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Factors affecting oil biodegradation

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Key elements influencing rates of oil biodegradation

  1. oil a) composition that defines its chemical and physical properties and b) concentration;
  2. abiotic factors (temperature, salinity, presence of water, etc.);
  3. biotic factors (composition of microbial community).

The above factors are usually interdependent, for example, abiotic factors affect oil composition and concentration, and biotic factors are highly dependent on abiotic factors.

Oil composition and concentration

Petroleum hydrocarbons can be divided into four classes: (1) saturates, (2) aromatics, (3) asphaltenes (phenols, fatty acids, ketones, esters, and porphyrins), and (4) resins (pyridines, quinolines, carbazoles, sulfoxides, and amides). In general, hydrocarbons have been ranked in the following order of decreasing susceptibility to biodegradation: n-alkanes > branched alkanes > low molecular weight aromatics > cyclic alkanes, with high molecular weight aromatics and polar compounds being extremely recalcitrant. However under some conditions (composition of microbial community, abiotic factors), the order of biodegradation can vary greatly.

It was shown that recalcitrant hydrocarbons can be oxidized and consequently degraded in the presence of other hydrocarbons, such as n-alkanes, that can be readily consumed by microorganisms. This phenomenon is termed co-oxidation.

Biodegradation also depends on dispersibility and weathering dynamics of the oil. In general, oils with high asphaltene and wax content are characterized by high viscosities and pour points and tend to be much less bioavailable.

The rates of uptake and mineralization of many organic compounds by microbial populations in the aquatic environment are proportional to the concentration of the compound, conforming to Michaelis-Menten kinetics. Michaelian kinetics have been demonstrated for low-molecular weight highly soluble oil components such as toluene. The microbial degradation of high-molecular weight hydrocarbons, such as long (>C12) alkanes with solubility less than 0.01 mg/liter, occurs at rates that exceed the rates of their dissolution and are a function of the hydrocarbon surface area available for emulsification or physical attachment by cells, and therefore do not display the dependence on concentration.

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Abiotic factors

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Biotic factors

Hydrocarbons in the environment are biodegraded primarily by bacteria and fungi. Algae and protozoa are important members of the microbial community in both aquatic and terrestrial ecosystems, but the extent of their involvement in hydrocarbon biodegradation is largely unknown and most likely is minor.

There are three mechanisms for adaptation of microbial communities to chemical contaminants: (1) induction and depression of enzymes,(2) genetic changes (mutations, horizontal gene transfer), and (3) selective enrichment.

The third mechanisms has been most documented. Prior exposure of microbial community to hydrocarbons, either from anthropogenic sources (spills, oil disposals, etc.) or from natural sources (seeps and plant-derived hydrocarbons) is important in determining how rapidly subsequent hydrocarbon inputs can be biodegraded. Generally, turnover rates of common oil components can be from 10 to 400 times slower than in chronically contaminated environment.

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References

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More about oil spill bioremediation at MetaMicrobe

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