Siliceous frustules: which algal group is defined by glass-like cell walls? Identify the group whose cells are enclosed in two interlocking silica shells known collectively as a frustule.

Introduction / Context:
Diatoms are iconic microalgae recognized by their intricate glass-like walls. Knowing the term “frustule” and linking it to diatoms is a frequent exam checkpoint in microbiology, limnology, and environmental monitoring.

Given Data / Assumptions:

• Frustules are composed largely of hydrated silica.
• Two overlapping halves (epitheca and hypotheca) form the rigid enclosure.
• Morphology varies (centric and pennate forms), aiding taxonomy.

Concept / Approach:
Among algal groups, only diatoms have the diagnostic silica frustule. Other algae may have cellulosic walls (for example, green algae) or other coverings (for example, euglenoids with a pellicle). Therefore, associating “frustule” with “diatom” is the key mapping.

Step-by-Step Solution:
Recall definition: frustule = biogenic silica cell wall.Link to group: diatoms (Bacillariophyta).Exclude alternatives: euglenoids have pellicles; desmids are green algae; “seaweeds” is too broad.Choose “Diatoms.”

Verification / Alternative check:
Microscopy images show ornate silica patterns unique to diatoms, used in paleoecology and water quality assessment.

Why Other Options Are Wrong:

• Euglenoids: flexible proteinaceous pellicle, not silica frustules.
• Desmids: cellulose walls typical of green algae.
• Seaweeds: macroscopic forms spanning several divisions, not specifically siliceous.
• Dinoflagellates: armored cellulose plates (theca), not silica shells.

Common Pitfalls:
Confusing diatom frustules with dinoflagellate thecal plates. The materials and architecture differ markedly.

Final Answer:
Diatoms.