Environmental Microbiology of Aquatic and Waste Systems

7.3 Pollution by Petroleum in Oceans and Seas: Role of Microorganisms in Oil Degradation and Remediation 175

referred to as a naphthenic crude. These crudes contain
mainly naphthenic and aromatic hydrocarbons.

Sweet versus Sour
Crude oils may be referred to as either sweet or sour
depending upon the level of hydrogen sulfide present.
A sweet crude has very little H 2 S, whereas a sour crude
has larger quantities of H 2 S present.

Light versus Heavy
Crude oil may be divided into light and heavy on the
basis of their gravity. The API (American Petroleum
Institute) gravity is determined as:

or

Crude oils with gravity > 33°API are considered as

light crudes. Such crudes with a high percentage com-

position of hydrogen are usually more suitable for pro-

cessing for gasoline production. Heavy crudes, that is,

those with gravity < 28°API tend to contain more

asphaltenes and are usually rich in aromatics. These

heavy crudes require more steps in their processing

(Anonymous 2003 ).

The major components of crude oil are hydrocar-

bons, ranging from very volatile, light materials such

as propane and benzene to more complex heavy com-

pounds such as bitumens, asphaltenes, resins, and

waxes. They are separable into four fractions: the satu-

rates, the aromatics, the resins (pyridines, quinolines,

carbozoles, sulfoxudes, and amides), and asphaltenes

(phenols, fatty acids, ketones, esters, and porphyrins).

Saturates are hydrocarbons containing no double

bonds. They are further classified according to their

chemical structures into alkanes (paraffins) and

cycloalkanes (naphthenes). Alkanes have either a

branched or unbranched (normal) carbon chain(s), and

have the general formula CnH 2 n + 2. Cycloalkanes have

one or more rings of carbon atoms (mainly cyclopen-

tanes and cyclohexanes), and have the general formula

CnH 2 n. Most of the cycloalkanes in crude oil have an

alkyl substituent(s) (Fig. 7.6).

Aromatics have one or more aromatic rings with or

without an alkyl substituent(s). Benzene is the simplest

one (Fig. 7.6), but alkyl-substituted aromatics gener-

ally exceed the nonsubstituted types in crude oil. In

contrast to the saturated and aromatic fractions, both the

resin and asphaltene fractions contain non-hydrocarbon

141.5

API 131.5

Specific gravity

=

API 141.5 / Specific gravity 131.5=−( )

Table 7.7 (continued)

Method Advantages Disadvantages
Gene-specific PCR • Can be adapted to quantify gene copy number• Require enrichment of target organism

• Virulence genes may be targeted;
  providing direct evidence that potentially
  harmful organisms present
  
  
  • Sufficient quantity of target genes may not
    be available requiring enrichment or large
    quantity of sample
  
  
  
  


• Does not require reference library • Requires training of personnel
  
  
  • Primers currently not available for all
    relevant hosts
    Host-specific PCR • Does not require cultivation of target
    organism
  
  
  
  


• Little is known about survival and distribu-
  tion in water systems


• Rapid; easy to perform • Primers currently not available for all


• Does not require a reference library relevant hosts
  Virus specific PCR • Host specific • Low in numbers, requires large sample size


• Easy to perform • Not always present even when humans present


• Does not require reference library

Fig. 7.5 Typical MST results (From Callaghan 2005 )