| Prevention and treatment of mold in library collections with an emphasis on tropical climates: A RAMP study |
|2.2 Environmental and nutritional factors in growth and survival|
Most of the information available on the growth and development of mold is derived from laboratory cultures rather than on site studies. This information is therefore not always relevant to the growth and development of the same organism in the library environment. It is however, accurate to say that three factors are essential for the growth and survival of molds: the correct temperature, adequate moisture, and proper nutrients. St. George9 notes that it is a common misconception that light is required for mold growth. Unlike most plants, virtually all molds lack chlorophyl and therefore, light plays no role in their development. Colonies thrive in the dark, since for some varieties, exposure to ultra-violet light is injurious or lethal.10
There are three critical temperatures for mold, the temperature below which no growth occurs, the temperature above which no growth occurs, and the temperature at which most rapid growth takes place. Most microbial forms grow in temperatures ranging from 59º to 95º F (15º to 35° C), although there are forms which will grow at almost freezing and others which thrive at over 150º F. The average optimum for mold growth is usually stated to be in the vicinity of 86° F. The optimum temperature for the growth of specific molds is difficult to determine, in part because of variables in other environmental conditions, and in part because the culturing of organisms in the laboratory is a very different matter than the growth of the same organism in more natural surroundings.
It should be noted that the temperature below which no growth occurs is not synonymous with the temperature at which the potential for growth is destroyed. Many molds can survive periods of several months at sub-zero temperatures, but are less tolerant of alternating below-freezing and above-freezing temperatures.11
Sykes, speaking of bacteria, says:
Refrigeration at low temperatures...is popularly considered to be fatal to all forms of life. Whilst this may be true for the larger forms of organized life, it is certainly not true for the smaller plant life, including micro-organisms....sometimes the death rate is as high as 99% but once frozen at a sufficiently low temperature the surviving cells can be preserved for long periods.12
Given the existence of the "hold-over" spores, this undoubtedly applies to molds as well.
The amount of moisture required for mold development is seldom addressed in the microbiological literature. In the laboratory molds are cultured in media with a high moisture content, but the precise level is seldom mentioned in their reports. The covered petrie dish creates a microclimate where the mold can flourish undisturbed. With regard to the growth of mold outside the laboratory, sources do indicate that the hygoscopic nature of materials affects the growth of mold. Materials which absorb and hold moisture from the air require lower levels of ambient relative humidity than do less hygroscopic materials. Thus, in a non-laboratory environment, the mold has at its disposal two sources of moisture, the air surrounding the item and the moisture held by the item itself.
The elements required for the growth of fungi include carbon, hydrogen, oxygen, nitrogen, sulfur, potassium, and magnesium. Trace elements such as iron, zinc, copper, manganese, and in some cases, calcium may also be required. Certain of the vitamins are also needed. Most naturally occuring compounds can be utilized by fungi as sources of carbon and energy. Cellulose provides many of these elements, as do animal and vegetable fats and their component acids and glycerine.13