Biomedical Engineering Reference
In-Depth Information
enable complete decomposition of lignin to CO
2
or partially modify or degrade
lignin. In the past, fungus was regarded as the only microorganism that had the
ability to degrade lignin. Since the application of
14
C-labeled lignin and coniferyl
alcohol dehydrogenation polymer, many lignin-degradable microorganisms have
been found (actinomycetes and bacteria).
In nature, lignin is degraded completely by fungi, bacteria, and the corresponding
microbial communities. Once lignocellulosic materials are degraded by wood-rot
fungi when the land's humidity conditions are suitable, other microorganisms (e.g.,
bacteria) can further rapidly degrade lignin that has been decomposed to a certain
degree and aromatic compounds derived from the lignin degradation. Moreover, part
of decomposed lignin can bind with the amino of the amino acid, one of the micro-
bial metabolites, and form humic acid and humus, which are excellent fertilizers.
However, usually only white-rot fungi can completely decompose lignin into CO
2
and water. Generally, other rot bacteria and actinomycetes can only partially change
the structure of the lignin molecules [
14
], such as by demethylation. According to
different ways microbes decompose timber, lignin-degradable microorganisms can
be divided into soft-rot bacteria and fungi, stain fungus, brown-rot fungi, and white-
rot fungi.
3.1.3.1
Soft-Rot Microorganisms
Microorganisms that cause wood to soften and rot include bacteria, ascomycete
fungi, and imperfect fungi. This rot usually occurs in wet timbers. Soft-rot bacteria
are more likely to grow on hardwood than softwood, which may be because of
differences in the structure of lignin. Soft-rot fungi preferentially degrade the
polysaccharide of lignocellulosic materials; the metabolism of lignin is slow and
incomplete, which usually results in softening of the wood surface, hence its name,
and is considered to be involved in the formation of humus. The study of soft-rot
bacteria is usually from the perspective of plant pathology [
15
]. Soft-rot fungi are
mostly ascomycetes and adelomycete; for example, some strains of
Papulospora
spp.,
Graphium
spp.,
Thielavia
spp., and
Paecilomyces
spp. can decompose lignin.
Chaetomium
spp. genus also has slight lignin-decomposing ability.
3.1.3.2
Stain Fungus
One category of staining is caused by fungi. The mycelium deeply penetrates
into the parenchyma of wood and directly stains the wood. Ascomycetes and
adelomycete, such as
Ceratostomella ips
Rumbold and
Endoconidiophora bunae,
cause wood staining. The other category of staining is caused by the growth of
fungi that only grow on the surface of the wood, such as
Mulor
spp.,
Rhizopus
spp.,
Penicillium
spp.,
Aspergillus
spp.,
Cladosporium
spp.,
Ascospore myxomycetes
,
Fusarium oxysporum
, and so on [
5
].
