The role of microorganisms in natural material circulation: There are many kinds of microorganisms, fast reproduction, strong environmental adaptability and wide distribution, so they play an important role in natural material circulation. The material circulation in nature is the unity of two opposing processes, synthesis and decomposition, which mainly includes the circulation of four elements, C, N, S and P. Microorganisms are important producers of biosphere and main decomposers of organic matter, and their activities are the basis of normal circulation of natural materials. The role of microorganisms in carbon cycle is mainly embodied in assimilation and CO2 production. Autotrophic microorganisms can use CO2 to synthesize organic matter, while heterotrophic microorganisms can decompose organic matter to produce CO2. Most of the nitrogen in nature exists in the form of N2, which cannot be directly utilized by most organisms. Microorganisms play an important role in the transformation and synthesis of nitrogen. NH3 in nature is mostly synthesized by microorganisms. The mutual transformation between different nitrogen also needs the participation of microorganisms; Only microorganisms can decompose nitrogen in organic matter. The main forms of microorganisms in the natural nitrogen cycle are nitrogen fixation, ammonification, nitrification, denitrification and assimilation. Most of the sulfur in nature cannot be directly utilized by most organisms, and can only be absorbed and utilized by other organisms through microbial transformation; The decomposition of sulfur in organic matter is also inseparable from microorganisms. The main ways of microbial utilization and transformation of sulfur are desulfurization, assimilation, vulcanization and anti-vulcanization. There are many insoluble inorganic phosphides in nature, which can not be used by ordinary plants. The activities of microorganisms can promote the effective utilization of phosphorus in the biosphere. Many microorganisms have strong ability to decompose organophosphorus compounds such as nucleic acid, lecithin and phytic acid, and the phosphoric acid released by their transformation can be absorbed and utilized by other organisms. Introduction to microbial natural substance circulation 0 Natural substance circulation mainly includes two aspects: one is the organic of inorganic substances, that is, biosynthesis; The other is the inorganic nature of organic matter, that is, mineralization or decomposition These two processes complement each other and form a material cycle in nature. Microorganism is one of the three members of the biosphere. They have many kinds, diverse metabolic pathways, high enzyme activity, rapid reproduction and strong adaptability to the environment. They are widely distributed in nature, whether it is land, water, air, animals and plants, some organs outside and inside the human body, or even some extreme environments. In short, microorganisms are important members of the biosphere and play an important role in the material cycle of nature. To sum up, it has the following two functions: first, microorganisms are one of the producers in the biological food chain; Secondly, it is the main decomposer of organic matter (Huang Xiuli, 1998). Microorganisms, such as photoautotrophic algae, cyanobacteria, photosynthetic bacteria, etc., can directly use CO2 in the air to synthesize organic matter through photosynthesis, which plays an important role in the organic process of inorganic matter. Decomposers, mainly heterotrophic microorganisms, play a major role in organic matter mineralization. Specifically, the role of microorganisms in the natural material circulation is reflected in the following four aspects (Xu Xiaohua, 199 1). 1 the role of microorganisms in carbon cycle carbon is the most basic element of various organisms and the structural skeleton of organic matter and biological cells. Without carbon, there would be no life. Carbon cycle includes CO2 fixation and CO2 regeneration. 1. 1 The role of microorganisms in CO2 fixation Some photoautotrophic microorganisms, such as algae, photosynthetic bacteria, cyanobacteria, etc., can directly utilize CO2 in nature, synthesize organic carbides through photosynthesis, and then convert them into various organic substances; Chemotactic autotrophic bacteria can assimilate CO2 through chemical energy. Although the organic matter synthesized by microorganisms is far less than that of green plants in quantity and scale, it plays an important role in some special environments (such as waters where plants are difficult to survive) (Wang Jialing et al., 1988). 