The PAHs represent a large family of compounds, ranging from the two-ringed naphthalenes to the ten-ringed derivatives of naphthalene. They can be modified by the presence of alkyl side chains or various functional groups which produce a large variety of physical-chemical properties and toxicities. While the most toxic components of oil, the MAHs, are relatively water soluble, they evaporate quickly after oil is spilled. At the other end, the nonvolatile high molecular weight PAHs cannot effectively dissolve in water. Therefore, only intermediate-sized PAHs (such as acenaphthene, phenanthrene, and fluoranthene) significantly influence the toxicity of oil to pelagic organisms in the water column. PAHs can lead to both carcinogenic and noncarcinogenic effects such as oxidative stress, suppression of the immune system, and impairment of endocrine regulation and development. They are metabolized by cytochrome P450 isozymes to reactive metabolites such as epoxides, which target biologically important macromolecules, such as DNA and proteins (Figure 3). The ability to generate reactive metabolites varies with species, mainly due to the varying types and amount of P450s within organisms. In general, invertebrates have lower enzymatic activity compared with vertebrates, but species variation is also significant. A few biological responses to PAHs have been suggested, such as an increased content of bile metabolites, induction of hepatic cytochrome P450s, increased DNA adducts in liver, and increased prevalence of liver cancer.
PAHs can degrade quickly via exposure to sunlight -in contrast, PAH half-lives in marine sediments range from months to years. In general, PAHs exposed to solar radiation can result in greater toxicity (phototoxi-city), as free radicals which react with oxygen to form reactive oxygen species such as singlet molecular oxygen are generated. These reactive species can then target and damage important macromolecules such as nucleic acids (i.e., DNA, RNA) and proteins. The reactive singlet molecular oxygen may potentially destroy gill or skin membranes of fish, impairing respiration. Photoenhanced toxicity has been reported in bivalve embryos, marine invertebrates, and fish.
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