Potential Hydrocarbonoclastic Bacteria:
Hydrocarbono-clastic means degrading hydrocarbons or simply 'eating' or 'breaking' the hydrocarbon molecules. 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. 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 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. Hydrocarbons are utilized by catabolic pathways like β-ketoadipate, Phenyl acetate and alkane oxidation reactions resulting in formation of fatty acids and carbon dioxide as chief end products. 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. HCB from soil, rhizosphere area, freshwater and sediments include species of Bacillus, Acinetobacter, Pseudomonas, Burkholderia, Arthrobacter, Nocardia, Micrococcus, Actinomyces, Alcaligenes, Brevibacterium, Aeromonas, Enterobacter and Flavobacterium. Occurrence of Cellulomonas, Sphingomonas, Spherotilus, Clostridium, Staphylococcus, Chromobacterium, Corynebacterium Erwinia, Desulfovibrio and Serratia has also been reported. 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. HCB from marine environment are mainly Gram negative, halotolerant, strictly hydrocarbonoclastic and aerobic γ-proteobacteria. 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. jadensis are also obligate HCB and rarely metabolize substrates other than hydrocarbons. 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.
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