Batrachospermum is a fresh water filamentous Rhodophycea alga, commonly called frog spawn alga. The colour depends upon depth at which alga occur - blue-green to purple-violet and pink....

Introduction:
Batrachospermum is a genus of red algae that is commonly found in freshwater environments. Despite its name, it is not actually red in color but rather appears blue or purplish. The reason behind this discrepancy lies in the presence of pigments and the way they interact with light.

Presence of Pigments:
Batrachospermum contains pigments called phycobilins, which are responsible for its blue coloration. Phycobilins are water-soluble pigments that absorb light in the blue and red regions of the electromagnetic spectrum. They are commonly found in red algae and cyanobacteria.

Phycoerythrin Pigment:
The primary pigment responsible for the blue coloration of Batrachospermum is phycoerythrin. Phycoerythrin absorbs light in the blue region of the spectrum and reflects or transmits light in the red region, giving the appearance of a blue color. This phenomenon is known as selective absorption. The blue light that is not absorbed by the phycoerythrin pigment is reflected back, resulting in the blue coloration of the algae.

Interaction with Light:
When light passes through the water, it gets absorbed or scattered by water molecules and other particles present in the environment. The blue light, being of shorter wavelength, tends to scatter more than the red light, which has a longer wavelength. This scattering of blue light contributes to the overall appearance of Batrachospermum as blue.

Color Perception:
The perception of color is a complex process that involves the interaction between light and the cells in our eyes. The cells in our retina, called cone cells, are responsible for color vision. These cone cells have different types of photopigments that are sensitive to different wavelengths of light. The blue light reflected by Batrachospermum stimulates the cone cells in our eyes that are sensitive to blue light, resulting in the perception of a blue color.

Conclusion:
In conclusion, Batrachospermum is classified as a red algae due to its phylogenetic relationship with other red algae. However, its blue coloration is a result of the presence of the pigment phycoerythrin, which selectively absorbs blue light and reflects or transmits red light. The scattering of blue light by water molecules and the perception of color by cone cells in our eyes further contribute to the blue appearance of Batrachospermum.