DISCUSSION POINTS

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					Biofilms, consisting of both microorganisms and
their extracellular polysaccharides, are highly diverse and
variable in both space and time. They are common on all surfaces
in both terrestrial and aquatic environments (Caldwell
et al., 1997; Fletcher, 1996; Ford et al., 1991; Ford, 1993;
Geesey and White, 1990; Gehrke et al., 1998; Neu, 1996).

Because virtually all surfaces may act as substrate
for bacterial adhesion and bio6lm formation (Busscher et
al., 1990; Costerton et al., 1995; Geesey and White, 1990;
Geesey et al., 1996; Marshall, 1980), attack of materials by
microorganisms can take place either directly or indirectly,
depending on the specific microorganisms, chemical and
physical properties of the materials, and their environmental
conditions (Gu et al., 2000d).

Both metal and non-metallic materials immersed in
aqueous environments or under high humidity conditions
are equally susceptible to biofouling and biodeterioration
(Characklis, 1990; Gu and Mitchell, 1995; Gu et al.,
1998b, 2000d; Jones-Meehan et al., 1994a, b; Knyazev
et al., 1986; Little et al., 1990; Thorp et al., 1994). Speci
6c examples include medical implants (Dobbins et al.,
1989; Gu et al., 2001a–c; McLean et al., 1995; Mittelman,
1996), water pipes (Rogers et al., 1994), arti6cial coatings
(Edwards et al., 1994; Gu et al., 1998a; Jones-Meehan et
al., 1994b; Stern and Howard, 2000; Thorp et al., 1997),
rubber (Berekaa et al., 2000), ultrapure systems (Flemming
et al., 1994; Mittelman, 1995), porous media (Bouwer,
1992; Cunningham et al., 1990, 1991; Mills and Powelson,
1996; Rittman, 1993; Vandevivere, 1995; Vandevivere and
Kirchman, 1993; Williams and Fletcher, 1996),

Biodegradability of a Polymer

The biodegradability of any polymer depends on its molecular weight, crystallinity and physical forms
(Gu etal.,2000). Generally, higher molecular weight results in greater resistance to degradation by
microorganisms, whereas monomers, dimmers and oligomers of a polymer’s repeating units are degraded
and mineralized more easily. High molecular weights lead to sharply decreased solubility, making the
polymer resistant to microbial attack because microorganisms need to assimilate the substrate through
their cellular membrane and then degrade the substrate further by means of intracellular enzymes. At least
two categories of enzymes, namely extracellular and intracellular depolymerizers, are actively involved in
biodegradation of polymers(Doi, 1990; Gu etal.,2000). It is commonly recognized that the closer a
polymer’s structure to that of a natural molecule, the more easily it is degraded and mineralized (Gu and
Gu, 2005). Polymers such as cellulose,chitin and poly b-hydroxybutyrate (PHB) are all biologically
synthesized and can be completely and rapidly biodegraded by heterotrophic microorganisms in a wide
range of natural environments (Be´renger etal.,1985; Byrom, 1991; Frazer,1994; Gamerithetal.,1992;
Gujer and Zehnder,1983; GunjalaandSulflita,1993; Hamiltonetal.,1995; HespellandO’Bryan-Shah, 1988).
It is also known that polymers with carbonated linear chains, such as polyolefins, are unlikely to be
attacked by microorganisms. However some research has demonstrated that ligninolytic and cellulolytic
fungi can degrade oxidized polymeric chains (Lee etal.,1991; Manzuretal.,1997; Weilandetal.,1994).
Several attempts have been made to induce carbon-to- oxygen bonds into the polymeric chain, most of
them involving photo-and thermo-oxidative treatment of polyolefins, principally polyethylene (PE); a few
of the attempts report the degradation of the polymer after a long period of incubation with selected fungi
or in mature compost(ChielliniandCorti,2003; Chiellini et al.,2003; Manzuretal.,2004; Pandey and Singh,
2001; Volke-Sepulvedaetal.,2002).


The extracellular polymers produced by microorganisms can act as surfactants that facilitate the
exchanges between hydrophilic and hydrophobic phases. These interactions favor the penetration
rate of microbial species (Lucas, et al., 2008). Microbes can release active chemicals to facilitate
biodegradation and the resulting degradation products can further:

Concurrently, the pH inside pores is modified by the degradation products, which normally have
some acid–base characteristics (Göpferich, 1996; Lucas, et al., 2008). Organic acids are more
efficient than mineral acids to fix cations. They are considered as one of the main causes of
biodeterioration (Warscheid and Braams, 2000). Also, some microorganisms as filamentous
bacteria and fungi are able to use these organic acids as carbon sources to extend their mycelia
framework (cf. § physical way) (Hakkarainen et al., 2000).

