Biodegradation of xenobiotics

biodegradation of xenobioticc by microorganisms

1. BIODEGRADATION OF XENOBIOTICS HYDROCARBONS, PLASTICs & PESTICIDES DONE BY SUSHMITA PRADHAN II SEM M.S c Microbiology

2. XENOBIOTICS It is derived from a greek word “ XENOS ” meaning ‘ foreign or strange ’. Xenobiotics are those chemicals which are man-made and do not occur naturally in nature. They are usually synthesized for industrial or agricultural purposes e.g. aromatics , pesticides , hydrocarbons, plastics , lignin etc. They are also called RECALCITRANTS as they can resist degradation to maximum level.

3. biodegradation According to the definition by the International Union of Pure and Applied Chemistry , the term biodegradation is “Breakdown of a substance catalyzed by enzymes in vitro or in vivo. In other words, defined as the ability of microorganisms to convert toxic chemicals ( xenobiotics ) to simpler non-toxic compounds by synthesis of certain enzymes Biodegradation of xenobiotics can be affected by substrate specificity, nutrition source, temperature, pH etc.

4. Sources of xenobiotics Petrochemical industry : - oil/gas industry, refineries . - produc es basic chemicals e.g. vinyl chloride and benzene 2 . P lastic industry : - closely related to the petrochemical industry - uses a number of complex organic c om p oun d s - such as anti-oxidants, plasticizers, cross-linking agents

5. 3 . Pesticide industry : - most commo n ly foun d. - structures are benzene and benzene derivatives, 4 . Paint industry : - major ingredient are solvents, - xylene, toluene, methyl ethyl ketone, methyl 5 . Others : - Electronic industry, Textile industry, Pulp and Paper industry , Cosmetics and Pharmaceutical industry , Wood preservation

6. BIODEGRADATION OF PESTICIDES Pesticides  are substances meant for destroying or mitigating any  pest. They are a class of  biocide . The most common use of pesticides is as plant protection products (also known as crop protection products ). It includes:  herbicide, insecticide,  nematicide , termiticide, molluscicide , piscicide, avicide, rodenticide ,  insect repellent, animal repellent,  antimicrobial, fungicide,  disinfectant, and  sanitizer .

7. DIFFERENT METHODS - 4 a) Detoxification :   Conversion of the pesticide molecule to a non-toxic compound. A single moiety in the side chain of a complex molecule is disturbed(removed), rendering the chemical non-toxic.  b ) Degradation :   Breakdown or transformation of a complex substrate into simpler products leading to mineralization. E .g . Thirum (fungicide) is degraded by a strain of Pseudomonas   and the degradation products are dimethylamine , proteins, sulpholipids , etc (Raghu et al., 1975).

8. c ) Conjugation (complex formation or addition reaction) :   An organism makes the substrate more complex or combines the pesticide with cell metabolites. Conjugation or the formation of addition product is accomplished by those organisms catalyzing the reaction of addition of an amino acid, organic acid or methyl crown to the substrate thereby inactivating the pestcides d) Changing the spectrum of toxicity :   Some pesticides are designed to control one particular group of pests , but are metabolized to yield products inhibitory to entirely dissimilar groups of organisms, for e.g. the fungicide PCNB is converted in soil to chlorinated benzoic acids that kill plants.

9. There are many mechanisms involved on the biodegradation of pesticides and other contaminants. These may be summarised as follows: Dehalogenation - nitrofen , DDT, cyanazine , propachlor . Deamination - fluchloralin Decarboxylation - DDTc , biofenox , dichlorop -methyl M ethyl oxidation - bromacil Hydroxylation - benthiocarb , bux insecticide

10. BIODEGRADTION OF PLASTICS Plastic is a broad name given to different polymers with high molecular weight, which can be degraded by various processes . The biodegradation of plastics by microorganisms and enzymes seems to be the most effective process . It consist of two steps- fragmentation and mineralization . But at the core, reaction occurring at molecular level are oxidation and hydrolysis . The decomposition of major condensation polymers (e.g. polyesters and polyamides) takes place through hydrolysis, while decomposition of polymers in which the main chain contains only carbon atoms ( e.g. polyvinyl alcohol, lignin) includes oxidation which can be followed by hydrolysis of the products of oxidation.

11. METHOD HYDROLYSIS - The process of breaking these chains and dissolving the polymers into smaller fragments is called hydrolysis. E.g. Pseudomonas sp s P olymeric Chains is broken down into constituent parts for the energy potential by  microorganisms. Monomers are readily available to other  bacteria and is used. Acetate and hydrogen produced is used directly by methanogens . Other molecules, such as volatile fatty acids (VFAs) with a chain length greater than that of acetate is first catabolized into compounds that can be directly used by methanogens . ACIDOGENESIS - This  results in further breakdown of the remaining components by acidogenic (fermentative) bacteria into ammonia ,  ethanol, carbon dioxide, and hydrogen sulfide. E.g Streptococcus acidophilus.

