Mass cultures of algae cells in open ponds do not use sunlight very efficiently, and their productivity can be limited by light-induced damage. Cells at the surface of the culture hog all the sunlight and end up wasting most of it because the absorption of excess sunlight makes the cells more susceptible to photoinhibition.

—Krishna Niyogi, Berkeley Lab

Niyogi, who holds joint appointments with Berkeley Lab and the University of California, Berkeley, is one of two corresponding authors on a paper published in the journal Nature entitled: “An ancient light-harvesting protein is critical for the regulation of algal photosynthesis.” Co-authoring the paper with Niyogi were Graham Peers and Thuy Truong from Berkeley Lab and/or UC Berkeley; Elisabeth Ostendorf, Andreas Busch and Michael Hippler (also a corresponding author) from the University of Münster; and Dafna Elrad and Arthur Grossman from the Carnegie Institution.

Light is necessary for photosynthesis, but its absorption by pigment molecules such as chlorophyll can cause severe oxidative damage and result in cell death. The excess absorption of light energy by photosynthetic pigments has led to the evolution of protective mechanisms that operate on the timescale of seconds to minutes and involve feedback-regulated de-excitation of chlorophyll molecules in photosystem II (qE). Despite the significant contribution of eukaryotic algae to global primary production, little is known about their qE mechanism, in contrast to that in flowering plants.

—Peers et al.

In 2008, Niyogi and Graham Fleming, a physical chemist with Berkeley Lab and UC Berkeley, were part of a collaboration that identified the light-harvesting protein CP29 as a valve that either permits or blocks the critical release of excess solar energy in green plants during photosynthesis. CP29 was shown to drain energy off from chlorophyll and into the carotenoid zeaxanthin, which Fleming and his research group earlier identified as the safety valve for the photo-protection of green plants.

We believe that CP29 is one of several light-harvesting proteins involved in dissipating excess light energy for green plants, with another protein, PsbS, acting as a light sensor that turns on the dissipation mechanism when needed. In green algae, the LHCSR protein, which also binds chlorophyll and zeaxanthin, appears to perform both the sensing and dissipating functions.

—Krishna Niyogi

For the green algae study, Niyogi and his collaborators worked with an algal organism called Chlamydomonas, used as a genetic model for other eukaryotic algae. They compared an energy-quenching mutant of Chlamydomonas, in which two of the three LHCSR-coding genes were absent, to a wild type of Chlamydomonas in a series of light exposure tests. While cells in the two cultures had a statistically identical rate of survival when exposed to low levels of light, the mutant culture showed a 40% reduction in cell survival compared to the wild culture when exposed to high levels of light.

We’ve shown that for green algae, and probably most other eukaryotic algae, the LHCSR protein is used to dissipate excess light energy and protect the photosynthetic apparatus from damage...It was surprising to see how nature has used related proteins in different ways for light harvesting and light dissipation. LHCSR, CP29 and PsbS are proteins that share a common ancestor with the main light-harvesting proteins in algae and plants, but they are like distant cousins. This suggests that the photoprotection function arose early in the evolution of the light-harvesting protein family.

—Krishna Niyogi

Niyogi and his research group are now active in another collaboration with the Fleming group to investigate exactly how the LHCSR protein gets rid of excess light. Preliminary results indicate that the biophysical process is much the same as the process for green plants even though the protein environment is different. Having identified LHSCR as the key dissipater of light energy and understanding how it does the job should not only be a boon for biofuels research, but should also help in the effort to design artificial photosynthesis systems that would be sustainable and secure sources of energy.

This research was supported by the US Department of Energy’s Office of Science through its Basic Energy Sciences program.

Resources

• Graham Peers, Thuy B. Truong, Elisabeth Ostendorf, Andreas Busch, Dafna Elrad, Arthur R. Grossman, Michael Hippler, Krishna K. Niyogi (2009) An ancient light-harvesting protein is critical for the regulation of algal photosynthesis. Nature 462, 518-521 doi: 10.1038/nature08587