POLYSTYRENE
Some materials considered as recalcitrant polymers (e.g. polyethylene, polystyrene and
polyurethane) are nevertheless subject to microbial biodeterioration (Shimao, 2001; Howard,
2002; Szostak-Kotowa, 2004; Shah et al., 2008). The microbial vulnerability of these polymers is
attributed to the biosynthesis of lipases, esterases, ureases and proteases (Flemming, 1998;
Lugauskas et al., 2003). Enzymes involved in biodeterioration require the presence of cofactors
(i.e. cations present into the material matrix and coenzymes synthesised by microorganisms) for
the breakdown of specific bonds (Pelmont, 2005). The biodeterioration of thermoplastic
polymers could proceed by two different mechanisms, i.e., bulk and surface erosion (von
Burkersroda et al., 2002; Pepic et al., 2008). In the case of bulk erosion, fragments are lost from
the entire polymer mass and the molecular weight changes due to bond cleavage. This lysis is
provoked by chemicals (e.g. H2O, acids, bases, transition metals and radicals) or by radiations
but not by enzymes. They are too large to penetrate throughout the matrix framework. While in
surface erosion, matter is lost but there is not change in the molecular weight of polymers of the
matrix. If the diffusion of chemicals throughout the material is faster than the cleavage of
polymer bonds, the polymer undergoes bulk erosion. If the cleavage of bonds is faster than the
diffusion of chemicals, the process occurs mainly at the surface of the matrix (von Burkersroda
et al., 2002; Pepic et al., 2008; Lucas, et al., 2008).
REASONS WHY DEGRADED SILA:

Recent studies show that atmospheric pollutants are potential sources of nutrients for some
microorganisms (Zanardini et al.,2000; Nuhoglu et al., 2006). Mitchell and Gu (2000) report the
deposition of sulphur dioxide, aliphatic and aromatic hydrocarbons from the urban air on several
polymer materials. These adsorbed pollutants may also favour the material colonisation by other
microbial species. Organic dyes are also potential nutrients for these microorganisms
(Tharanathan, 2003; Faÿ et al., 2007; Lucas, et al., 2008).


   Many fungi reproduce sexually only on occasions, such as the plant pathogens that undergo
    sexual reproduction once a year after death of their plant hosts.
   However it must be emphasized that the life cycles of many fungi are so poorly known that
    some connections between asexual and sexual morphs undoubtedly remain to be made.



From a preliminary ecology study on wood colonization and polyethylene colonization, it is
evident that these fungi possess some intriguing mechanisms which allow them to rapidly
acquire access to a resource and then to exploit it efficiently (Schoeman, 1996).


The discovered Xylaria sp. by Cuevas and Manaligod (1997) inhabited plastic along with litter
and organic soil. Xylariaceous fungi are often isolated as their anamorphs, like Nodulisporium
sp. from soil litter. Most of these xylariaceous litter inhabitants, which grow on litter and organic
soil, do not form the teleomorph in culture, thus, have not been identified (Petrini & Rogers,
1986; Rogers, 2000). Many sexually reproducing ascomycetes that we know well usually are
seen only as asexual fungi (Alexopoulos, et al., 1996).Thus, fungi cultured in the laboratory are
anamorphs, since fungi appear in the asexual stage most of the times, and only rarely in their
sexual/teleomorph stage. Hence, there is a consistency between species form as regards to its
natural occurrence and when grown in the laboratory. Anamorphs and teleomorphs are not only
different sexually, but most likely, physiologically as well. Then perhaps the teleomorph of
Xylaria sp., if further tested to reveal its presence or absence, would exhibit a different
biodegrading ability for the given pollutants.




Fungi reproduce both sexually and asexually, though not necessarily at the same time. Asexual
reproduction is more significant for the colonization of the species, because it occurs several
times during the season, thereby producing numerous individuals, while the sexual stage of many
fungi may occur only once a year. The fact that many species may be found in one stage than the
other (i.e. mostly asexual stage), greatly cause difficulties in identification of the species. And it
is often impossible to predict if the stage currently at hand would produce another stage. A more
common problem encountered is that of fungi with an asexual stage but no known sexual stage.
Also, many fungi have no distinctive asexual reproductive stage.



It was suggested that water conservation is the initial impetus for colonization of xylariaceous
fungi of various hosts and substrates. Thus, diverse lifestyles of Xylaria sp. have developed to
save water on occasionally dry sites. (Rogers, 1979; Rogers, 2000). The relationship developed
between perithecial stromata to substrate for Xylaria, Hypoxylon, Kretzschmaria, Podosordaria,
and others, is to have a massive stromata lacking in conspicuous gelatinous tissue which only
develop superficially on the substrate (Rogers, 2000).

Most xylariaceous fungi that have been assessed have a strong capacity to degrade cellulose and
lignin, causing physiological white rots, and the most efficient of them rival basidiomycetes in
substrate degradation (Nilsson et al., 1989; Rogers, 2000).

				
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