Thermal classes of microbes Organisms that grow best at temperatures above 45°C are referred to as:

Introduction / Context:
Temperature tolerance categories help predict microbial habitats and guide industrial bioprocesses. Recognizing which organisms thrive at elevated temperatures supports bioreactor design, enzyme selection, and food sterilization strategies.

Given Data / Assumptions:

• >45°C indicates a heat-loving preference.
• Common categories: psychrophiles, mesophiles, thermophiles, hyperthermophiles.
• Terminology is standardized in microbiology texts.

Concept / Approach:
Thermophiles prefer growth above ~45°C, often displaying optima between 50°C and 65°C. Hyperthermophiles extend this range, thriving at ≥80°C, while mesophiles grow best at moderate temperatures (about 25–40°C) and psychrophiles at cold temperatures (≤20°C). Therefore, the appropriate label for organisms growing best above 45°C is thermophiles.

Step-by-Step Solution:
Identify threshold (>45°C) as the defining clue.Relate threshold to standard categories.Select 'Thermophiles' as the correct class.

Verification / Alternative check:
Examples include Thermus aquaticus (source of Taq polymerase) and certain Bacillus species adapted to hot springs or compost heaps.

Why Other Options Are Wrong:

• Psychrophiles: prefer cold, not >45°C.
• Mesophiles: prefer moderate temperatures.
• Any of these: incorrect—only thermophiles fit.
• Hyperthermophiles: typically require even higher temperatures (~80–110°C).

Common Pitfalls:
Confusing thermophiles with hyperthermophiles; overlooking that thermotolerant mesophiles are not the same as thermophiles.

Final Answer:
Thermophiles.