Scientists discover first new chlorophyll in 60 years

University of Sydney scientists have
stumbled upon the first new chlorophyll to be discovered in over 60
years and have published their findings in the international journal Science.
Found by accident in stromatolites from Western Australia's Shark Bay, the new pigment named chlorophyll f can utilise lower light energy than any other known chlorophyll.

The historic study published online in Science, challenges our
understanding of the physical limits of photosynthesis - revealing that
small-scale molecular changes to the structure of chlorophyll allows
photosynthetic organisms to survive in almost any environment on Earth.


The new chlorophyll was discovered deep within stromatolites -
rock-like structures built by photosynthetic bacteria, called
cyanobacteria - by lead author Dr Min Chen from the University of
Sydney.

A team of interdisciplinary scientists, including Dr Martin Schliep
and Dr Zhengli Cai (University of Sydney); Associate Professor Robert
Willows (Macquarie University); Professor Brett Neilan (University of
New South Wales) and Professor Hugo Scheer (University of Munich),
characterised the absorption properties and chemical structure of chlorophyll f, making it the fifth known type of chlorophyll molecule on Earth.


Chlorophyll is the essential molecule in oxygenic photosynthesis -
the process that enables plants, algae and some bacteria to convert
carbon dioxide into sugar and oxygen by using free energy from sunlight.
Until recently, oxygenic photosynthesis was thought only to occur in
light that is visible to human eyes, between 400nm to 700nm, as
chlorophyll was strictly limited to absorbing light in this range.

This was overturned in 1996 when scientists found a cyanobacterium
that could photosynthesise using light just outside the visible
spectrum - at 710nm, in the infrared region - using a modified
chlorophyll molecule, named chlorophyll d. Since this discovery, scientists around the world have been puzzled by how chlorophyll d is able to get enough energy from infrared light for photosynthesis.
Now the rules of photosynthesis need to be rewritten again, with the
discovery of a new chlorophyll that can absorb light of even lower
photon energy - 720nm - making it the most red-shifted chlorophyll to
date.

In ecological terms, chlorophyll f allows cyanobacteria living
deep within stromatolites to photosynthesise using low-energy infrared
light, the only light able to penetrate into the structure, which
challenges further our understanding of the physical limits of
photosynthesis.
Dr Chen, from the School of Biological Sciences, explains:

"Finding  the new chlorophyll was totally unexpected - it was one of those
serendipitous moments of scientific discovery.
"I was actually looking for chlorophyll d, which we knew could
be found in cyanobacteria living in low light conditions. I thought
that stromatolites would be a good place to look, since the bacteria in
the middle of the structures don't get as much light as those on the
edge."

After obtaining a sample of stromatolite from Hamelin Pool, Dr Chen looked for chlorophyll d
by culturing the cyanobacterial sample in infrared light of 720nm. This
ensured only the survival of cyanobacteria that had chlorophylls able
to absorb and use infrared light.
High performance liquid chromatography of the cultured sample
performed six months later revealed not only trace amounts of
chlorophyll d, but also a new chlorophyll not seen before.

Testing the optical absorption spectrum of the new chlorophyll
revealed that it could absorb much longer wavelengths of light than any
other known chlorophyll - 10nm longer than chlorophyll d and more than 40nm longer than chlorophyll a.
Sequential mass spectral analysis revealed the molecular weight of
the new pigment to be 906 mass units. Then nuclear magnetic resonance
(NMR) spectroscopy was performed to determine the chemical structure of
the chlorophyll. Results indicated that chlorophylls a, b, d and f
have very similar chemical structures, differing only in the position
of a substitution. Yet these tiny differences in structure give the
chlorophylls very different spectral properties, and hence can function
in very different light environments.

"Discovering this new chlorophyll has completely overturned the
traditional notion that photosynthesis needs high energy light," Dr Chen
said.


"It is amazing that this new molecule, with a simple change to its
chemical structure, can absorb extremely low energy light. This means
that photosynthetic organisms can utilise a much larger portion of the
solar spectrum than we previously thought and that the efficiency of
photosynthesis is much greater than we ever imagined.

"Chlorophyll f, and its ability to absorb infrared light, can have numerous applications to industries like plant biotechnology and bioenergy.


"For us, the next challenge is to work out the function of this new chlorophyll in photosynthesis.
"Is its job to capture additional red light and pass it on to another
chlorophyll, like chlorophyll a, in the reaction centre for
photosynthesis?

"Or is it the only chlorophyll responsible for photosynthesis in the
cyanobacterium? And if it is, then we will be speechless wondering how
this molecule can get enough energy from infrared light to make oxygen
from water.


"Whatever happens next, the fact that we have discovered a
cyanobacterium that exploits a tiny modification in its chlorophyll
molecule to photosynthesise in light that we cannot see, opens our mind
to the seemingly limitless ways that organisms adapt to survive in their
environment."