The discovery that an ancient light harvesting protein plays a pivotal role
in the photosynthesis of green algae should help the effort to develop algae
as a biofuels feedstock. Researchers with the Lawrence
Berkeley National Laboratory (Berkeley Lab) have identified the protein
LHCSR as the molecular "dimmer switch" that acts to prevent green algae from
absorbing too much sunlight during photosynthesis and suffering oxidation damage
as a consequence.
"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," says Krishna Niyogi, a biologist
who holds joint appointments with Berkeley Lab's Physical Biosciences
Division and the University of California (UC) Berkeley's Department of
Plant and Microbial Biology. "We describe LHCSR as an ancient member of
the family of light harvesting proteins because it seems to have been one of
the first to branch off from a common ancestor shared long ago by both algae
Niyogi is the corresponding author 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, Thuy Truong, Elisabeth Ostendorf, Andreas Busch, Dafna Elrad, Arthur
Grossman and Michael Hippler.
It is a popular misconception that algae are simply aquatic plants. While algae
share with plants a reliance on photosynthesis, the protein machinery differs.
Understanding the photosynthetic machinery unique to algae is important because
algae are considered prime candidates to serve as feedstocks for future biofuels.
Algae boast high energy content and yield, rapid growth, and the ability to
thrive in seawater or wastewater. Also, the oil extracted from algae can be
refined into biodiesel, and jet fuel or ethanol, in addition to gasoline.
Cultivating algae on a scale for commercial biofuels production, however, has
proven to be a major challenge. Growing algae in closed photobioreactors is
effective but probably too expensive for commercial scale production. Growing
algae in open ponds has proven problematic primarily because of photosynthesis.
"Mass cultures of algae cells in open ponds do not use sunlight very
efficiently, and their productivity can be limited by light-induced damage,"
says Niyogi. "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."
The findings of Niyogi and his colleagues could enable researchers to engineer
green algae cells that make better use of light. Like green plants, algae use
photosynthesis to harvest energy from sunlight and convert it to chemical energy.
Also like green plants, when pigment molecules such as chlorophyll absorb an
excess of solar energy, the result is severe oxidative damage that can be fatal
to the cells. However, both plants and algae have evolved a photo-protective
mechanism-called energy-quenching-by which excess energy can be safely dissipated
from one molecular system to another for routing down relatively harmless chemical
"Despite the significant importance of aquatic photosynthesis for determining
the influences of oceans and lakes on climate and biogeochemistry, little has
been known about the energy-quenching mechanism in algae," says Niyogi.
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,"
Niyogi says. "In green algae, the LHCSR protein, which also binds chlorophyll
and zeaxanthin, appears to perform both the sensing and dissipating functions."
For the green algae study, Niyogi and his collaborators worked with an algal
organism called Chlamydomonas, which is considered "the fruit fly of the
algae world," in terms of being 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 percent reduction in cell survival compared to the wild culture
when exposed to high levels of light.
"It was surprising to see how nature has used related proteins in different
ways for light harvesting and light dissipation," says Niyogi. "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
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.
"The LHCSR protein provides us with another blueprint from nature that
might be used to control the harvesting of sunlight by future artificial photosynthesis
systems," Niyogi says.
This research was supported by the U.S. Department of Energy's Office
of Science through its Basic Energy Sciences program.
Berkeley Lab is a U.S. Department of Energy national laboratory located in
Berkeley, California. It conducts unclassified scientific research for DOE's
Office of Science and is managed by the University of California. Visit our
Website at www.lbl.gov/