The contemplation of the inherent capabilities of microorganisms to promote the biodegradation and eventually the utilization of polyethylene (PE) as carbon source seems to
be more controversial nowadays that in fairly recent past years. Conflicting results have been indeed obtained that are suggesting from one side the ability of specific microbial strains to degrade high molecular weight PE, whilst many studies are yet claiming the inertness to biological attack of this polyolefin and structurally alike polymers also when exposed to very stressing and active microbial environments such as the composting windrows. Degradation of PE by microorganisms has been generally monitored in terms of microbial growth, biometry (e.g. biochemical oxygen demand or carbon dioxide emission, including 14CO2 generation from radio-labelled samples), sample weight loss, mechanical properties, variation of structural features (e.g. molecular weight, spectroscopic features, mechanical properties).
Preliminary investigations are dating back to the early 1960s. At that time, the increase of microbial cell numbers was taken as an indication of PE assimilation by bacteria in comparison with low molar mass paraffins (Jen-hou and Schwartz 1961). From these studies it was suggested that several bacteria can utilize as carbon source low molecular mass PE fractions slightly below 5000, whereas no microbial activity was detected on higher Mw fractions. Potts and co-workers assessed that linear paraffin with Mw below 500 were utilized by different microorganisms (Potts 1972). Similar results were obtained later by Albertsson and Banhidi that recorded the utilization of short oligomeric fraction of HDPE by microorganisms after 2 years biodegradation experiments (Albertsson and Banhidi 1980). Indeed, it can be suggested from a theoretical point of view that since PE is a nominally straight-chain hydrocarbon, it should be metabolized according to the biochemical pathway holding true for linear alkanes (Fig.1.14). On the other hand, it has been established that there is a molecular weight upper limit for the utilization of n-alkanes as a carbon source by microorganisms. Haines and Alexander established that linear hydrocarbons with more than 44 carbon atoms (tetratetracontane) cannot be metabolized by soil micro-organisms ( Haines and Alexander 1974). More recently in a study carried out by using single bacterial strains (Heat et al. 1997), this dimensional limit has been extended to 720 Dalton corresponding to an hydrocarbon chain constituted by 60 carbon atoms. In any case these limits are thought to be related to the bacterial metabolism of n-alkanes that need the accessibility to methyl chain ends by extra cellular oxidizing enzymes to start the biodegradation process.
RCH3 RCH2OH RCHO RCOOH
H3CRCH3 H3CRCOOH HOCH2RCOOH OCHRCOOH HOOCRCOOH
RCH2CH2CH3 RCH2CH(OH)CH3 RCH2COCH3 RCH2OC(O)CH3 RCH2OH + CH3COOH
RCH3 RCH2OOH RCHO
RCH2OH
RCOOH
RCOOCR Terminal oxidation (Pseudomonas)
Diterminal oxidation (Candida)
Subterminal oxidation (Nocardia)
Finnerty's pathway (Acinetobacter)
The first step is known as hydroxylation ( -oxidation) which give rise to the corresponding primary alcohols which are further enzymatically oxidized to aldehyde and hence to carboxylic acids. The resulting carboxylated n-alkanes can be metabolized according to the β-oxidation process in analogy with the catabolism of fatty acids. Thus, the rate and eventually the ultimate extent of biodegradation of solid n-alkanes can be strongly affected by the availability of CH3 chain ends susceptible to enzymatic oxidation. It follows that the number of chain ends present at the surface of a solid n-alkane decreases with the molecular weight increase and hence with extremely low values in the case of high molecular weight polyethylene.
On the other hand, it can not be at least hypothetically ruled out that oxidizing biological processes other than the -oxidation of terminal methyl groups such as the random chain cleavage as mediated by dehydrogenation/oxidation leading to carbonyl groups might play an important role in the biodegradation of PE.
Taking into account these suggestions Kawai and her collaborators tried to establish a numerical simulation model for the biodegradability of PE starting from experimental data relevant to the biodegradation of PE-wax having 2900 Mw and 1100 Mn, respectively (Kawai et al. 2002; Kawai et al. 2004). The computational model for the numerical simulation was set up regarding to two main factors:
i) sample weight loss due to -oxidation;
ii) fast consumptions of low molecular weight fractions. Both factors were substantiated by experimental data.
In particular the increase of both Mw and Mn of PE wax was observed after cultivation as an indication of the microbial consumption, in the meanwhile the time profile of molecular weight variation suggested the fast assimilation of the lower Mw fractions, as well as the gradual decrease of assimilation rates with increase in molecular weight. By applying the numerical model, which was validated by experimental results, to the biodegradation of PE, the authors provide suggestions supporting the terminal oxidation and -oxidation as the main processes involved in the microbial degradation of PE. In addition, by using this approach it was possible to distinguish between the process of biodegradation as mediated by bacteria such as Sphingomonads or Aspergillus sp. and Penicillium sp. fungi. It was therefore evidenced that bacteria exhibited higher biodegradation rates that were attributed to the higher affinity toward PE of Gram-negative bacteria cell walls with respect to the more hydrophilic chitin walls of fungi (Kawai et al. 2004). Sphingomonas species were thought to metabolize PE wax, preferentially with Mw below 2000, throughout primary terminal oxidation followed by -oxidation with enzymatic systems located in the cell membrane fraction, thus suggesting for the transport of oxidized PE wax into the periplasmic space through outer membranes (Kawai 1999).
Regarding the capabilities of single microbial species to attack PE as carbon substrate, only a few reports are so far available. Nevertheless, in the recent years evidences in the occurrence of soil microorganisms directly involved in the biodegradation processes of PE have been reported by Ohtake and collaborators (Ohtake et al. 1998; Watanabe et al. 2009).