pit membranes such as bordered pits and ray
parenchyma pits. In these fungi, bore holes are
also typically apparent as the fungal hyphae
have the capability of boring transversely into
the wood cell walls to pass from one fiber/tracheid to another. The hyphae must ramify
across the surface of the wood in the case of
molds, or on the surface and throughout interior cells of wood in the case of stain and decay
fungi, in order to obtain nutrients, which are
required for fungal survival. Extracellular
enzymes, and low molecular weight (LMW)
fungal metabolites, are secreted by the fungal
hyphae to solubilize compounds ranging from
simple oligosaccharides to polymerized lignin.
The goal of most fungi is to obtain sugars or
short-chain oligosaccharides that can be
absorbed or actively transported into the fungal
thallus. Degraded and modified fragments of
lignin can also be metabolized by some fungi,
and the hyphae of these types of fungi again are
responsible for secretion of metabolites that
can depolymerize lignin. But lignin typically is
either segregated from cellulose via a biochemical mechanism (brown rots) or is metabolized
(white rots) primarily as a means to access the
carbohydrate fractions of wood by the fungi.
Fungal Decay and Moisture Content
All fungi require moisture to grow in or on the
surface of wood. Generally, the moisture content (MC) required for decay fungi to grow into
wood and secrete metabolites into the wood cell
wall must be above the fiber saturation point
(Zabel and Morrell 2020). This is because
enzymes and other degradative components of
the fungal LMW metabolites involved in fungal
decay processes must be able to diffuse from
the fungus through an extracellular fungal
matrix (ECM) or biofilm layer which surrounds
all fungal hyphae. The ECM is the gateway
through which all extracellular metabolites
must pass to then further diffuse into the
wood cell wall. Liquid water must therefore be
present in the lumen of the wood cells for this
type of diffusion to occur at the surface of the
fungal hyphae. The wood cell wall must also be
at, or near, fiber saturation to allow LMW fungal metabolites and ions to diffuse within the
wall as a prerequisite for some types of decay
initiation. Mold fungi only require moisture to
be present on the surface of wood and other
substrates, to grow. Many architectural manuals indicate that mold will grow on wood as
low as 20% MC. Technically, mold growth is
unlikely to occur at less than 30% MC (the fiber
saturation point) because, as with all degradative fungi, some amount of liquid water is
needed for extracellular enzyme secretion.
Practically speaking however, the 20% MC figure is useful because, particularly when the
wood MC is in equilibrium with moisture in
the air in an enclosed or poorly ventilated
space, then liquid water can condense out in
an enclosed or poorly ventilated area. Depending on temperature, the relative humidity (RH)
required to maintain 20% MC in wood is close
to 90% RH. Over a broad range of temperature
ranging from 3 to 32
C a decrease in temperature of only ~ 2
C will allow the dew point to be
reached, and condensation of liquid water at
the surface of wood under such conditions can
then occur. Although small amounts of surface
water on wood would not allow decay in any
form to advance, some molds and even surface
stains could establish growth, as staining and
mold can develop in a matter of a few days at
moderate temperatures above approximately
10
C.
As a caveat to the discussion on MC limitations for fungal growth, very wet wood (from
150 to 200+ % MC) will also limit or stop fungal
growth, and most decay fungi have a MC optimum for decay in the range between 50 and
100% MC (Fig. 15.2) (Zabel and Morrell 2020).
MC optima exist for other fungal species as
well. For many soft rot fungi, MC optima for
decay have been reported to be much higher,
“near saturation”; however, aeration of wood
samples undergoing soft rot attack has also
been assessed to be critical for optimal soft rot
decay. This suggests that some soft rot fungi
may be able to survive and thrive under conditions where they can pull dissolved oxygen
from water. Relative to MC optima for wood
decay fungi in different density wood species, it
has been observed that lower density woods will
continue to decay at high MC levels that stop
decay in high-density species. This is because
lower-density wood species have proportion15 Fungi Involved in the Biodeterioration and Bioconversion of Lignocellulose Substrates
371
parenchyma pits. In these fungi, bore holes are
also typically apparent as the fungal hyphae
have the capability of boring transversely into
the wood cell walls to pass from one fiber/tracheid to another. The hyphae must ramify
across the surface of the wood in the case of
molds, or on the surface and throughout interior cells of wood in the case of stain and decay
fungi, in order to obtain nutrients, which are
required for fungal survival. Extracellular
enzymes, and low molecular weight (LMW)
fungal metabolites, are secreted by the fungal
hyphae to solubilize compounds ranging from
simple oligosaccharides to polymerized lignin.
