Archaea domain membership: which group listed is NOT included among Archaea?

Introduction / Context:
Archaea are a domain of prokaryotes that are distinct from Bacteria and Eukarya. Classic Archaea include organisms adapted to extreme environments, such as high salt, high temperature, or anaerobic niches where methane is produced. The question asks for the group that is not archaeal.

Given Data / Assumptions:

• Methanogens are associated with methane production in anaerobic environments.
• Halophiles thrive at very high salinities.
• Thermoacidophiles tolerate high temperatures and low pH.
• Cyanobacteria are photosynthetic prokaryotes commonly called blue green bacteria.

Concept / Approach:
Methanogens, extreme halophiles, and many thermoacidophiles are well known archaeal groups. In contrast, cyanobacteria are bacteria, not archaea. They are oxygenic photoautotrophs that perform photosynthesis using chlorophyll a and are major contributors to global oxygen cycles.

Step-by-Step Solution:

Verification / Alternative check:
Any standard taxonomy reference divides prokaryotes into Archaea and Bacteria. Cyanobacteria fall under Bacteria; the others are archetypal Archaea.

Why Other Options Are Wrong:

• Methanogens: archaeal anaerobes producing methane.
• Halophiles: archaeal extremophiles of high salinity habitats.
• Thermoacidophiles: archaea adapted to hot, acidic environments.

Common Pitfalls:
Confusing cyanobacteria with archaeal extremophiles because both can occupy unusual environments; however cyanobacteria are bacterial phototrophs.

Final Answer:
cyanobacteria