Class notes
Fungal Growth
- Institution
- University Of KwaZulu-Natal (UKZN)
Understanding concepts of fungal growth.
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Fungal GrowthGrowth of individual hyphae can be measured microscopically on solid
medium as an increase in hyphal length. Hyphae are usually grown on the
surface of a cellophane membrane, overlaying the agar, to maintain
hyphae in a single plane of focus. Changes in total hyphal length can also be
measured and used to estimate biomass formation if biomass density and
hyphal diameter are assumed to be constant.
Fungal growth is typically in response to nutrient availability as well as other
environmental factors such as temperature, pH, moisture etc.
Typically, a single unit of fungus will grow rapidly in the beginning. The
exponential phase will be followed by a plateau, and then a decline phase
as the organic nutrients become depleted.
These stages are typical of any organism growing in fixed quantities of
nutrients, particularly under laboratory conditions.
The graph above shows the growth phases of fungi.
On agar, initial hyphal growth results in undifferentiated mycelium. The
hyphae at the margin are exposed to abundant fresh organic nutrients while
the hyphae at the center to much less.
Eventually, hyphae at the center become a sink for nutrients from the
periphery and have to respond to the less suitable environment.
, 4 Zones can be differentiated:
- Outer zone (extension zone) is extending into fresh nutrients.
Hyphae adopt a pattern which appears to be very efficient at
extracting nutrients, but the mechanism of control is unclear. In some
cases, the angle of branches is acute.
Fungal growth is isotropic (growth proceeds evenly in all directions).
- Behind the leading edge is the productive zone, where the majority of
increase in biomass takes place. Hyphal branching is less acute, and
hyphae are more likely to be upright or submerged than flat on the
agar surface.
- Fruiting (fruiting zone) follows the storage period. Fruiting is commonly a
stationary phase, and nutrients are redistributed within the mycelium
from storage in the spores.
- The fourth stage is the aged zone. Hyphae are commonly vacuolate or
empty and hyphal walls are degenerating (autolysis).
Ideally, the fungus disappears from the aged zone, but in practice, some
evidence of their presence usually remains.
Fungi are heterotrophs. They obtain their nutrients from their surrounds since
they cannot make their own.
They do this either as decomposers (obtains nutrients from dead organisms)
or as parasites (obtains nutrients from other living organisms).
Carbon metabolism
- Metabolizes by the processes of glycolysis and the carboxylic acid
cycle.
- Fungi are usually aerobic however, some species (e.g. yeasts), are
capable of living in low oxygen-tension environments and utilizing
fermentative pathways of metabolism.
- Recently, truly anaerobic fungi have been discovered within animal
rumen and in anaerobic sewage-sludge digesters.
Water, pH and temperature
- Fungi require water for nutrient uptake and are therefore restricted to
damp environments.
- They tend to occupy acidic environments between pH 4 to 6, and by
their activity further acidify it.
- Most fungi are mesophilic, growing around 20 to 40 ℃. Some are
psychrophilic (can grow under 5 ℃) while others are thermophilic (can
grow at over 50 ℃).
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