1. Biodegradation refers to the decomposition process of complex compounds under biocatalysis. In the process of petroleum degradation, microorganisms first produce catabolic enzymes through their own metabolism to crack heavy hydrocarbons and crude oil and reduce the viscosity of petroleum. In addition, in the process of their growth and reproduction, they can produce solvents, acids, gases, surfactants, biopolymers and other effective compounds that are beneficial to oil displacement, and then they are further oxidized and decomposed into small molecules by other microorganisms to achieve the purpose of degradation.
2. The most important degrading bacteria in the ocean belong to achromobacter, Acinetobacter, Alcaligenes, Arthrobacter, Bacillus, Flavobacterium, Corynebacterium, Microbe, Micrococcus, Pseudomonas, Actinomycetes and Nocardia. In most marine environments, these bacteria are the main degrading bacteria. Among the fungi, Basidiomycetes aurea, Candida, Rhodotorula and Trichosporon are the most common marine petroleum hydrocarbon degrading bacteria. Some filamentous fungi such as Aspergillus, Mucor, Fusarium and Penicillium should also be classified as marine degrading bacteria. In addition to the bacteria mentioned above, the main degrading bacteria in soil also include mycobacteria and a large number of filamentous fungi. Some species of Aspergillus and Penicillium are distributed in both marine and soil environments. Some species of Trichoderma and Mortierella are soil-degrading bacteria.
3. The key to control oil pollution is to degrade hydrocarbons. According to the chemical structure characteristics of hydrocarbons, the degradation pathways of hydrocarbons are mainly divided into two parts: the degradation pathway of chain hydrocarbons and the degradation pathway of aromatic hydrocarbons. There are three main degradation modes of linear alkanes: terminal oxidation, sub-terminal oxidation and ω oxidation. In addition, alkanes can sometimes form olefins under the action of dehydrogenase, and then form alcohols at double bonds for further metabolism. Regarding the degradation pathway of aromatic hydrocarbons, it is firstly converted into catechol or its derivatives under aerobic conditions, and then further degraded. Therefore, the key step of bacterial and fungal degradation is the process of substrate oxidation by oxidase, which requires the participation of molecular oxygen.
The specific mechanism is as follows:
1. Under the action of n-alkane oxidase, n-alkanes are first converted into carboxylic acids, and then deeply degraded by β-oxidation, producing short-chain fatty acids with two carbon units and acetyl coenzyme A, and releasing CO2. N-alkane oxidase is a kind of dioxygenase, which can catalyze the conversion of n-alkane into hydroperoxide of n-alkane. The reaction needs O2, but NAD(P) H is not needed. Alkanes can also be converted into ketones first, but it is not the main metabolic mode. Multi-branched olefins are mainly converted into dicarboxylic acids and then degraded, and methyl groups will affect hydrolysis. The chemical formula is as follows:
2. The degradation of cycloalkane requires the synergistic oxidation of two oxidases. One oxidase first oxidizes it into cyclic alcohol, then dehydrogenates it to form cyclic ketone, and the other oxidase oxidizes cyclic ketone again, and the ring is deeply degraded after being broken. The chemical formula is as follows:
3. Aromatic hydrocarbons are generally alkylated to form diols, the ring is broken, and catechol is degraded into the intermediate product of tricarboxylic acid ring. Both fungi and microorganisms can oxidize aromatic substrates from benzene to benzoanthracene. At first, bacteria combined two oxygen atoms of molecular oxygen into the substrate through the catalytic action of dioxygenase, thus oxidizing aromatic hydrocarbons into dihydrodiphenol with cis configuration. Cis -2- dihydrohydroquinone is further oxidized to catechol, which is further oxidized and cracked by another dioxygenase that catalyzes the cracking of aromatic rings. Contrary to bacteria, fungi oxidize aromatic hydrocarbons to trans -2- dihydrodiphenol under the catalysis of monooxygenase and cyclic hydrolase. (Take naphthalene degradation as an example) Fungi degrade petroleum hydrocarbons into trans-diols, while bacteria almost degrade them into cis-diols (many trans-diols are potential carcinogens, while cis-diols are not toxic). The chemical formula is as follows:
The following table summarizes them briefly:
Specific degradation processes and products of various hydrocarbons
N-alkanes N-alkanes → carboxylic acids → short-chain fatty acids with two carbon units+acetyl coenzyme A+CO2.