12. ACETOGENESIS - Simple molecules created through the acidogenesis phase are further digested by Acetogens to produce largely acetic acid, as well as carbon dioxide and hydrogen . METHANOGENESIS- Here , methanogens use the intermediate products of the preceding stages and convert them into methane, carbon dioxide, and water. These components make up the majority of the biogas emitted.  Methanogenesis  is sensitive to both high and low  pHs and occurs between pH 6.5 and pH 8. The remaining, indigestible material the microbes cannot use and any dead bacterial remains constitute the digestate .

13. Some of the microorganism that can degrade plastics are:- Aliphatic Polyesters PolyEthylene Adipate (PEA ) - lipases from  R. arrizus , R. delemar , Achromobacter  sp. and  Candida cylindracea Poly ( β- Propiolactone ) PPL - estereases from   Acidovorax   sp .,  Variovorax paradoxus ,  Sphingomonas paucimobilis . Aromatic Polyesters Poly-3-Hydroxybutyrate ( PHB ) – estereases from Pseudomonas lemoigne ,  Comamonas  sp.  Acidovorax faecalis , Aspergillus fumigatus Poly Lactic Acid ( PLA ) - proteinase K from  Tritirachium album, Amycolatopsi s sp Strains of Actinimycetes has been reported to degrade polyamide (nylon), polystyrene, polyethylene .

14. BIODEGRADATION OF HYDROCARBONS A  hydrocarbon  is an organic compound consisting entirely of hydrogen and carbon. The majority of hydrocarbons found on  earth naturally occur in crude oil . Aromatic hydrocarbons  (arenes), alkanes , alkenes , cycloalkanes and alkyne-based compounds are different types of hydrocarbons .

15. BIODEGRADATION OF PETROLEUM Petroleum compounds are categorized into 2 groups 1. Aliphatic hydrocarbon e.g. alkane, alcohol, aldehyde 2. Aromatic hydrocarbon e.g. benzene, phenol, toluene, catechol Aromatic hydrocarbons are degraded aerobically and anaerobically.

16. AEROBIC DEGRADATION A re metabolized by a variety of bacteria, with ring fission . A ccomplished by mono- and dioxygenases . Catechol and protocatechuate are the intermediates. M ostly found in aromatic compound degradation pathway.

17. 19 OTHER MECHANISMS 1) Photometabolism : in bacteria , this light-induced “bound oxygen” (OH • ) i s used to oxidize substrates

18. 20 2) under nitrate-reducing condition : Nitrate-reducing bacteria couple the oxidation of org anic compound with water to the exergonic reduction of nitrate via nitrite to N 2 .. 3) dissimilation through sulfate respiration : Sulfate- reducing bacteria couple the oxidation of org anic compound with water to the exergonic reduction of sulfate via sulfite to sulfide .

19. 21 Some m icroorganisms involved in the biodegradation of hydrocarbons Org anic Pollutants Organisms Phenolic Achromobacter, Alcaligenes, compound Acinetobacter , Arthrobacter , Azotobacter, Flavobacterium , Pseudomonas putida Candida tropicalis Trichosporon cutaneoum Aspergillus , Penicillium Benzoate & related Arthrobacter , Bacillus spp., compound Micrococcus , P. putida

20. 22 Org anic Pollutants Organisms Hydrocarbon E . coli, P. putida, P. A eruginosa , Candid a Surfactants Alcaligenes , Achromobacter, Bacillus , Flavobacterium, Pseudomonas , Candida Pesticides P . Aeruginos a  DDT B . sphaericu  Linurin Arthrobacter, P. cepacia  2,4-D P . cepacia  2,4,5-T , Parathion

21. 23 Genetic Regulation of Xenobiotic Degradation plasmid-borne mostly in the genus Pseudomonas PLASMID SUBSTRATE TOL Toluene, m -xylene, p -xylene CAM Camphor OCT Octane, hexane, decane NAH Napthalene pJP1 2,4-Dichlorophenoxy acetic acid pAC25 3-Chlorobenzoate SAL Salicylate

22. POLYCYCLIC AROMATIC HYDROCARBONS (PAH ) Bacteria, fungi, yeasts, and algae have the ability to metabolize both lower and higher molecular weight PAHs found in the natural environment. Most bacteria have been found to oxygenate the PAH initially to form dihydrodiol with a cis -configuration, which can be further oxidized to catechols . Most fungi oxidize PAHs via a cytochrome P 450 catalyzed mono - oxygenase reaction to form reactive arene oxides that can isomerize to phenols. White-rot fungi oxidize PAHs via ligninases (lignin peroxidases and laccase ) to form highly reactive quinones .

