9. Microbial Growth and Biosynthesis
9.6 Temperature and Microbial Growth
Learning Objectives
- Illustrate and briefly describe minimum, optimum, and maximum temperature requirements for growth
- Identify and describe different categories of microbes with temperature requirements for growth: psychrophile, psychrotrophs, mesophile, thermophile, hyperthermophile
- Give examples of microorganisms in each category of temperature tolerance
When the exploration of Lake Whillans started in Antarctica, researchers did not expect to find much life. Constant subzero temperatures and lack of obvious sources of nutrients did not seem to be conditions that would support a thriving ecosystem. To their surprise, the samples retrieved from the lake showed abundant microbial life. In a different but equally harsh setting, bacteria and archaea grow at the bottom of the ocean around deep sea vents (Figure 9.38), where temperatures can reach 340 °C (700 °F). In fact, some of those bacteria and archaea are the primary producers of the vent ecosystem, providing fixed carbon for the other organisms.
Unlike other environmental conditions such as pH or osmolarity, microbes have no way to regulate their temperature: their internal temperature matches that of their environment. Changes in temperature have the biggest effect on enzymes and their activity, with an optimal temperature that leads to the fastest metabolism and resulting growth rate. Temperatures below optimal will lead to a decrease in enzyme activity and slower metabolism, while higher temperatures can actually denature proteins such as enzymes and carrier proteins, leading to cell death. As a result, microbes have a growth curve in relation to temperature with an optimal temperature at which growth rate peaks, as well as minimum and maximum temperatures where growth continues but is not as robust. For a bacterium, the growth range is typically around 30 degrees (Figure 9.39).
The upward trend of growth rates between psychrophiles and hyperthermophiles is attributed to the relationship between thermal energy and the rate of chemical reactions and has been termed the “Arrhenius effect”. This is a reference to the Arrhenius equation that describes the relationship between temperature and the velocity of a chemical reaction.
Mesophiles
Organisms categorized as mesophiles (“middle loving”) are adapted to moderate temperatures, with optimal growth temperatures ranging from room temperature (about 20 °C) to about 45 °C. As would be expected from the core temperature of the human body, 37 °C (98.6 °F), normal human microbiota and pathogens (e.g., E. coli, Salmonella spp., and Lactobacillus spp.) are mesophiles. The mesophiles occupy the same environments that humans do, in terms of foods that we eat, surfaces that we touch, and water that we swim in and drink.
Psychrophiles
The psychrophiles are the cold lovers, with an optimum of 15oC or lower and a growth range of -20oC to 20oC. Most of these microbes are found in