Persistence recalcitrance

PAHs have been found all over the globe in all compartments of the environment. They are ubiquitous because the are persistent. Recalcitrance in PAHs may stem, in part, from the delocalized electrons in the planar pi orbitals of the aromatic structure. Their relatively high octanol-water partition coefficients, Kows, make them rather lipophilic. The lipophilicity of PAHs forces them from the dissolved phase to particles and also into lipid rich organisms, but they can be metabolized in higher organisms. However, these metabolites are often more toxic than their parent PAHs. When combined with other stressors, particularly ultraviolet radiation, PAHs can exert enhanced toxicity. 

The refractory nature of some pollutants, notably, persistent polyhalogenated compounds, has raised problems of bioremediation of contaminated sites (e.g., sediments and dumping sites). There has been interest in the identification, or the production by genetic manipulation, of strains of microorganisms that can metabolically degrade recalcitrant molecules. For example, there are bacterial strains that can reductively dechlorinate PCBs under anaerobic conditions. 

Fewson CA (1988) Biodegradation of xenobiotics and other persistent compounds the causes of recalcitrance. Trends Biotechnol 6 148-153... 

The higher is this index, the more recalcitrant and persistent can be considered the environmentally examined compound. 

Although the rate at which components of plant and animal residues are decomposed by the soil microbial biomass varies widely (Stout et al., 1981), none of the classes of naturally produced organic compounds persist in the soil indefinitely as there are always species or a succession of species that can degrade them. Jenkinson and Ladd (1981) pointed out that if it were not so, the completely recalcitrant SOM fractions would accumulate indefinitely in the soil and by now would cover the surface of the earth. 

Black C, produced by wild fires and humic substances (HS), the natural by products of SOM decomposition in soil and water systems, are certainly the classes of organic compounds that most closely approximate this recalcitrant behavior. HS occur widely, being found in large amounts not only in the soil and sediments but also in lakes, rivers, ground waters, and even the open ocean (Stevenson, 1994). Besides these relatively refractory substances, more labile compounds can persist in soil for a much longer time than would be predicted from their inherent recalcitrance to decomposition. SOM stabilization (Figure 5.2) is generally considered to occur by three main mechanisms (i) physical protection, (ii) chemical stabilization, and (iii) biochemical stabilization (Six et al., 2002). 

Since black C has a highly aromatic structure with a low level of substitution with functional groups, it is highly recalcitrant and therefore contributes to the stable fraction of soil C. At the global scale, formation of black C rapidly transfers fast-cyclable C from the biosphere to much slower-cyclable forms that may persist in the soil for millennia. It therefore represents an effective pathway for C sequestration. 

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