In this article we will discuss about the microbial degradation of petroleum and xenobiotic.
1. Microbial Degradation of Petroleum (Hydrocarbon):
Petroleum and its products are the hydrocarbons. It is a rich source of organic matter and is oxidised if comes in contact with air and moisture. There are some microorganisms which cleave the hydrocarbons into simpler molecules.
Bioremediation is an important process now-a-days used for abatement of pollution. In this process oil or other pollutants are utilized by microorganisms if added with inorganic nutrients. The importance of bioremediation in oil spill of marine (sea) environment is widely studied.
Fungi and bacteria are the main agents which decompose oil and oil products. Besides, cyanobacteria, yeast and algae have shown to oxidise hydrocarbons. The simplest hydrocarbon pollutant is methane. It is degraded by a specialized group of bacteria called methanotrophic bacteria.
As you know, oil is insoluble in water and is less dense; it floats on the surface and forms slicks or oil films. Hydrocarbon-oxidising microorganisms develop rapidly in such films. Oil is present both in anoxic (absence of O2) as well as oxic (presence of O2) environment as is evidenced by the presence of natural oil deposits.
Many pseudomonads, different cyanobacteria, various corynebacteria and mycobacteria are able for degradation of petroleum products. Initially, the nonvolatile components are oxidised by bacteria and in the later process, certain fractions of branched-chain and polycyclic hydrocarbons are degraded slowly. Sometimes, it has an impact on fisheries.
It is important to mention that addition of inorganic nutrients such as phosphorus and nitrogen to oil spill increase bio-remediation rates significantly.
The microbial production of hydrocarbon occurs in colonial alga, Botryococcus braunci, by secretion of long-chain hydrocarbons (C30 to C36) that have the consistency of oil. An increasing interest has been shown in using this type of microbe as renewable sources of production of petroleum.
Biotechnological Approaches for Abatement of Pollution:
In recent years experimentations on biotechnology have been done for the production of potential microbial strains capable of degrading pollutants. Dr. Anand Mohan Chakraborty (an India-borne American Scientist) succeeded in producting a genetically engineered strain of Pseudomonas putida that utilized complex chemical compounds. It was called as super bug. Gene Cloning in Microorganisms.
Microbial biogeochemistry has far reaching importance in the area of bioleaching of metals. The bioleaching process allows recovery of about 70% of the mineral from low grade ores as in the case of copper. The application of Thiobacillus ferrooxidans population helps to recover this metal. It involved the biological oxidation of copper present in these ores to produce soluble copper sulphate.
The copper sulphate can be recovered by reacting the leaching solution. Sometimes, bioleaching process requires addition of nitrogen and phosphorus, if these are low in ores. These added minerals enhance the solubilisation process. Vernadskii (1934) thought about the possibility of solubilization of silicates by soil microorganisms to liberate various cations of silicate elements.
Aleksandrov and Zak (1950) isolated certain bacteria capable of decomposing alumino silicate, which were named as “silicate bacteria” such as Proteus mirabilis.
Microorganisms interact with silicate materials either by decomposing and solubilizing of silicate materials or utilize silica in a dissimilatory fashion by incorporating it into their cells or bodies in the dissolved form, releasing it as free silicic acid. They also assimilate silica by taking it up in dissolved form. Sarcina ureae is known to release silicon from quartz occurred as a result of alkalification of the medium.
Significant quantities of Si, Ai, Fe and Mg were reported to be solubilized by Penicillium simplissimum WB-28 from dunite, peridotite, basalt, granite and quartzite rocks due to the production of citric acid. The destruction of apophyllite, olivine and Ca and Zn silicate by Pseudomonas and other soil organisms was found to be accompanied by the releases of ketogluconic acid and other organic acids.
Solubilization of silica from diatoms has been found to be associated with bacterial activity which in many cases has been reported to be due to hydrolytic enzymes. Biodegradation of different aluminosilicates for the recovery of aluminium has been widely studied.
Acidolysis, complexolysis and alkalolysis are considered to be the acting mechanisms depending upon the type of the metabolite secreted. The action of silicate bacteria’ on aluminosilicates has been connected with the formation of mucilagenous capsules as well as with the production of different metabolites such as organic and amino acids.
The ability of heterotrophic bacteria, B. mucilaginous to degrade silicate and aluminosilicate minerals has been used to develop a technological scheme for the dressing of low grade magnesite and bauxite.
Bacillus lichenoformis, isolated from magnesite ore deposit, uptake silica and silicon which was restricted to adsorption onto bacterial cell surface rather than an internal cell surface uptake through membrane. Recently, Haider (1993) reported different strains of root nodule bacteria bacteria, Rhizobium and Bradyrhizobium, capable of solubilising silicates from different synthetic silicates.
