Assertion: Agar is used to grow microbes and in preparation of ice-creams and jellies.

Reason: Agar is a hydrocolloid which is produced by certain marine brown algae.

Assertion is true, but Reason is false.

The correct answer is Option (3)- Assertion is true, but Reason is false.

Algae are useful to man in a variety of ways. At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis.

Being photosynthetic they increase the level of dissolved oxygen in their immediate environment. They are of paramount importance as primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals.

Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food. Certain marine brown and red algae produce large amounts of hydrocolloids (water holding substances), e.g., algin (brown algae) and carrageen (red algae) which are used commercially.

Agar, one of the commercial products obtained from Gelidium and Gracilaria are used to grow microbes and in preparations of ice-creams and jellies. Chlorella a unicellular alga rich in proteins is used as food supplement even by space travellers.

Gracilaria and Gelidium are  members of rhodophyceae .  Rhodophyceae are commonly called red algae because of the predominance of the red pigment, r-phycoerythrin in their body. Majority of the red algae are marine with greater concentrations found in the warmer areas. They occur in both well-lighted regions close to the surface of water and also at great depths in oceans where relatively little light penetrates.