1.2 Role of microorganisms in CO2 regeneration Heterotrophic microorganisms can utilize organic matter in animals, plants and microbial carcasses, and secrete highly active enzymes to decompose lignocellulose and chitin which are difficult to decompose in other organisms (Liang Xiaobing et al., 2001; Huang Fuzhen, 1996), bacteria can decompose particulate organic matter into biodegradable soluble organic matter (Song, 2000). Bacteria are the main users of DOM. While using these organic substances, they decompose them continuously to obtain the energy needed for growth, and at the same time produce a large amount of CO2(Munster, 1993). The decomposition of organic matter in nature is dominated by microorganisms, and the secondary production of DOM by aquatic bacteria can consume 30 ~ 60% of the primary production (cola, 1998). 2 The role of microorganisms in the nitrogen cycle Nitrogen is the main component of nucleic acids and protein, and it is an essential element for organisms. Although the gas accounting for 78% of the atmospheric volume is N2, all animals and plants and most microorganisms can't directly use N2. As the most important primary producers in nature, plants have little demand for inorganic nitrides such as ammonium salts and nitrates. Only by transforming and recycling N2 in the atmosphere can they meet the demand of plants for nitrogen. Nitrogen cycle includes nitrogen fixation, ammonification, nitrification, denitrification and assimilation, and each process is inseparable from the participation of microorganisms. 2. 1 The process in which nitrogen is reduced to ammonia or other nitrides is called nitrogen fixation. There are two ways to fix nitrogen in nature. One is abiotic nitrogen fixation, that is, nitrogen fixation by lightning, volcanic eruption and ionizing radiation and artificial ammonia synthesis. The amount of nitrogen formed by abiotic nitrogen fixation is far from meeting the needs of biological growth in nature. The second is biological nitrogen fixation, that is, nitrogen is fixed by the action of microorganisms, and most of the nitrogen needed for biological growth in nature is provided through this action. Biological nitrogen fixation is not only economical, but also does not damage the environment, and plays an important role in the transformation of N2. Lake sediments contain a large number of nitrogen-fixing bacteria (Peptea, 1993), and the microorganisms that can fix nitrogen are prokaryotes, mainly bacteria, actinomycetes and cyanobacteria (Xu Xiaohua, 199 1). 2.2 Ammonification The process in which microorganisms decompose nitrogen-containing organic matter to produce ammonia is called ammonification. Ammonification is very important in agricultural production. All kinds of animal and plant residues and organic fertilizers, including green manure, compost and manure, are rich in nitrogen-containing organic matter. These organic substances must be ammoniated by various microorganisms before they can be absorbed and utilized by plants. The ammonifying bacteria in water contribute to the circulation of nitrogen in water and the cleaning of water body, and the ammonifying bacteria in lake sediments are quite active (Genovese, 1994). 2.3 Nitrification The process by which microorganisms oxidize ammonia into nitrate is called nitrification. Nitrification is an indispensable part of nitrogen cycle in nature. Nitrification is carried out in two stages, and each stage is inseparable from the role of microorganisms. In the first stage, ammonia is oxidized to nitrite by nitrifying bacteria. In the second stage, nitrite is oxidized into nitrate by nitrifying bacteria. The number of nitrogen-fixing bacteria in soil is more than that of nitrifying bacteria (gold, 199 1). 2.4 Assimilation Ammonium salts and nitrates are good inorganic nitrogen nutrients for plants and microorganisms, which can be absorbed and utilized by plants and microorganisms to synthesize nitrogen-containing organic substances such as amino acids, protein and nucleic acids. The assimilation of bacteria in the lake is helpful for freshwater fish to utilize protein (Shivokene, 1996). 