RIKEN researchers determined the structure of the “antenna” that a blue-green alga uses to harvest light and compared it to that of four other species. This research could help in the development of efficient photoreactive compounds, as well as providing clues about the evolution and diversity of cyanobacteria.
The findings were published in Nature Communications.
Phycobilisomes are bundles of proteins and chromophores that act as antennas for capturing light during photosynthesis and are found on the surface membranes of cyanobacteria (or blue-green algae) and other algae.
“Phycobilisomes absorb light at wavelengths that other light-absorbing photosynthetic proteins, such as photosystems I and II, find difficult to use,” says Keisuke Kawakami of the RIKEN SPring-8 Center. “Every algal species has a distinct phycobilisome structure. Understanding these structures will help us learn more about efficient solar energy production.”
Thermosynechococcus vulcanus, a cyanobacterium, has the most common shape of phycobilisomes, which is a half-sphere with a fan-like arrangement of rods protruding from the core. Light energy is absorbed and then transferred to photosystems I and II via the rods and cores.
Koji Yonekura, also of the RIKEN SPring-8 Center, and his colleagues analysed and compared the structure and function of T. vulcanus’s phycobilisome to those of four previously reported species using cryo-electron microscopy.
The researchers discovered that the core is made up of five protein cylinders rather than the more common three-cylinder structure. They also discovered a new type of cylinder. Each phycobilisome has several rods protruding from its core. These are made up of stacked proteins and chromophores linked together by linker proteins. T. vulcanus has only one type of chromophore, phycocyanobilin, whereas some algae have multiple.
“Understanding unidirectional energy transfer requires understanding the structure of linker proteins and amino acids surrounding each phycocyanobilin, as well as their interactions,” says Kawakami. “We were surprised to discover that amino-acid residues surrounding specific phycocyanobilins can manipulate the absorption wavelengths of those chromophores. It will enable continuous and rapid energy transfer even under changing light conditions.”
The researchers also discovered that these amino-acid residues differ subtly between algal species, most likely due to differences in light conditions in different growth environments.
“Exploring the phycobilisome structures in various species will help us understand the diversity and evolution of algae,” says Kawakami. “Phycobilisomes could inspire the development of optical devices that use the mechanism of unidirectional energy transfer with a single chromophore.”