Reasons for changes in the CMN structure

microbial community may shift to a poor and uneven composition. Among these are stressors, such as competition, based on habitat alteration, limited resources, or differential growth rates (Tilman, 1993; Langworthy et al., 2002; Zhou et al., 2002); exposure to toxic compounds; or selection based on inherent or acquired metabolic capability (MacNaughton et al., 1999; Langworthy et al., 2002; Viñas et al., 2002).

Creosote is a non-aqueous phase liquid that, when applied to soil, may cause many physico-chemical changes in the microbial environment. Tar “droplets” have been observed in PAH-contaminated soils, including the CMN, that may cause sticky aggregates to form (Johnsen et al., 2005). Zones of anaerobicity may form due to poor oxygen diffusion or water retention, or a decrease in moisture content due to the impermeability to water. These conditions can lead to habitat fragmentation, in which larger communities may be divided at the microscale. This results in a loss of population interaction and an induction of competition between the remaining species for the limited access to resources such as oxygen or water with soluble nutrients (Tilman, 1993).

Substrate competition is the most common type of competition between

microorganisms in nature (Lengeler et al., 1999). It is possible that competition for carbon sources once occurred in the CMN. Other components of creosote, e.g. alkanes or simple aromatics, and natural organic matter in the soil would have been degraded in preference to PAHs. Once their supply had diminished, competition may have ensued. Theoretically, there is still a sufficiently high concentration of “bioavailable” carbon, including low molecular weight PAHs, in the CMN soil to support the growth of many species without inducing competition. It seems more likely that this overabundance of carbon itself could be a source of stress to the community by causing differential and unbalanced growth rates of certain groups to the detriment of others (Stephen et al., 1999).

The effects of PAH toxicity to indigenous microbes are probably manifested early after an exposure and may result in significant damage to or complete loss of certain populations. Because of community interdependency, it is likely that other populations

suffer as a result. Toxic effects are also concentration dependent. At low or intermediate levels, PAHs may stimulate the growth or activity of microorganisms in soil (Langworthy et al., 2002; Johnsen et al., 2002). However, given the high levels of PAH in the CMN, it is not unreasonable to assume that toxicity played a large role in affecting diversity (DelPanno et al., 2005).

Apart from toxicity, the most likely reason for the dramatic shift in community structure is the selection of specific groups that are capable of metabolizing PAHs. Low substrate specificity and functional complexity may provide advantages to certain organisms. Over time, a community may become adapted to an environmental insult, especially a contaminant. Organisms may be selected that already possess the metabolic capabilities to deal with the compound or that can acquire this capability, for example through horizontal gene transfer. Repeated exposure to a contaminant results in quick enzyme induction, if the required enzymes are not constitutively expressed.

The organisms that have survived in the CMN soil fall within the Proteobacteria. This phylum is the largest and phenotypically most diverse of all Bacteria, with widely varied habitats and means of energy acquisition (Kersters et al., 2005). Members of the

-subclass include the Sphingomonas species, some of which are known PAH-degraders. Burkholderia species, members of the -subclass, are ubiquitous in nature, and some members are also known aromatic degraders. The -subclass is the largest within the Proteobacteria and comprises the Pseudomonas species. This genus is well known for its “metabolic diversity and genetic plasticity” (Moore et al., 2005). (The - and - subclasses are now considered one broad complex called the Chromatibacteria, which may explain the difficulty in assigning PMN95 to a class).

One reason for the selection of Proteobacteria is the phenotypic diversity exhibited by this group. If a community loses member populations, it can benefit from the

functional redundancy of the remaining members. The greater the functional complexity of a community, the more resilient it is in response to disturbance (Tilman and Knops, 1997).

Microbial community succession in soil proceeds from r-selected to K-selected organisms (Garland et al., 2001). r-Strategists are opportunists with a broad niche width, who expend their energy on rapid reproduction during times of high resource availability. K-strategists are considered equilibrium organisms, or those who predominate when resources are limited and who prefer to direct their energy toward maintenance by adaptation and niche specialization (Andrews, 1984, as cited in Atlas and Bartha, 1993).

This theory may help to partially explain the resulting community in the CMN soil. Many Proteobacteria, especially members of the -subgroup, such as Pseudomonas sp., are considered to be r-strategists. Gram-positives, such as the Mycobacteria, are

theorized to be the K-strategists. The presence of only r-organisms and the complete absence of K-organisms in the CMN soil suggest that K-organisms were eliminated in the past due to periodic desorption, or “flushes”, of PAH from the soil matrix into the

microenvironment. It is assumed that these “flushes” continue today. Otherwise, one might expect to find Mycobacteria dominating in this soil as in other aged, PAH- contaminated soils (Leys et al., 2005; Uyttebroek et al., 2006a; Johnsen et al., 2007).