The goal of most fungi is to obtain sugars or
short-chain oligosaccharides that can be
absorbed or actively transported into the fungal
thallus. Degraded and modified fragments of
lignin can also be metabolized by some fungi,
and the hyphae of these types of fungi again are
responsible for secretion of metabolites that
can depolymerize lignin. But lignin typically is
either segregated from cellulose via a biochemical mechanism (brown rots) or is metabolized
(white rots) primarily as a means to access the
carbohydrate fractions of wood by the fungi.
Fungal Decay and Moisture Content
All fungi require moisture to grow in or on the
surface of wood. Generally, the moisture content (MC) required for decay fungi to grow into
wood and secrete metabolites into the wood cell
wall must be above the fiber saturation point
(Zabel and Morrell 2020). This is because
enzymes and other degradative components of
the fungal LMW metabolites involved in fungal
decay processes must be able to diffuse from
the fungus through an extracellular fungal
matrix (ECM) or biofilm layer which surrounds
all fungal hyphae. The ECM is the gateway
through which all extracellular metabolites
must pass to then further diffuse into the
wood cell wall. Liquid water must therefore be
present in the lumen of the wood cells for this
type of diffusion to occur at the surface of the
fungal hyphae. The wood cell wall must also be
at, or near, fiber saturation to allow LMW fungal metabolites and ions to diffuse within the
wall as a prerequisite for some types of decay
initiation. Mold fungi only require moisture to
be present on the surface of wood and other
substrates, to grow. Many architectural manuals indicate that mold will grow on wood as
low as 20% MC. Technically, mold growth is
unlikely to occur at less than 30% MC (the fiber
saturation point) because, as with all degradative fungi, some amount of liquid water is
needed for extracellular enzyme secretion.
Practically speaking however, the 20% MC figure is useful because, particularly when the
wood MC is in equilibrium with moisture in
the air in an enclosed or poorly ventilated
space, then liquid water can condense out in
an enclosed or poorly ventilated area. Depending on temperature, the relative humidity (RH)
required to maintain 20% MC in wood is close
to 90% RH. Over a broad range of temperature
ranging from 3 to 32
C a decrease in temperature of only ~ 2
C will allow the dew point to be
reached, and condensation of liquid water at
the surface of wood under such conditions can
then occur. Although small amounts of surface
water on wood would not allow decay in any
form to advance, some molds and even surface
stains could establish growth, as staining and
mold can develop in a matter of a few days at
moderate temperatures above approximately
10
C.
As a caveat to the discussion on MC limitations for fungal growth, very wet wood (from
150 to 200+ % MC) will also limit or stop fungal
growth, and most decay fungi have a MC optimum for decay in the range between 50 and
100% MC (Fig. 15.2) (Zabel and Morrell 2020).
MC optima exist for other fungal species as
well. For many soft rot fungi, MC optima for
decay have been reported to be much higher,
“near saturation”; however, aeration of wood
samples undergoing soft rot attack has also
been assessed to be critical for optimal soft rot
decay. This suggests that some soft rot fungi
may be able to survive and thrive under conditions where they can pull dissolved oxygen
from water. Relative to MC optima for wood
decay fungi in different density wood species, it
has been observed that lower density woods will
continue to decay at high MC levels that stop
decay in high-density species. This is because
lower-density wood species have proportion15 Fungi Involved in the Biodeterioration and Bioconversion of Lignocellulose Substrates
371