bacteria which can ferment sugar to produce lactic acid. It can inhibit the activity of harmful microorganisms, the proliferation of pathogenic bacteria and the decay of organic matter. Lactic acid bacteria can decompose lignin and cellulose, which are not easy to decompose at room temperature, so that organic matter can be fermented and converted into active and plant nutrients.
1.6 saccharomycetes
Yeast is a group of single-celled organisms belonging to fungi, containing higher amino acids, vitamins and other nutrients. Under aerobic conditions, yeast can convert water-soluble sugars into carbon dioxide and water. Under the condition of hypoxia, saccharides can be used as carbon source for yeast fermentation and reproduction. So yeast can effectively decompose the sugar dissolved in the pool water, quickly reduce the biological oxygen consumption in the water. In the experiment, Wu Wei et al. treated nitroso nitrogen in water with Candida yeast, the degradation rate of nitroso nitrogen reached 0.036 mg/L·h, and found that the content of nitroso nitrogen, chemical oxygen demand and the ratio of calcium to magnesium had influences on the degradation rate.
1.7 Gram-positive actinomycetes
Gram-positive actinomycetes belong to aerophilic flora. It can obtain the matrix from the photosynthetic bacteria, produce various antibiotics and enzymes, directly inhibit the bacteria, promote the proliferation of beneficial microorganisms. The combination of actinomycetes and photosynthetic bacteria works better. It can also decompose under normal conditions not easy to decompose lignin, cellulose, chitin, conducive to the absorption of animals, plants. Wu Wei et al. (2000) achieved good results when nocardia was used to treat ammonia nitrogen in aquaculture water, and the maximum removal rate of ammonia nitrogen reached 3.5mg /L·h.
1.8 thiobacillus
Sulfide bacteria is a kind of autotrophic bacteria that can oxidize inorganic sulfide to sulfuric acid and obtain energy from oxidizing inorganic sulfur. Sulfide bacteria are widely distributed in pond sediment and water body, and their oxidation provides sulfuric acid sulfur elements available to aquatic plants, reducing the content of sulfur and hydrogen sulfide in the pond.
1.9 EM bacteria
EM is the abbreviation of effective microbiome. It is a microbial community composed of photosynthetic bacteria, lactobacillus, yeast and other beneficial bacteria, which can form a complex and relatively stable microecosystem through symbiotic and proliferative relationship. Beneficial microorganisms in EM, through a series of decomposition and synthesis functions such as nitrogen fixation, photosynthesis, can make the organic matter in the water form various nutrient elements for the growth and reproduction of their own and feed organisms, at the same time increase the dissolved oxygen in the water, reduce the content of ammonia, hydrogen sulfide and other toxic substances, and maintain the balance of the aquaculture water environment. In addition, EM bacteria can also form dominant bacteria groups in the intestinal tract to inhibit the activities of E. coli, and promote the digestion and absorption of feeds by the body, so as to reduce the content of ammonia nitrogen in excreta, play a role in purifying water quality and promoting growth. The author once tested that EM bacteria were applied in the macrobrachium Rosenbergii aquaculture pond every 20 days, the dissolved oxygen in the water after the application was basically stable at about 5 mg/L, the breeding water quality was maintained good, there was no harmful shrimp disease, the market time of the experimental group was more than 10 days earlier than the control group, the yield per unit area increased by 10%.
2 The function of microecological preparation in healthy aquaculture
2.1 Decompose organic pollutants and purify the environment
Lactobacillus, Bacillus, yeast, EM and other microecological modifiers in vivo can improve the utilization rate of protein and other components in the feed, inhibit the propagation of spoilage microorganisms, reduce the production of ammonia and other spoilage substances, and thus improve the water environment; Micro ecological water quality improver of photosynthetic bacteria, nitrifying bacteria, bacillus can participate in the degradation of organic matter, such as play ammoniation, nitrosation, nitration, oxidation, sulfide, nitrogen fixation, secrete extracellular enzymes, eliminate water in organic matter, ammonia nitrogen, hydrogen sulfide and other harmful substances, reduce the chemical oxygen demand (COD) and biological oxygen demand (BOD), and indirectly increase the dissolved oxygen in water, improve water environment.
2.2 Supplement nutrients and promote the healthy growth of cultured animals
Many bacteria in microecological preparations are rich in protein, amino acids, vitamins and other nutrients, which are good feed additives or good bait for animal larva. Aquatic animals the existence of the microbial flora in the digestive tract and the balance between them with the host, guaranteed the normal metabolism of the host, as the body's growth and development provides a rich nutrients, has promoted many kinds of amino acids in the digestive tract, vitamin and a series of effective utilization synthesis and absorption of nutrients, thereby promoting body growth and development. For example, photosynthetic bacteria are rich in protein, vitamins and trace elements. Bacillus can also produce amylase, lipase, protease and other digestive enzymes, assist animal digestion bait, improve feed utilization.
