Source of agar used in microbiology media Agar, the widely used solidifying agent in bacteriological culture media, is derived from the cell walls of which algal division?

Introduction / Context:
Agar is indispensable in microbiology because it melts at high temperature and solidifies firmly while remaining largely indigestible for most bacteria. Knowing its biological source helps connect laboratory practice with natural products chemistry.

Given Data / Assumptions:

• Agarose and agaropectin are the principal polysaccharides in agar.
• Extraction is from certain red algae such as Gelidium and Gracilaria.
• Thermal properties (high melting, low re-melting hysteresis) make agar ideal for plates and slants.

Concept / Approach:
Red algae (Rhodophyta) possess sulfated galactans in their cell walls. Processing yields agar that provides a stable matrix for microbial growth without supplying readily metabolizable carbon to most organisms.

Step-by-Step Solution:
Identify the polymer: agar obtained from red algal cell walls.Map to division: Rhodophyta.Eliminate other options: green, golden/diatom, dinoflagellate, and brown algae are not the standard sources of agar.Select “Rhodophyta (red algae).”

Verification / Alternative check:
Commercial agar suppliers list Gelidium/Gracilaria sources, both red algae, confirming the provenance used in culture media.

Why Other Options Are Wrong:
Other algal divisions provide different compounds (for example, alginates from brown algae) but not agar as used in routine microbiology.

Common Pitfalls:
Confusing agar with alginate or carrageenan. While carrageenan also comes from red algae, agar specifically refers to agarose-rich preparations favored for microbial culture.

Final Answer:
Rhodophyta (red algae).