Extremophiles – hyperthermophiles: A microbe that reliably grows at temperatures above 95°C is most likely to be classified as which type of organism?

Introduction / Context:
Life at near-boiling and even superheated temperatures is a hallmark of hyperthermophiles, which revolutionized our understanding of the limits of life and fueled advances in biotechnology (e.g., thermostable enzymes). Identifying which domains harbor such organisms is central to extremophile biology.

Given Data / Assumptions:

• Growth above 95°C indicates extraordinary thermal tolerance.
• Most known hyperthermophiles inhabit hydrothermal vents or hot springs.
• Eukaryotes (fungi, protozoa) generally cannot grow at these extremes.

Concept / Approach:
Hyperthermophiles predominantly belong to Archaea (e.g., Pyrolobus, Thermococcus, Sulfolobus), with membrane lipids, protein folding, and DNA repair systems adapted to extreme heat. Bacteria include some thermophiles, but the most extreme growth temperatures are characteristic of Archaea. Eukaryotic microbes rarely exceed ~60°C for growth.

Step-by-Step Solution:
Match the temperature threshold (>95°C) with known domains of life. Recognize Archaea as the domain containing the highest-temperature growers. Exclude fungi and protozoa due to thermal limits. Select “An archaean.”

Verification / Alternative check:
Isolation records from deep-sea vents document archaeal growth near 113°C under high pressure; enzymology of archaeal DNA polymerases underpins PCR technology.

Why Other Options Are Wrong:
Fungi/protozoa lack the adaptations for such temperatures; cyanobacteria are thermotolerant in some springs but not at ≥95°C for growth.

Common Pitfalls:
Assuming “thermophilic bacteria” reach all extremes; the upper limits are largely archaeal.

Final Answer:
An archaean (hyperthermophilic archaeon).