23. 25

24. 26 Compound Organisms Metabolite Naphthalene Acinetobacter calcoaceticus , Alcaligenes denitrificans, Mycobacterium sp . , Pseudomonas sp ., Pseudomonas putida , Naphthalene cis -1,2 – dihydrodiol, 1,2 – dihydroxynaphthalene, 2 - hydroxychromene - 2 – carboxylic acid, trans – o – hydroxybenzylidene pyruvic acid, salicylaldehyde, salicylic acid, catechol, gentisic acid, naphthalene trans – 1,2 – dihydrodiol . Acenaphthene Beijerinckia sp ., Pseudomonas putida, Pseudomonas fluorescens, Pseudomonas cepacia 1- Acenaphthenol, 1- acenaphthenone, acenaphthene – cis – 1,2 – dihydrodiol, 1,2 – acenaphthenedione, 1,2 – dihydroxyacenaphthylene, 7,8 – diketonaphthyl – l – acetic acid, 1,8 – naphthalenedicarboxylic acid, 3 – hydroxyphthalic acid . Bacterial strain degrading

25. 27 Compound Organisms Metabolite Fluoranthene Alcaligenes denitrificans , Mycobacterium sp . , Pseudomonas putida , Pseudomonas paucimobilis, Pseudomonas cepacia , Rhodococcus sp . 7- Acenaphthenone, 1- acenaphthenone, 7- hydroxyacenaphthylene, benzoic acid, phenylacetic acid, adipic acid, 3- hydroxymethyl – 4,5- benzocoumarin, 9- fluorenone – 1 – carboxylic acid, 8- hydroxy – 7- methoxyfluoranthene, 9- hydroxyfluorene , 9- fluorenone, phthalic acid, 2- carboxybenzaldehyde Pyrene Alcaligenes denitrificans , Mycobacterium sp . , Rhodococcus sp. Pyrene cis - and trans - 4,5 – dihydrodiol , 4 – hydroxyperinaphthenone , phthalic acid, 4- phenanthroic acid, 1,2 - and 4,5 – dihydroxypyrene , cinnamic acid, cis – 2 – hydroxy – 3 – ( perinaphthenone -9-yl ) propenic acid Chrysene Rhodococcus sp. None determined Benz [ a ] anthracene Alcaligenes denitrificans , Beijerinckia sp . , Pseudomonas putida Benz [ a ] anthracene cis – 1,2, cis - 8,9-, and cis – 10,11- dihydrodiols , 1- hydroxy – 2 – anthranoic acid, 2- hydroxy – 3 – phenanthroic acid, 3- hydroxy – 2 – phenanthroic acid . Benz [ a ] pyrene Beijerinckia sp ., Mycobacterium sp. Benz [ a ] pyrene cis -7,8 - and cis -9,10 – dihydrodiols .

26. POLYCHLORINATED BIPHENYLS (PCB s ) Synthesized chemicals from petro-chemical industry used as lubricants and insulators in heavy industry. First manufactured in 1929 by Monsanto . Manufacture and unauthorized use banned in 1978 by USEPA Used because- Low reactivity Non-flammable High electrical resistance Stable when exposed to heat and pressure Used as Hydraulic fluid, Casting wax, Carbonless carbon paper, Compressors, Heat transfer systems, Plasticizers, Pigments, Adhesives, Liquid cooled electric motors, Fluorescent light.

27. RISKS - Causes reproductive disabilities in animals, human, birds. Carcinogenic Bioaccumulation Soluble in almost all the solvents, fats, oils Nervous system damage Endocrine gland malfunction

28. Methods for PCB removal Natural Attenuation : Microbes already in the soil are allowed to degrade as they can naturally and the site is closely monitored . Biostimulation : Microbes present in the soil are stimulated with nutrients such as oxygen, carbon sources like fertilizer to increase degradation. Bioaugmentation : Microbes that can naturally degrade PCB’s are transplanted to the site and fed nutrients if necessary.

29. Pathways for pcb removal Fungal DEGRADATION – Aspergillus niger : fillamentous with cytochrome p450 that attacks lower chlorinated PCB’s Phanerochaete chrysosporium : White rot fungi can attack lignin (PCB) at low conc entration with the help og ligninases . BACTERIAL degradation- Soil bacteria breaks down PCBs via dioxygenase pathways. Most identified seem to be Pseudomonas species, Achromobacter , Acinetobacter , Alcaligenes , Arthrobacter , Corynebacterium , Rhodococcus , Burkholderia .

30. reference BOOKS - S ullia S.B and S hantharam S .; General microbiology, Second edition Page No. 348-350 Dubey R. C and M aheshwari D.K; A textbook of microbiology, second revised edition 2009; Page No. 832-836 ARTICLES Biodegradation of polymers- Dr R olf Joachim Miller Biodegradation of pesticides- Andre Luiz Porto, Marcia N itcshke and G leiseda Melgar Microbial Degradation of Petroleum Hydrocarbon - Nilanjana Das and   Preethy Chandran