Question

Blue colour of blue-green algae is due to
A. Phycocyanin and allophycocyanin
B. Phycoerythrin
C. Anthocyanin
D. Anthoxanthin

Answer 1

Hint: Blue-green algae are also called cyanobacteria. They appear blue-green due to the pigments that maximally absorb the light of 621 nm wavelength.

Complete answer:
Blue-green algae are the photoautotrophic prokaryotes and are classified under eubacteria. They perform oxygenic photosynthesis which means that they use water molecules as a source of electrons during photosynthesis. It produces molecular oxygen as the by-product.

Since they are autotrophic, blue-green algae have pigments in their cell. Like green plants, they have chlorophyll-a molecules as the primary photosynthetic pigments. However, in addition to the chlorophyll-a, blue-green algae also have phycoerythrobilin and phycocyanobilin pigments in their cells. These pigments also serve during the process of photosynthesis by trapping the sunlight.

Each pigment absorbs light in a particular portion of the electromagnetic spectrum and reflects the light of the other colours. The colour of the light reflected by a pigment determines its colour. For example, phycoerythrin imparts red colour to the red algae while the water-soluble anthocyanin gives red, pink, blue-purple colour to flowers and fruits of angiosperms. Anthoxanthin pigment gives white-cream colour to the petals of the flowers.

Similarly, the phycocyanobilin of the blue-green algae has blue coloured phycocyanin and allophycocyanin. The combination of chlorophyll-a (green) and phycocyanin and allophycocyanin (blue) imparts their characteristic appearance.

Hence, the correct answer is option (B).

Note: Blue-green algae were first classified with green plants due to their ability to perform photosynthesis and the presence of chlorophyll-a pigment. However, they are prokaryotic organisms.