The vast majority of life on our planet uses red light (680 to 700 nm) in the process of photosynthesis, but the new type uses near-infrared light (750 nm) instead: it was detected in a wide range of cyanobacteria, one of the largest groups of bacteria on Earth, where they have existed for several billion years. A paper reporting this discovery is published in the journal Science.

The standard, near-universal type of photosynthesis uses the green pigment, chlorophyll-a, both to collect light and use its energy to make useful biochemicals and oxygen.

The way chlorophyll-a absorbs light means only the energy from red light can be used for photosynthesis.

Since chlorophyll-a is present in all plants, algae and cyanobacteria that we know of, it was considered that the energy of red light set the ‘red limit’ for photosynthesis; that is, the minimum amount of energy needed to do the demanding chemistry that produces oxygen.

However, when some cyanobacteria are grown under near-infrared light, the standard chlorophyll-a-containing systems shut down and different systems containing a different kind of chlorophyll, chlorophyll-f, takes over. Until now, it was thought that chlorophyll-f just harvested the light.

The new study shows that instead chlorophyll-f plays the key role in photosynthesis under shaded conditions, using lower-energy infrared light to do the complex chemistry. This is photosynthesis ‘beyond the red limit.’

“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,” said co-lead author Professor Bill Rutherford, from the Department of Life Sciences at Imperial College London, UK.

The chlorophyll-f based photosynthesis reported today represents a third type of photosynthesis that is widespread.

However, it is only used in special infrared-rich shaded conditions; in normal light conditions, the standard red form of photosynthesis is used.

It was thought that light damage would be more severe beyond the red limit, but the new study shows that it is not a problem in stable, shaded environments.

“Finding a type of photosynthesis that works beyond the red limit changes our understanding of the energy requirements of photosynthesis,” said co-lead author Dr. Andrea Fantuzzi, also from the Department of Life Sciences at Imperial College London.

“This provides insights into light energy use and into mechanisms that protect the systems against damage by light.”

“Low-light adapted cyanobacteria could be used to colonize Mars and other planets, to produce oxygen and create a biosphere,” said co-author Professor Elmars Krausz, from the Research School of Chemistry at Australian National University.

“This might sound like science fiction, but space agencies and private companies around the world are actively trying to turn this aspiration into reality in the not-too-distant future.”

“Photosynthesis could theoretically be harnessed with these types of organisms to create air for humans to breathe on Mars.”

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Dennis J. Nürnberg et al. 2018. Photochemistry beyond the red limit in chlorophyll f–containing photosystems. Science 360 (6394): 1210-1213; doi: 10.1126/science.aar8313