Conceived and designed the experiments: MT. Performed the experiments: MT KO. Analyzed the data: MT KO. Wrote the paper: MT KO.
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By: Sonali Bhawsar Category: Applications Date: Views: Article Summary: Hydrocarbons are principle constituents of petroleum, related byproducts like coal tar, diesel, petrol, gasoline, kerosene and major environmental pollutants. Petroleum is composed of straight or branched saturated aliphatic, alicyclic, aromatic and unsaturated olefinic hydrocarbons.
Hydrocarbons are recalcitrant because of their oily nature and poor water solubility. They pers.. Petroleum, crude oil or oil and hydrocarbons are sometimes used synonymously. Hydrocarbons are principle constituents of petroleum, related byproducts like coal tar, diesel, petrol, gasoline, kerosene and major environmental pollutants. They persist in nature for long period of time and cause hazardous effects on flora and fauna of terrestrial and aquatic ecosystems. Oil spills, oil transportation, drilling operations, refineries and local fuel filling stations are some of the reasons responsible of hydrocarbon contamination.
Hydrocarbon degrading bacteria are known as hydrocarbonoclastic bacteria HCB. They utilize hydrocarbons as carbon and energy source for growth. HCB are important constituents of oil degrading consortia and are usually present in hydrocarbon contaminated sites. They have been exploited for their biodegradation potential and have been used successfully for cleanup of oil contaminated soils and aquatic systems.
Not only bacteria but some fungi and yeasts are also known to be hydrocarbonoclastic. Hydrocarbonoclastic mechanism: Contact and attachment to the hydrocarbon molecule is thought to be pre-requisite step for oil degradation. Some bacteria contain catabolic plasmids that can transform hydrocarbon pollutant into simple organic molecule; plasmid genes encode for enzymes for ring cleavage and oxidation reactions.
Bacterial production of biosurfactants or bioemulsifiers also determines their oil degradation potential. Bacterial cells first attach to oil droplets. Droplets are eventually disintegrated to smaller size for their efficient utilization and uptake inside the cell. Larger oil droplets cannot be taken up by bacterial cells.
Emulsification at this stage increases oil-water interface and faster rate of utilization and decomposition of oil substrates. Similarly, biosurfactants produced by HCB disperse hydrophobic hydrocarbon molecules thereby increasing their surface area to enhance the growth and efficiency of oil utilization.
HCB from different habitats: The most striking feature of HCB is their ability to proliferate rapidly in presence of hydrocarbon. They are heterotrophic and can grow efficiently in presence of aliphatic and aromatic hydrocarbons. Most of the HCB are obligate hydrocarbonoclastic and occasionally utilize non-hydrocarbon substrate like acetate and pyruvate.
They are rarely present in unpolluted areas! Therefore their sampling is always done from hydrocarbon contaminated sites. Indigenous species bloom after the hydrocarbon pollution. However, redox potential, temperature, light intensity and concentration of hydrocarbon substrate largely affect growth and activity of HCB. Consortia of HCB can have broad spectrum hydrocarbonoclastic potential than individual strains. Consortia can degrade naphthalene, fluorene, phenanthrene, anthracene, chrysene, fluoranthrene, benzopyrene, indenol and hundreds of polyaromatic hydrocarbons in shorter period.
Single HCB may take months to degrade hydrocarbon but consortia requires only few days to accomplish the task. Other important characteristics of HCB are that they have very small genome, few rRNA operons and cytoplasm contains very small number of proteins.
Hydrocarbons are water insoluble and remain adsorbed to clay, humus particles and debris. Hydrocarbon degradation in soil is therefore influenced by their availability for uptake by HCB. Acinetobacter, Pseudomonas and Bacillus respectively produce lipopolysaccharide, rhamnolipids and lipopeptide type of bioemulsifiers and biosurfactants. Important role of these polymeric byproducts in the conversion of hydrocarbons to bioavailable form is principally responsible for their hydrocarbonoclastic potential in the soil.
Micrococcus, Arthrobacter, Burkholderia and Serratia also produce biosurfactants for hydrocarbon degradation. In freshwater ecosystems, hydrocarbons form a thin hydrophobic layer after pollution. This oil layer is subject to photodegradation and also degraded by HCB community of freshwater which suddenly blooms up after pollution. Native soil bacteria like nocardias and actinomycetes are nutritionally versatile and can utilize different hydrocarbons as carbon and energy source.