In a country like India with its vast unexploited mineral potential, bioleaching assumes a great national significance.
2. Microbial Degradation of Xenobiotic:
Xenobiotic are those chemicals which do not exist in nature. These are man-made, synthesized compound such as pesticides. Pesticides are toxic chemicals which act by interfering with microbial reactions in the target organisms.
Since most of the time, these are added in soil and may affect those microorganisms which are important in maintaining soil fertility. Such organisms also detoxify pesticides in soil. Thus any chemical which seriously affects the soil microflora may harm soil fertility and crop production. Pesticide loss can also occur by volatilization, leaching or spontaneous break down.
The microorganisms that are able to metabolize pesticides and herbicides are given in Table 33.3.
These are diverse group of microorganisms which are able to metabolize pesticides and herbicide, including genera of both fungi and bacteria.
(i) Characteristics of Microbial Metabolism:
Most of the metabolic activities in the microbial world are meant for production of energy. Most of the organic molecules can serve as a source of energy to atleast some microbes. However, a few groups of chemicals are foreign to microorganisms. Among insecticidal compounds, the halogen-containing chemicals must be regarded as foreign or unusable material as such for microorganisms.
Microorganisms, if mutated may have adaptability towards chemicals that are initially toxic to them. In such cases the pesticide-degrading metabolic activities become higher. In general microbes alter the pesticide degradation process by using several mechanisms. Some of them are given below (Table 33.4).
(a) Enzymatic process:
It is most important to know whether the microbe derives energy from the process or not. It is generally possible that incidental metabolism is more prevalent form of microbial metabolism when the amount of pesticide is low in comparison with other carbon sources.
The catabolic metabolism could occur when the amount of pesticide is high, coupled with favourable chemical structure of pesticide that allows it to be microbially degradable and utilizable as a carbon source.
Through the studies of chlorinated aromatic pesticides, it may be possible to select a microbial strain capable of degrading the pesticides by enriching the medium with a non-chlorinated analogue of the pesticide. By such approach, even very stable and usually non-degradable pesticides might be made susceptible to microbial attack.
(b) Non-enzymatic process:
Some pesticides are photo chemically altered in the environment. There are two ways in which microbial products can promote photochemical reactions. In the first case, microbial products can act as photosensitizers by absorbing the energy from light and transmitted to insecticidal molecule. In another case, microbial products can facilitate photochemical reactions by serving as donors or acceptors.
Very limited information’s are available on the importance of the microbial formation of organic products capable of reaching with pesticides. Such reactants can be postulated to include amino acids, peptides, alkylating agent’s organic acids, vitamins, etc. Insecticidal chemicals are known to react with amino acid particularly with an -SH moiety.
Finally, microbial production of cofactors which are used in both enzymatic and non-enzymatic reactions should not be overlooked. Co-factors are those which promote the overall reactions involving an organic chemical without becoming a part of the chemical reaction product derived from that chemical.
(ii) Common Processes of Insecticidal Metabolism:
(a) Hydrolytic processes:
Most of the microbial activities are based on hydrolytic processes. It does not occur in other biological group. For example, major metabolic product in Trichoderma viride is the phenol of mexacarbate (i.e. the hydrolysis product) as compared with metabolism in animals which gave various oxidation products.
The reason for such hydrolytic reactions being common in the microbial world is that many of the microbes secrete hydrolytic enzymes exogenously as in the case of fungi. Nearly all the exoenzymes secreted by microbial cells seen to be related to the metabolism of large molecules.
(b) Reductive systems:
The major conversion product of parathion is aminoparathion. This is due to microorganisms. On the other hand, products of oxidative reactions are such as para-oxon diethyl-thiophosphoric acid which occurs due to animal metabolism.
Another important microbial reaction on insecticide is reductive dechlorination. The reaction proceeds by replacing a chlorine atom on a non-aromatic carbon with a hydrogen atom. The best known case being the conversion of DDT to TDE and DDE (Fig. 33.11).
Other insecticides which are known to go through such dechlorination reactions are gamma- BHC.
There are several oxidative reactions such as epoxidation of cyclodienes such as altruis and hepatachlor to corresponding epoxides, oxidation of thioethers to sulphoxides and sulphones, oxidative dealkylation of alkylamines, aromatic ring opening, decarboxylation, etc.
The important aspect is to identify one by one the key metabolic reactions and the stable end products in order to provide the necessary information’s to understand the processes, tendencies and role of microorganisms in altering the character of the important group of environmental pollutants.