Which of the following algae contains mannitol as reserve food material ?

The correct answer is Ectocarpus.

Algae store reserve food materials to survive periods when photosynthesis is not possible. Different groups of algae use different compounds for this purpose.

• Volvox and Ulothrix are primarily freshwater green algae that store starch as their main reserve carbohydrate.

• Gracilaria is a red alga that typically accumulates floridean starch rather than sugar alcohols.

• Ectocarpus belongs to the brown algae (Phaeophyceae). Brown algae uniquely synthesize and store the sugar alcohol mannitol as a compatible solute and energy reserve. Mannitol is accumulated in the cytoplasm and can be mobilized when the alga requires carbon.

Therefore, among the listed options, only Ectocarpus contains mannitol as its reserve food material.