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Approaches to oil spill bioremediation

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There are several approaches to bioremediation of oil waste such as bioreactors, landfarming, biopiling, and bioventing. They are usually used in somewhat controlled and restricted environment. In case of oil spill in open sea because of tanker's crash or well explosion, gigantic scale of such accidents and uncontrolled and sometimes unpredictable spread of the oil, in situ bioremediation can be the only option.

Most efforts to alleviate oil spill bioremediation are directed on optimization of natural biotic and abiotic factors affecting oil bioremediation.

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Surfactants/emulsifiers/dispersants in bioremediation

A major question in the biodegradation of hydrocarbons is how the microbes actually contact the substrates. Three mechanisms are often invoked, and different bacterial species may use one or more of them:

Surfactants are amphiphilic (possessing both hydrophilic (water-loving) and lipophilic (fat-loving) properties) compounds which can reduce surface and interfacial tensions by accumulating at the interface of immiscible fluids and increase the solubility, mobility, bioavailability and subsequent biodegradation of hydrophobic and insoluble organic compounds.

The high diversity of surfactants produced by numerous microorganisms is noteworthy. Microbial biosurfactants/bioemulcifiers are divided into 2 broad categories: low molecular weight molecules (for example, rhamnolipids produced by Pseudomonas species) and high molecular weight polymers (for example, RAG-1 emulsan, produced by Acinetobacter calcoaceticus RAG-1).

According to their chemical structure, bioemulsifiers may be classified into the following main groups:

  1. Glycolipids, in which carbohydrates such as sophorose, theralose or rhamnose are attached to a long-chain aliphatic acid or lipopeptide (for example, rhamnolipids synthesized by P. aeruginosa);
  2. Aminoacid-containing like surfactin produced by Bacillus subtilis;
  3. Polysaccharide-lipid complexes (for example, emulsan produced by Acinetobacter calcoaceticus;
  4. Protein-like substances such as liposan synthesized by Candida lipolytica.

The efficacy of the natural biosurfactants has prompted a search for chemically synthesized, cheaper surfactants.

The interaction of surfactants / emulsifies with oil, media and microbial community is very complex. Addition of surfactants alleviates contaminant degradation by some native microorganisms while at the same time inhibiting viability and productiveness of others. Net effect is sometimes very difficult if not impossible to predict.

Points of consideration for surfactant usage (each of these factors can operate on local and very specific level as well as on the scale of the whole experiment / rescue operation):

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Application of fertilizers

Hydrocarbon-degrading microorganisms usually exist in very low abundance in marine environments where the majority of large-scale spills occur. Influx of hydrocarbons stimulate rapid growth of these organisms. In natural environment, the amount of bioavailable nitrogen and phosphorus quickly become insufficient to support the growth. In soil and sediments, not only nitrogen and phosphorus but also oxygen become exhausted rapidly.

Considerations for fertilizers application

There are two types of fertilizers: inorganic and organic. Applications are performed either on open water or in vicinity of the shoreline and/or on the shore. In most experiments biodegradation of oil in fertilized areas was somewhat greater than in control areas. In some experiments there were no differences.

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Field application of surfactants and nutrients

Many commercial formulations that were used for enhancement of oil biodegradation are a combination of surfactants and fertilizers.

By far the largest and best documented use of bioremediation on a marine spill to date was application of Customblen and Inipol EAP22 in Prince William Sound, Alaska when grounding of the Exxon Valdez on March 24, 1989, released approximately 11 million gallons of crude oil, and stranded oil contaminated shores of the islands in the Sound.

The U.S. Environmental Protection Agency (EPA) and Exxon entered into cooperative agreement to develop bioremediation as part of the spill clean-up, and extensive laboratory and field studies were begun. After success of the pilot trials, full-scale application began in August 1989. More than 70 miles of beaches were treated with Inipol. Visual observations as well as intensive statistical analysis concluded that the application of nutrients and dispersants accelerated the biodegradation of the oil by a factor of 3-5 times. Also, no acute toxicity was observed.

Inipol EAP22 was first used on large scale in 1978 in response to the Amoco Cadiz spill on the Brittany coast, France. The optimal dose of the preparation was determined to be 10% (wt/wt) of the oil. Later, the dose was lowered to 5% (wt/wt). Simple calculations show that amount of the surfactant/fertilizer mixture required for successful bioremediation is huge. Cost of its production, transportation and dispersion is enormous.

Inipol EAP 22 (elemental composition 7.4% N and 0.7% P)
Customblen (elemental composition 28.0% N and 3.5% P)

The Joint Information Center has announced that the two dispersants being used to combat the spill after the Deepwater Horizon oil rig exploded off the coast of Louisiana in the U.S. are Corexit 9500 and Corexit (R)EC9527A, both made by the water treatment company Nalco in Napierville, U.S. Their chemical composition is proprietary (Web ref.).

More about dispersant use in Deepwater Horizon oil spill (Schmidt CW 2010. Environ Health Perspect)

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Using non-native microorganisms

Pollution by petroleum hydrocarbons stimulates preferential growth of microorganisms that can use them for biomass growth and for energy. Identification of the key organisms that play roles in pollutant biodegradation is important for understanding, evaluating and developing in situ bioremediation strategies. For this reason, many efforts have been made to characterize bacterial communities, to identify responsible degraders, and to elucidate their catalytic potential.

Seeding (bioaugmentation) is the introduction of allochthonous microorganisms into natural environment for the purpose of increasing the rate and/or extent of oil biodegradation. Some of the organisms not only serve as biodegraders but also may produce biosurfactants. There are several criteria to be met by the seed organisms: ability to degrade a wide spectrum of oil pollutants, genetic stability, viability during storage, a high degree of enzymatic activity and growth in the environment, the ability to compete with indigenous microorganisms, nonpathogenicity, and inability to produce toxic metabolites.

Most microorganisms considered for seeding are obtained by enrichment cultures from previously contaminated sites. Also, many attempts at bioengineering of "super-degrader" were made. Specifically, a multiplasmid-containing Pseudomonas strain capable of oxidizing aliphatic, aromatic, terpenic, and polyaromatic hydrocarbons has been reported (Chakrabarty AM et al. 1978). The use of such strain as an inoculum would preclude the problems associated with competition between strains in a mixed culture.

Today, many companies are developing and/or marketing hydrocarbon-degrading seed cultures.

There were several field evaluations of the effects of seeding on contaminated shorelines and in soil. In most cases, nutrient addition alone had a greater or comparable effect on microbial oil decomposition.

Among commercial biodegraders, Pseudomonas spp. (P. putida, P. fluorescens, P. alcaligenes, etc.) occupy the most prominent place. Other microorganisms include bacteria such as Rhodococcus, Arthrobacter, Acinetobacter, Moraxella, Bacillus, Alteromonas and fungi such as Candida and Trichoderma.

Arguments against seeding
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References

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More about oil spill bioremediation at MetaMicrobe

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