What Happened to the Phycobilisome?

Beverley R Green

doi:10.3390/biom9110748

Abstract

The phycobilisome (PBS) is the major light-harvesting complex of photosynthesis in cyanobacteria, red algae, and glaucophyte algae. In spite of the fact that it is very well structured to absorb light and transfer it efficiently to photosynthetic reaction centers, it has been completely lost in the green algae and plants. It is difficult to see how selection alone could account for such a major loss. An alternative scenario takes into account the role of chance, enabled by (contingent on) the evolution of an alternative antenna system early in the diversification of the three lineages from the first photosynthetic eukaryote.

Highlights

Gene loss is a major factor in evolutionary change [1,2,3]
Certain losses can provide a selective advantage in adaptation to novel environmental challenges, e.g., drought tolerance in wild Arabidopsis [4] and thermophily in basal red algae [5]
How could selection alone account for the loss of an entire macromolecular complex like the phycobilisome? The phycobilisome (PBS) is a highly efficient light-harvesting complex essential for photosynthesis in cyanobacteria, rhodophyte algae, and glaucophyte algae [6,7,8,9]. but it has been completely lost in the chlorophyte lineage (Figure 1)

Summary

Introduction

Gene loss is a major factor in evolutionary change [1,2,3]. While new genes are continually being acquired by duplication, recombination, horizontal transfer, endosymbiotic transfer, and de novo origin, other genes are being lost by the gradual accumulation of deleterious mutations (pseudogenization), insertions, deletions, and chromosomal reorganization. The phycobilisome is generally a very large and elaborate macromolecular structure made up allophycocyanin (APC) Attached to this core are a number of rods, each made up of one or more of pigment-binding proteins that absorb visible light, and funnel that energy into photosynthetic phycocyanin (PC) hexamers. In some species, these rods are extended by the addition of phycoerythrin reaction centers to drive the central processes of photosynthesis [6,7,16,17]. The loss was contingent on the presence of a pre-existing gene or genes that could diversify into an alternative antenna family

Light-Harvesting in the Three Primary Lineages of Photosynthetic Eukaryotes

Advantages and Disadvantages of Losing the PBS

PBS Loss after Secondary Endosymbiosis

Conclusions