Think of the first chapter you came across in a primary school science textbook that made you feel you were studying real science. For most of us, that was the chapter on how plants take sunlight from the sun and turn it into energy for itself, and eventually for all other life on Earth.
Now, in a discovery that can change our understanding of the fundamental process of photosynthesis, scientists have found a new way that bacteria can absorb infrared light and turn it into energy. This could not only change the way humans hunt for extra-terrestrial life and lead to new ways of developing crops, but also rewrite science textbooks.
Plants use photosynthesis to generate energy from naturally occurring carbon dioxide and water, which they use as fuel. This was thought to be driven only by red light, but a team of scientists at Imperial College London discovered a type of photosynthesis that uses near-infrared light instead.
"The new form of photosynthesis made us rethink what we thought was possible. It also changes how we understand the key events at the heart of standard photosynthesis. This is textbook changing stuff," lead researcher Professor Bill Rutherford, at the Department of Life Sciences at Imperial College, said.
This form of photosynthesis was detected in blue-green algae when they grow in near-infrared light — these microbes are found in shaded conditions in Yellowstone National Park in the United States and in beach rock found in Australia. Scientists in Imperial College, London have now discovered that the process also occurs in a cupboard fitted with infrared LEDs.
The photosynthesis we always knew of involves the famous green pigment — chlorophyll-a — which plants use to collect light to make useful biochemicals and oxygen out of. The manner in which chlorophyll-a absorbs light from the Sun means only the energy from red light can be used for photosynthesis.
All plants, algae and "green microbes" that we are aware of contain chlorophyll-a, and it was assumed that there was a ‘red limit’ for photosynthesis; that is the minimum amount of energy needed to start the chemistry that produces oxygen. The concept of 'red limit' also appears in astrobiology, where scientists have used it to judge whether complex life could have evolved on planets elsewhere in the universe. The new study suggests that these scientific concepts can now be rendered redundant.
The scientists at Imperial College found that when some cyanobacteria are grown under near-infrared light, the standard chlorophyll-a containing species do not respond, but different systems containing a different kind of chlorophyll — chlorophyll-f — do.
The pigment chlorophyll-f is not unknown to man; until now, it was thought that the pigment only harvested the light. The new study shows that chlorophyll-f actually plays the key role in photosynthesis under certain shaded conditions. This is now known as photosynthesis ‘beyond the red limit’.
Co-author Dr Andrea Fantuzzi from Imperial College said, "Finding a type of photosynthesis that works beyond the red limit changes our understanding of the energy requirements of photosynthesis. This provides insights into light energy use and into mechanisms that protect the systems against damage by light."
The new insights, beyond redirecting our search for alien life, could also be useful in engineering crops to perform more efficient photosynthesis by using a wider range of light.
"I did not expect that my interest in cyanobacteria and their diverse lifestyles would snowball into a major change in how we understand photosynthesis. It is amazing what is still out there in nature waiting to be discovered," Dr Dennis Nürnberg, initiator of the study, said.
The discovery, published on 15 June in Science, was led by Imperial College London, and was supported by the BBSRC. It involved groups from the ANU in Canberra, the CNRS in Paris and Saclay and the CNR in Milan.