2.3 Inhibit pathogenic bacteria, improve the body immunity
There exists a dynamic balance system of microorganisms in the internal and external environment of aquaculture animals, in which beneficial and harmful bacteria live together. When aquatic animals are affected by a variety of adverse factors, such as the deterioration of the breeding environment, inadequate nutrition and long-term drug use, this balance will be broken, and the original community structure will change, causing a large number of pathogenic bacteria to multiply. Microecological preparation has biological activity, and can reproduce rapidly and become a dominant population after application. It competes with harmful bacteria for space and limited nutrition, and puts harmful bacteria in an unfavorable living environment and inhibits their growth and reproduction. Microecological preparation is also a good immune activator. Some beneficial microorganisms can not only competitively reject pathogens, but also produce or contain antibacterial substances and a variety of immune promoting factors, which can activate the immune system of the body and strengthen the stress response mechanism of the body.
3 Precautions for the use of microecological preparations
3.1 To reduce the adverse effects of external factors
The effect of water quality microecological improver is easily affected by environmental factors, such as water temperature, pH value, dissolved oxygen, light, organic content, etc., and different strains are also affected by environmental factors differently. If the photosynthetic bacteria were used on rainy days, the effect was not obvious. The effect of bacillus preparation in water with high nitrite content and pH value is not obvious. Adding antibiotics and other substances in water will also reduce the effect of microecological preparations. In vivo microecological improver is easily affected by drying, high temperature, high pressure, oxidation and other factors during product processing, storage and transportation. Therefore, the use of microecological preparations should proceed from the reality, select the corresponding products, and through improvement measures, to minimize the adverse effects of external factors.
3.2 Use early and for a long time
Microecological preparations can be used in the whole growth process of the cultured objects, but the microecological balance in vitro and in vivo of the cultured objects has not been completely established, and the disease resistance is relatively weak. At this time, the microecological preparations are easy to form the dominant bacteria group, and the effect is the best. On the other hand, the beneficial bacteria had a growth cycle in which the quantity increased to the peak and then decreased, and the preventive effect of microecological preparation was better than that of treatment. Therefore, only by regularly and continuously releasing microecological preparations can the population dominance and microecological balance be stabilized for a long time.
3.3 We should pay attention to the quality of microecological preparations
It is important to pay attention to the number and vitality of beneficial bacteria when using microecological preparations. The effect of microecological preparation is realized through a series of physiological activities of probiotics, and its final effect is closely related to the number and vitality of probiotics applied. The number or vitality of probiotics is insufficient or not strong enough to form microbial community superiority, which makes it difficult to play a role. At the same time, the experiment shows that with the extension of the storage period, the number of live bacteria of the microecological preparation decreases gradually, and its effect is obviously weakened, so the storage period should not be too long.
4 Problems and Suggestions in the application of microecological preparations
Probiotics although because of its nature, no residue, no resistance, non-toxic side effects of aquatic farming animals, and many other advantages, can partly replace or replace antibiotics, are more and more widely used in aquaculture production, but its application in aquaculture time is shorter, there are still some problems to be solved.
4.1 The pertinence of microecological preparation
At present, there are only 12 kinds of bacteria suitable for probiotics in China, and different kinds of microecological preparations have different pertinence and effects. For example, photosynthetic bacteria with high mass fraction have the best oxygenation effect. EM bacteria and nitrifying bacteria had better effect on ammonia nitrogen elimination. Bacillus has the obvious effect of reducing nitrite content. EM helps raise the pH of the water. Many scholars and manufacturers have tried to take advantage of the different characteristics of different strains and combine them to give full play to their comprehensive effectiveness. However, the growth and reproduction conditions of different strains are different, so it is difficult for the same environmental conditions to meet the requirements for all the composite strains to play a role at the same time. Therefore, in the actual production, appropriate preparations should be selected according to different needs, and different products should be designed according to different characteristics of strains and aquatic breeding animals as far as possible. It is necessary to strengthen the research, further expand the microbial species that can be applied, and develop the excellent bacterial species with strong acid resistance and many metabolites. By means of molecular biology, the existing strains can be modified to meet people's needs.
4.2 Stability of microecological preparations
Maintaining the stability of bacterial species is the most practical problem in the research and production of microecological preparations. The main factors affecting the stability of microecological preparation are temperature, humidity, ph, storage time, mechanical friction and extrusion, etc. For example, microecological modifiers in vivo tend to decrease or lose their activity in the process of feed processing and storage, so that the acid resistance is not strong after entering the digestive tract, and it is difficult to form the dominant bacteria group. The application of water quality microecological improver is easily affected by application time, application period and environmental factors. Therefore, in the actual production, it is necessary to strengthen the research on the dosage form of microecological preparations, reduce the damage of processing and other processes to the bacterial cells, explore the reasonable use methods, and improve the quality fraction of viable bacteria.
4.3 Safety of microecological agents
Many microecological agents are derived from environmental protection agents. When they are used in aquaculture, attention should be paid to the specificity of strains. They should be identified by authoritative institutions and regularly tested for safety, so as to ensure the safety of cultured animals and consumers. At present, in addition to the photosynthetic bacteria, for other probiotics there is no unified product standards of the state, the content of active microbes in probiotics is also no special strict rules, and many domestic manufacturers and some simple equipment, small scale of production, distribution exists in the operation, generic, and even the phenomenon such as fraud, coupled with the imperfect detection means, the good and bad are intermingled of probiotics on the market. Therefore, fishery producers should choose the products of those manufacturers with research and development strength and mature technology to ensure the safety of the products. The government departments should establish the testing system of microecological preparations as soon as possible, gradually improve the industry standard system, strengthen publicity, and regulate the production and market management of microecological preparations.
