Accessory pigments in algae- Phycobilins

Pigments which are not directly involved in photosynthesis but have the ability to absorb light and transfer it to the principal photosynthetic pigments are called as Accessory pigments. Phycobilins are water soluble pigments that are fairly joined to a protein, which acts as a accessory pigment in photosynthesis. Phycocyanobilin and Phycoerythrobilin are major phycobilins.

1. Accessory pigments in Algae: Phycobilins Presented by: Megha Varshini Gowda B R

2. Contents Introduction Phycobilins Phycocyanin Phycoerythrin Comparison between phycocyanin and phycoerythrin Phycobilins in photosynthesis Conclusion Reference

3. Introduction Pigments are organic compounds which absorbs certain wavelength of visible light and reflects the other. Each pigment has its own characteristic color. The photosynthetic pigments in algae are classified into 3 categories: Chlorophylls Carotenoids and xanthophylls Phycobilins The pigments which are not directly involved in photosynthesis but have the ability to absorb light and transfer it to the principal photosynthetic pigments are called as Accessory pigments. F. E Fritsch classified algae based on their pigmentation.

4. Phycobilins Phycobilins are water soluble pigments that are fairly joined to a protein, the combination being referred to as biliproteins . Phycoblinins are tetrapyrroles , the four pyrroles in phycobilins occur in an open chain. 300-800 phycobilins assemble and organize into phycobilisomes which are about 40 nm in diameter and are associated with the outer surfaces of lamellar membranes in Cyanobacteria and Rhodophytes . They usually have their major absorption bands from 520-670nm. These pigments are higher in concentration in many Cyanophycean and Rhodophycean members and are responsible for the characteristic color of the organism. The major phycobilins are Phycocyanobilin Phycoerythrobilin

5. Phycocyanin Phycocyanobilin and the protein to which it is covalently attached together constitute Phycocyanin. It appears bluish color because it absorbs strongly from 610-660 nm. It absorbs green, yellow and red light and transmit blue. It is the main phycobilin in the Cyanophycean members and is also found in members of Rhodophyta . Phycocyanin are sub divided into 3 types: R- Phycocyanin ( Rhodophyta ) C- phycocyanin ( Cyanophyta ) Allophycocyanin

6. Figure 1: Arthrospira jenneri Figure 2: Oscillatoria sancta Cyanophycean members exhibiting blue-green color due to the presence of Phycocyanin pigment.

7. Molecular formula: C 33 H 38 N 4 O 6 It has 9 conjugated double bonds and its λ max occurs in the red region. Figure 3 : Phycocyanobilin

8. Phycoerythrin Phycoerthrobilin along with the protein to which it is covalently attached is called Phycoerythrin. Phycoerythrin is reddish because it absorbs green and has at least one main band between 530-570 nm. It occurs throughout the red algae and in some Cyanobacteria. Phycoerythrin are of three types R- Phycoerythrin B- Phycoerythrin C- Phycoerythrin ( Cyanophyta ) Due to photodestruction of phycoerythrin, many red alga do not appear reddish at all, and a full range of pigment is exhibited like violet, purple, black, yellow and greenish forms. ( Rhodophyta )

9. Figure 4 : Polysiphonia elongata Figure 5: Gelidium corneum Rhodophycean members exhibiting red color due to the presence of Phycoerythrin pigment.

10. Molecular formula: C 33 H 38 N 4 O 6 It has 7 double bonds in the main conjugated system and absorbs maximally in the green region of the spectrum. Spectroscopic and biochemical studies have shown that energy from light absorbed by phycoerythrobilin must first be transferred to phycocyanobilin before it can reach chlorophyll and contribute to photosynthesis. Figure 5: Structure of Phycoerythrin.

11. Comparison between Phycocyanin and Phycoerythrin Phycocyanobilin Absorbs green, yellow and red light and transmit blue Absorbs strongly from 610-660 nm. Main phycobilin in the Cyanophycean members and is found in members of Rhodophyta . 9 conjugated double bonds and its λ max occurs in the red region. Independently can harvest the light. Phycoerythrobilin Absorbs green and transmit red. Absorbs strongly from 530-570 nm. It occurs throughout the red algae and in some Cyanophycean members. 7 double bonds in the main conjugated system and its λ max occurs in the green region of the spectrum. Presence of  phycocyanobilin is necessary for light-harvesting function 

12. Phycobilins in photosynthesis The energy of photons of light absorbed by the phycobilin is transferred to chlorophyll- a molecules in the reaction centers by the Förster excitation transfer mechanism. The indirectly photoexcited chlorophylls in the reaction centers then undergo the photosynthetic ‘light reaction’ — essentially a photoionization. The phycobilisomes act as a light-absorbing antenna for photosynthesis, and are able to efficiently absorb light in the green and orange spectral region where chlorophyll absorbs poorly. The pathway of energy transfer is Phycoerythrin ( λ max =565) Phycocyanin ( λ max =620-638) Chlorophyll a

13. Conclusion Phycobilins are the water soluble pigments which give characteristic color to the organism. They play an important role in photosynthesis as they increase the range of absorption of wavelength. It enables the continuity of life in the benthic region of oceans where most of the Rhodophytes exists.

14. References Bold, H. C. and Wynne, M.J. 1978. Introduction to the algae- Structure & reproduction. Prentice Hall. Pp.451-454 Lee, R. E. 2008. Phycology . 4 th Edn . Cambridge Univ. press, New york . Pp.17-19 Singh, V., Pande , P.C. and Jain, D.K. 2014. A Textbook of Botany. 4 th Edn . Rastogi Pub.Co . New Delhi. Pp.6, 27-29. https://www.ncbi.nlm.nih.gov/compound/6443764 https://pubchem.ncbi.nlm.nih.gov/compound/6438349 www.algaebase.org