Soil bacteria mentioned above are also found in freshwater as they are carried by rainwater runoff, water drainage or via discharge of effluents. Hydrocarbonoclastic potential of HCB in freshwater is dependent on emulsification or surfactant activity of strains, pH, concentration of contaminant in that habitat.
They always grow in environment of high carbon and low nitrogen content which is ideally created by hydrocarbon pollution. Obligate marine hydrocarbon bacteria genera include Marinobacter, Marinobacterium, Alcanivorax, Neptunomonas, Thalassolituus, Cycloclasticus, Oleispira and Oleiphilus. Marinobacterium jannaschii and Oleispira antarctica are psychrophilic and halotolerant HCBs. Neptunomonas naphthovorans degrade polyaromatic hydrocarbons PAH especially naphthalene.
On the contrary, Thalassolituus oleivorans can grow on and degrade only aliphatic hydrocarbons. Cycloclasticus pugetii is present in deep sediments of Atlantic and Pacific oceans and carry out aerobic degradation of substituted and unsubstituted aromatic hydrocarbons. Different species of Marinobacter like alkaliphilus, aquaeolei, articus, hydrocarbonoclasticus, maritimus and squalenivorans are strictly HCB.
Alcanivorax borkumensis and A. They synthesize extracellular lipids like polyhydroxy alkanoates and wax esters when they are grown on sole carbon source like hexadecane under phosphorus and nitrogen limiting conditions. Hydrocarbon degradation in marine habitat is largely dependent on temperature, salinity, depth, latitude and redox potential. Degradation and removal of hydrocarbon pollutants from the environment via HCB is cheap, ecofriendly and very promising strategy.
Indigenous as well as in vitro formulated consortia of HCB are being used not only for oil spill mitigation but also for biopolymer production and in biocatalysis. We do not own any responsibility for correctness or authenticity of the information presented in this article, or any loss or injury resulting from it.
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Microbes Environ. Epub Feb 7. Most hydrocarbonoclastic bacteria in the total environment are diazotrophic, which highlights their value in the bioremediation of hydrocarbon contaminants. Eighty-two out of the hydrocarbonoclastic bacterial species that have been already isolated from oil-contaminated Kuwaiti sites, characterized by 16S rRNA nucleotide sequencing, and preserved in our private culture collection, grew successfully in a mineral medium free of any nitrogenous compounds with oil vapor as the sole carbon source. Fifteen out of these 82 species were selected for further study based on the predominance of most of the isolates in their specific sites.
E-mail: wk. Received Jun 19; Accepted Dec 5. This article has been cited by other articles in PMC. Abstract Eighty-two out of the hydrocarbonoclastic bacterial species that have been already isolated from oil-contaminated Kuwaiti sites, characterized by 16S rRNA nucleotide sequencing, and preserved in our private culture collection, grew successfully in a mineral medium free of any nitrogenous compounds with oil vapor as the sole carbon source. Fifteen out of these 82 species were selected for further study based on the predominance of most of the isolates in their specific sites. All of these species tested positive for nitrogenase using the acetylene reduction reaction.
Aromatic compounds are among the most persistent of these pollutants and lessons can be learned from the recent genomic studies of Burkholderia xenovorans LB and Rhodococcus sp. These studies have helped expand our understanding of bacterial catabolism , non-catabolic physiological adaptation to organic compounds , and the evolution of large bacterial genomes. First, the metabolic pathways from phylogenetically diverse isolates are very similar with respect to overall organization. Thus, as originally noted in pseudomonads , a large number of "peripheral aromatic" pathways funnel a range of natural and xenobiotic compounds into a restricted number of "central aromatic" pathways. Nevertheless, these pathways are genetically organized in genus-specific fashions, as exemplified by the b-ketoadipate and Paa pathways. Comparative genomic studies further reveal that some pathways are more widespread than initially thought. Thus, the Box and Paa pathways illustrate the prevalence of non-oxygenolytic ring-cleavage strategies in aerobic aromatic degradation processes.