pHtolerance

pHtolerance describes the capacity of an organism or system to withstand environmental pH levels outside its optimum. It is typically defined as the range of pH values over which survival, growth, or metabolic activity is maintained, often reported as minimum and maximum pH.

Measurement and context: The observed pHtolerance depends on many factors, including temperature, ionic strength, nutrient availability, and the specific activity being measured (growth, enzyme activity, or viability). Consequently, pHtolerance is often distinct from the pH optimum, which is the pH at which performance is highest. Different assays or conditions can yield different tolerance ranges for the same organism.

Biological mechanisms: Microbes and plants maintain pH by modulating proton transport across membranes (for example via H+-ATPases and antiporters), altering membrane composition, synthesizing buffering compounds, and adjusting metabolism. In microbes, acid tolerance and alkaline tolerance are well-described strategies; in plants, pH tolerance is influenced by soil chemistry, root uptake transporters, and organic acid exudation. Adaptations may involve changes to enzyme stability, membrane integrity, and cellular homeostasis.

Examples: Acid-tolerant bacteria used in dairy fermentation can operate at relatively low pH, while alkaliphiles thrive at high pH reported in some soda lakes. Many crop species have limited pHtolerance linked to soil pH, though some tolerate broader ranges due to root and rhizosphere adaptations.

Applications: Understanding pHtolerance informs strain selection for food and industrial fermentation, crop management and breeding for soil conditions, wastewater treatment, and ecological risk assessments of environmental pH changes due to pollution or acid rain.