2.5 Denitrification The process of microbial reduction of nitrate and release of molecular nitrogen and/or N2O is called denitrification or denitrification. Denitrification is one of the important reasons for soil nitrogen loss. Denitrification is generally only carried out under anaerobic conditions, and the method of intertillage and loosening soil is often used to inhibit denitrification in agricultural production. Denitrification is beneficial to the whole nitrogen cycle. Contribution of Denitrifying Bacteria in Water to Carbon Cycle (Song, 2000). Lake sediments contain a large number of denitrifying bacteria. Without denitrification, the natural nitrogen cycle will be interrupted, and nitrate will accumulate in water, which will pose a great threat to human health and the survival of aquatic organisms (Peptea, 1998). 3 The role of microorganisms in the sulfur cycle Sulfur is one of the essential elements of living matter, and it is also a component of some essential amino acids, some vitamins and coenzymes. Sulfur and H2S in nature are oxidized by microorganisms to generate sulfate ions, which are assimilated by plants and microorganisms and reduced to organic sulfide, one of the cell components. After the death of life, the organic sulfide in the corpse returns to nature in the form of H2S and S through the decomposition of microorganisms. In addition, sulfate ions can be reduced to H2S by microorganisms in anoxic environment. In a word, the circulating forms of sulfur in nature mainly include desulfurization, assimilation, vulcanization and anti-vulcanization (Xu Xiaohua, 199 1). 3. 1 Desulfurization The process of degrading sulfur-containing organic matter in the carcasses of animals, plants and microorganisms into H2S is called desulfurization. Sulfur-containing organic compounds mostly contain nitrogen. In the process of microbial decomposition, both H2S and NH3 are produced, so the sulfhydryl removal process of H2S and ammonia removal process of NH3 are often carried out at the same time. Generally, ammonifying microorganisms have this effect. 3.2 Sulfurization is a process in which H2S, sulfur or FeS are oxidized into H2SO4 under the action of microorganisms. In agricultural production, H2SO4 formed by microbial vulcanization can not only be used as a source of plant sulfur nutrition, but also help to dissolve mineral elements in soil and promote agricultural production. The microorganisms that can oxidize inorganic sulfides in nature mainly include sulfur bacteria and sulfide bacteria (Huang Xiuli, 1998). 3.2. 1 Sulfur bacteria can oxidize H2S into S and store it in bacteria. When H2S is lacking in the environment, sulfur particles stored in cells can continue to be oxidized into H2SO4. The main types are: 1) colorless sulfur bacteria, which contain no photosynthetic pigment; 2) Photoautotrophic sulfur bacteria, which contain carotenoids such as bacteriocin, carry out photosynthesis under anaerobic conditions (Zhou Deqing, 1993). 3.2.2 Sulfide bacteria can oxidize S or reductive sulfide into H2SO4, and there are no sulfur particles in the cells. They are obligate or facultative autotrophic bacteria, mainly some species of thiobacillus (Xia Shufen, Zhang Jiayao, 1988). 3.3 Assimilated plants and microorganisms can transform sulfates into reduced sulfides, and then fix them in protein and other components in the form of sulfhydryl groups. 3.4 Desulfurization Under anaerobic conditions, the process of sulfate being reduced to H2S by microorganisms is called anti-desulfurization. Desulfurization in poorly ventilated soil will increase the content of H2S in soil, which is harmful to the roots of plants. A large number of sulfide-resistant bacteria grow in the seabed sediments (Song,, 2000). The microorganism involved in this process is sulfate-reducing bacteria. Role of microorganisms in phosphorus cycle Phosphorus is also one of the important elements of organisms. There are many insoluble inorganic phosphides in nature, which can not be used by ordinary plants. Microbial activities can promote the effective utilization of phosphorus in the biosphere. Phosphorus cycle is mainly manifested in the process of effective conversion and ineffective conversion of phosphate. Insoluble phosphate minerals contained in rocks and soil can be converted into soluble phosphate under the action of organic acids and inorganic acids produced by many microorganisms. In the process of degrading organic matter, microorganisms will also degrade organophosphorus compounds contained in it. Many microorganisms have strong ability to decompose organophosphorus compounds such as nucleic acid, lecithin and phytic acid, and the phosphoric acid released by their transformation can be absorbed and utilized by other organisms (