Petroleum Hydrocarbons

 Petroleum- natural product resulting from anaerobic conversion of biomass under high temperature and pressure.

• Always present in biosphere by natural seepage, but in lower amounts
• Forced recovery by drilling increased its amount and concentrated around offshore production areas and major shipping routes
• Biodegradable but at slower rates
• Oil floating on water difficult to contain and collect – destructive to birds and marine life-economic and aesthetic damage-high cleaning costs
• Dissolved aromatic compounds disrupt the chemoreception of some marine organisms (even at low ppb)- Disrupts feeding and mating responses leading to elimination of some marine organisms
• Some derivatives resistant to biodegradation and are carcinogenic- may move up marine food chains and contaminate fish/shell fish

Petroleum

• complex mixture of aliphatic, alicyclic and aromatic hydrocarbons. 
• A smaller proportion of nonhydrocarbon compounds such as naphthenic acids, phenols, thiols, heterocyclic nitrogen and sulphur compounds as well as metallo porphyrins
• several hundred individual components in every crude oil- composition varies with its origin
• Most xenobiotic pollutants are substituted/modified hydrocarbons
• Susceptibility varies with the type and size of the hydrocarbon molecule

Aliphatic hydrocarbons

• Aliphatic hydrocarbons are composed of hydrogen and carbon, which can be linear, branched or cyclic.
• Aliphatic compounds can be saturated or unsaturated.
• There are several types of aliphatic hydrocarbons, including alkanes, alkenes and alkynes.
• Alkanes are the most abundant constituents in crude oil and are the first component to be degraded.

Aromatic hydrocarbons

• Polycyclic aromatic hydrocarbons (PAHs) - atmospheric, water and soil pollutants containing one or more aromatic rings.
• Among monoaromatic compounds, benzene, toluene and xylene were well reported and studied.
• Polycyclic aromatic hydrocarbons (PAHs), such as naphthalene, anthracene and phenanthrene, are also well documented. 
• Due to their complex structure, PAHs are highly resistant to degradation and remain persistent in the ecosystem

Alicyclic/Heterocyclic compounds

• Heterocyclic compounds are organic compounds containing at least one heterocyclic ring-oxygen, nitrogen and sulfur are incorporated into an organic ring structure 
• include polar compounds such as nitrogen (quinolines), sulfur (dibenzothiophenes) and oxygen (xanthene) atoms.
• Heterocyclic compounds are the most recalcitrant for degradation

Petroleum degradation, thus, in short, include degradation of :

• n-alkanes of intermediate chain length (C10-C24) -are degraded most rapidly
• short chain alkanes are toxic to many microorganisms, but these compounds evaporate rapidly
• very long chain alkanes become increasingly resistant to biodegradation
• As the chain length increases and alkanes exceed a molecular weight of 500, alkanes cannot be used as Carbon sources
• Branching reduces the rate of degradation, since tertiary and quarternary carbon atoms interfere with the degradation
• Aromatic compounds (condensed type) degrade slowly than the alkanes
• Alicyclic compounds do not serve as carbon source/not degraded unless they have a long aliphatic sidechain- Then, degraded by cometabolism