Question

Which of the following is not a diatom?
A) Nostoc
B) Navicula
C) Cyclotelle
D) Pinnularia

Answer 1

Diatoms a large group of microscopic algae, but careful consideration has to be taken of the frustule and the valves it forms. It would otherwise be very easy to confuse diatoms with blue-green algae.

Complete answer:
Diatoms are commonly found in fresh and salt water, on land, and within the soil. Being a very diverse group, they can be free living, or in colonies, be photosynthetic heterotrophs or autotrophs, and have radial or bilateral symmetry. However one distinctive feature of these organisms is that they are surrounded by a frustule: a stiff cell wall that has two sections – the epitheca and the hypotheca. The cell wall is made of pectin and silicon.
Nostoc is classified as blue - green algae, and also as a cyanobacterium. It is photosynthetic, and one of the oldest living organisms on this planet. Despite these features, Nostoc’s cell wall is made of peptidoglycans and an outer lipid and protein membrane. It is therefore not a diatom. Option A is correct.
Navicula is found in freshwater and soil living diatoms. It is autotrophic, and boat shaped, the name itself means “Little ship”. It is the largest genus of diatoms and secretes mucus to allow the organism to flow smoothly over the ground. Option B is incorrect
Cyclotella is a small diatom with polymorphic valves . it is found in marine and freshwater environments. The organisms resemble a wheel in shape, and the name comes from the Greek word for circle. The frustules contain openings called areolas that regular the passage of substances in and out of the cell. Option C is incorrect.
Pinnularia is a fresh water algal diatom. It is sometimes classified under Navicula. They are elongated, thin unicellular algae, and like all diatoms, have the siliceous frustule. Option D is therefore also incorrect.

Hence the correct answer is OPTION(A)

Note: All diatoms reproduce by simple mitosis, with each daughter cell receiving one of the thecal cell wall structures. The frustule is a transparent structure which may be covered by a slimy, mucin layer.