1.2. Bioremediation of Ecosystems Contaminated With Organic Pollutants

Crude petroleum and its refined products are the major sources of organic contamiants polluting the ecosytems. Petroleum is mainly composed by three hydrocarbon (paraffin, naphthenes, and aromatic) fractions. Each petroleum fraction is usually composed by hundreds of different hydrocarbon molecules rather than a defined composition. Thus, fractions are dissimilar in terms of volatility, bioavailability, toxicity, degradability, and persistence. Spills are difficult to avoid during the petroleum processing and delivery. This complex array of compounds presents a tremendous challenge for designing effective bioremediation strategies.

Once petroleum hydrocarbons reach an environment, damage can be the result of several causes. Primary biological impact is due to the blocking effect of oil layer to water, nutrients, O₂, and light access. Cytotoxic and mutagenic effects of hydrocarbons lead to long-term pollution consequences. A more bioavailable toxic compound not only shows increased noxious effects but also has higher accessibility for biodegradation. In contrast, strongly adsorbed fraction is less toxic but more difficult to take care of. This general rule is relevant for designing biological strategies for the cleanup of polluted soils or sediments because petroleum hydrocarbons tend to tightly adsorb to these matrices.

Selection of an appropriate strategy for remediation relies on the physicochemical properties of the polluted matrix and on the degree and age of the spill. The aim of bioremediation is to overcome the limiting factors that slow down biodegradation rates. Bioremediation of organic compounds can be accomplished either by in situ or ex situ treatments. During in situ applications, the organic contamination is treated at the site. The ex situ technologies involve the transport of the polluted soil to a place where a suitable treatment system can be engineered for the removal of organic contaminants. For petroleum hydrocarbons, four scenarios may arise:

1. The excess of carbon source due to hydrocarbon input results in limitation of other nutrients. Addition of nitrogen and phosphorus can restore the balance and increase biodegradation rates.

2. Insufficient oxygen availability decreases biodegradation rates. Air injection or simple stirring can overcome oxygen limitation during aerobic hydrocarbon degradation.

3. Low bioavailability of hydrocarbons. Addition of environmentally friendly surfactants (such as those non-toxic and biodegradable ones produced by microorganisms or plants) can improve solubility and thus bioavailability of hydrocarbons.

4. Non-efficient catabolic machinery from native microbial communities. Addittion of pure culture or microbial consortium hydrocarbon-degrading microorganisms can enhance degradation rates.