Fungal growth in both modern and historic buildings is attributed to environmental conditions such as water, humidity, temperature and lack of ventilation. Often, people will ask the laboratory if it is possible to tell how old the mold is and/or whether the mold was actively growing and what the source of the mold was. The simple answer to these questions is that currently it is not possible to say how old the mold is or the source of the mold. Although the lab may speculate that the mold was actively growing if mycelia/hyphae appear young, strictly speaking, it is difficult to tell whether the mold was actively growing by observing it under the microscope. In this article we shall discuss in details the phases of fungal growth and succession so that the reader can understand why it may be difficult to give definite answers to these questions.
Fungal growth succession
Before we discuss fungal growth phases let’s see how different groups of fungi interact in nature. Viable fungal spores are ubiquitous in indoor environments and are well adapted to inhabit this ecological niche if just sufficient water is available. Based on the water activity (or equilibrium relative humidity) requirements, fungi can be divided into 3 broad groups:
Primary colonizers (also referred to as xerophiles) are capable of growth in relatively dry conditions at water activities (aw) of between 0.6-0.8. These kind of conditions can be achieved through slow leaking pipes, condensation or other sources of moisture. At this low water activity viable propagules (spores and mycelial fragments) of species of Wallemia, Penicillium, Aspergillus and Eurotium would germinate and form growth (see the photo to the right).
As the water activity of the building material increases to between 0.8 and 0.9, the secondary colonizers would proliferate. Secondary colonizers include species of Cladosporium, Phoma, Ulocladium, Alternaria and also some species of Aspergillus such as A. flavus and A. versicolor. If nutrients are still available on the substrate and the water activity remains unchanged the secondary colonizers would gradually become the dominant fungi.
At water activity of 0.9 and above, the “water-loving” fungi (also called hydrophiles) would dominate as the tertiary colonizers. These levels of water activity are generally only met by incoming water as under flooding conditions and not just high humidity or condensation on indoor surfaces. Immediately after the flooding or serious water leakage, for example, all types of molds would colonize the wet building material but with those that require higher water activity dominating. Presence of tertiary colonizers in a building is an indication of a serious water problem. Tertiary colonizers include molds such as Stachybotrys, Chaetomium, Ulocladium, Trichoderma, Aureobasidium as well as actinomycetes and other bacteria.
This grouping can also be based on nutritional requirements of the fungi. For example, primary colonizers prefer simple sugars while tertiary colonizers prefer complex sugars such as cellulose.
Fungal growth may be restricted or unrestricted. Unrestricted fungal growth occurs when the substrate contains an excess of all growth factors. During unrestricted growth, the total hyphal length and the number of tips of a mycelium increases indefinitely. In nature unrestricted growth may only be possible within a short time due to unfavourable growth conditions. Restricted growth occurs when not all nutrients are present in excess or when conditions such as nutrient concentration, pH or mycelial morphology are changed sufficiently to affect the maximum growth rate. Growth of a fungus on a solid substrate for example a drywall, eventually results in conditions such as nutrient depletion, change in pH at the centre of the colony which are less favourable for growth than was initially the case. If there is no competition with other micro-organisms, growth of the hyphae on the peripheral ring occurs at approximately maximum growth rate but growth proceeds at below maximum growth rate elsewhere in the colony, often falling to zero or near zero at the colony centre, i.e., some parts of the fungus would be actively growing while others would not be growing at all or would be dead.
Fungal growth phases
From the time a spore or a hyphal fragment germinates to form a colony to the time the fungus dies, there are a number of growth phases. Although these phases have been determined under laboratory conditions, it is possible that the same occur in nature. In nature the duration of each phase would be determined by the environmental conditions including other competing micro-organisms.
- Lag phase
Once the growth conditions become favourable for the fungal propagules (i.e., viable spores or mycelial fragments) to germinate, new transport systems must be induced before growth commences. Thus growth starts slowly and accelerates gradually. This phase is referred to as the lag phase.
- Exponential or log phase
Exponential growth occurs only for a brief period as hyphae branches are initiated, and then the new hypha extends at a linear rate into un-colonized regions of substrate. The biomass of the growing fungus doubles per unit time. As long as the nutrients are in excess growth remains constant during the exponential phase.
- Stationary phase
As soon as the nutrients are depleted or toxic metabolites are produced growth slows down or is completely stopped. The biomass increases gradually or remains constant. During the stationary phase, hyphal growth stops and, in some molds, cell differentiation occurs, resulting in spore formation. During this process nutrients are transferred from the vegetative mycelium to the developing spores. The spores are dispersed by air movement to other areas of the building where they can start new mold growth once the conditions for growth are favourable.
- The death phase
During the death phase, the mycelium eventually dies off. The death phase is usually accompanied by breakdown of the mycelia through self-digestion. Some fungi form spores by fragmentation of the hyphae.
As noted earlier different sections of the same fungus are at different phases of growth.
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