Olefin olefin → dicarboxylic acid
Cycloalkane cycloalkane → cyclic alcohol → cyclic ketone
Aromatic hydrocarbon → diol → catechol → intermediate product of tricarboxylic acid ring
From the above, it can be seen that the degradation of some refractory chemicals by microorganisms is completed through the catalytic action of a series of oxidases. In nature, this process is usually completed by the synergy of various microorganisms, and the speed is relatively slow. In order to expand the range of substrate degradation by microorganisms, improve the degradation efficiency and completely mineralize these refractory chemicals, it should be possible to construct new functional strains by using the transfer of natural degradation particles. Degradable particles refer to a kind of plasmids encoding some chemical metabolic pathways. For example, in order to eliminate oil spill pollution at sea, Chakraany et al. in the United States transferred the combination of four degrading particles, CAM, OCT, XAL and Nah, from different strains of Pseudomonas into one strain, and constructed a "multi-plasmid superbacterium" which can degrade aromatic hydrocarbons, polycyclic aromatic hydrocarbons, terpenoids and aliphatic hydrocarbons at the same time. This kind of bacteria can shorten the oil slick that natural bacteria need more than one year to remove to several hours.
4. In the natural environment, the ability and speed of microbial degradation of petroleum hydrocarbons are closely related to their environment.
1, liquid petroleum hydrocarbons will form a water-oil interface in water, and microorganisms will degrade hydrocarbons on this water-oil interface, and the degradation speed is closely related to the area of the water-oil interface. It is emulsifier that can produce biological emulsifier, increase the area of water-oil interface and promote the degradation of hydrocarbons by microorganisms.
2. Microbial degradation of petroleum hydrocarbons can occur in a wide temperature range, and microorganisms that degrade petroleum hydrocarbons exist in the environment of 0℃ ~ 70℃. Most microorganisms are easy to degrade petroleum hydrocarbons at room temperature, while some low molecular weight petroleum hydrocarbons toxic to microorganisms are difficult to volatilize at low temperature, which will inhibit the degradation of petroleum hydrocarbons to a certain extent, so petroleum hydrocarbons are difficult to degrade at low temperature.
3. Most petroleum hydrocarbons degrade under aerobic conditions, because the degradation of many hydrocarbons requires oxygenase and molecular oxygen. However, some hydrocarbons can degrade under anaerobic conditions.
4. Nitrogen source and phosphorus source often become the limiting factors of hydrocarbon degradation by microorganisms. In natural water, it is also restricted to add water-soluble nitrogen and phosphorus sources to promote the degradation of petroleum hydrocarbons, because the limited nitrogen and phosphorus sources are diluted at high times in water, which is difficult to support the growth of microorganisms.
5. Microbial degradation of petroleum hydrocarbons is generally at neutral pH, and extreme pH environment is not conducive to microbial growth.
Its efficiency and quality also depend on the quantity, type and state of petroleum hydrocarbons. For example, Chaineau et al. used microorganisms to treat soil contaminated by petroleum hydrocarbons. After 270 days, it was found that 75% of the crude oil was degraded. Among saturated hydrocarbons, normal alkanes and branched alkanes were almost completely degraded in 16 d; 22% cycloalkanes were not degraded; 7 1% aromatics were assimilated; The asphaltene, which accounts for 10% of the total weight of crude oil, is completely retained. Generally speaking, the relative ability of petroleum hydrocarbons to be degraded by microorganisms is as follows: saturated hydrocarbons >; Aromatic hydrocarbons > gum and asphalt. Among saturated hydrocarbons, linear alkanes are the easiest to degrade; Among aromatic hydrocarbons, bicyclic and tricyclic compounds are easily degraded, while aromatic hydrocarbons containing five or more rings are difficult to be degraded by microorganisms. Gum and asphalt are extremely difficult to be degraded by microorganisms.
Conclusion: Although microorganisms can degrade oil, there is no effective method to completely degrade oil in a short time, so the research on microbial degradation of oil still has a long way to go. But with the further development of modern microbiology and genome project, the discovery of more microbial species and the application of biotechnology, the problem of oil pollution will be solved more effectively!
Reference: Soil and Environmental Microbiology edited by Chen Wenxin.
Research progress on degradation of organic pollutants by microorganisms such as Tian Lei.
Biodegradation of pollutants Zhu Huailan, Zhang Tong, Jin Zhigang
12 strains of high-efficiency oil-degrading bacteria were isolated from oil-contaminated soil and water. The oil reduction rate of a single strain was 40.3% ~ 57.6%, among which O-8-3, O-28-2 and O-46 strains could tolerate the temperature of 40℃ and the salinity of 1.5%. These three strains belong to Pseudomonas. , Bacillus. And Acinetobacter. Compared with single strain O-8-3, the oil degradation rate of mixed strain O-8-3/O-28-2/O-46 can be increased by 20. 1. The experimental results of treating oil production wastewater in Shengli Oilfield by inoculating mixed strains of O-8-3/O-28-2/O-46 in laboratory showed that the degradation rate of petroleum pollutants reached 96.9% within 72 h, which was 60.7% higher than that by inoculating natural flora.
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