6 Wood Discoloration

3.1 Nutrients 55
conditions in the soil, in lakes, and marine environments, mixed bacterial
populations of the erosion, cavitation and tunneling bacteria can degrade
wood (Schmidt and Liese 1994; Daniel and Nilsson 1998; Kim and Singh 2000).
Even a bacterial pure culture attacked woody cell walls (Schmidt et al. 1995)
(Fig. 5.3c).
Whereas the fungal cell wall with openings up to 10 nm hardly limits the
uptake of water and small molecules, the plasma membrane is a selectively
permeable barrier for the uptake and secretion of solutes. Water, non-polar
and small uncharged polar molecules, like glycerol and CO2, can diffuse freely.
Larger polar molecules and ions pass the membrane by means of diffusion
or active transport (Rayner and Boddy 1988; Jennings and Lysek 1999). The
uptake occurs mainly at the hyphal tips (Figs. 2.3, 2.4). Three main classes
of nutrient uptake and transport occur in fungi, facilitated diffusion, active
transport, and ion channels (Robson 1999). A constitutive low affinity transport
system of facilitated diffusion allows the energy-independent accumulation of
solutes like sugars and amino acids when present at a high concentration
outside of the hypha, but not against a concentration gradient. When the
solute concentration is low, carrier proteins are induced that have a higher
affinity for the solute and mediate the energy-dependent uptake of solutes
against a concentration gradient at the expense of ATP. During this process,
fungi create an electrochemical proton gradient by pumping out hydrogen ions
from the hyphae at the expense of ATP via proton pumping ATPases in the
plasma membrane. The proton gradient provides the electrochemical gradient
that drives nutrient uptake as hydrogen ions flow back down the gradient.
A number of ion channels that are highly regulated pores in the membrane
and allow influx of specific ions into the cell when open have been found
in fungi. Ca 2+ stimulated K + channels carry an inward flux of K + ions and
are thought to be involved in regulating the turgor pressure of the hypha.
A mechanosensitive or stretch-activated Ca 2+ channel is opened when the
membrane is under mechanical stress like during the generation of the high
calcium gradient at the hyphal tip.
During early growth, nutrients surrounding the young mycelium are in excess.
As the mycelium develops further, nutrients in the center are increasingly
utilized, nutrient depletion and accumulation of metabolic products occur
beneath the colony center. Therefore, growth becomes restricted to the periphery.
Different parts of the colony are at different physiological ages, with
the youngest hyphae at the edge of the colony and the oldest, non-growing
mycelium at the center (Robson 1999).
The movement of the nutrient over short distances from a food source on
to the regions devoid of the nutrient or nutrients required for growth can
occur by diffusion within the aqueous phase of the cytoplasm (Jennings and
Lysek 1999). As mycelial extension proceeds, nutrients are shifted from the
site of absorption to another part of the mycelium by translocation (Jennings
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