Photosynthesis under light conditions different from the Earth

Artists impressions of a habitable planet around M-dwarfs
(left) and primordial Earth (right). The surface of M-dwarf
planet is illuminated by visible light. On the other hand,
similar light conditions are expected underwater, since only
blue-green light can penetrate meters of water. Credit:
Astrobiology Center

Researchers at the Astrobiology Center (ABC) of National
Institutes of Natural Science (NINS) in Japan and their
colleagues have proposed that Earth-like red-edge reflection
patterns could be observed on exoplanets around M-dwarfs.
They point out that the first oxygenic phototrophs are most
likely to have evolved underwater to utilize visible light,
as occurred in the primordial ocean on Earth.

M-dwarfs or red dwarfs are small (0.5-0.1 solar-masses) and
cool ( ~3000 Kelvin) , and are abundant in
universe. The sun-like stars are considered plausible targets
for searching habitable exoplanets. However, nearby M-dwarfs
are becoming the most extensive targets for habitable planet
searches because they are the most abundant nearby stars and
thus could be the first candidate for detecting biosignatures
on exoplanets via transit or direct imaging observations in
near future.

One of the most important exoplanetary biosignatures is a
specific reflection pattern on the land surface called
“red-edge,” which is caused by vegetation such as forests and
grasslands. On the Earth, red-edge appears between red and
infrared (IR) wavelengths, since red is absorbed for photosynthesis while IR
radiation is reflected. In previous studies, it was predicted
that red-edge position on exoplanets should be decided by the
radiation spectrum of nearby stars. Around M-dwarfs, red-edge
was expected to be shifted to a longer wavelength, since
planets on the exoplanets use abundant IR radiation for
photosynthesis.

Researchers at the Astrobiology Center (ABC) of National
Institutes of Natural Science (NINS) and their colleagues have
proposed an alternative prediction that red-edge could be
observed as on the Earth even on exoplanets around M-dwarfs in
the online journal Scientific Reports. They point out
that the first oxygenic phototrophs are most likely to have
evolved underwater to utilize just as in the primordial ocean on
the Earth. They examined light adaptation mechanisms of visible
and IR radiation-using phototrophs required for adapting to
land habitats and found that IR-using phototrophs struggle to
adapt to changing light condition at the boundary of water and
land. Kenji Takizawa, read author of the study, said “It is too
risky to utilize IR-radiation during water-to-land evolution.”

Explore further:

Precision measurements of exoplanet velocities

More information: Kenji Takizawa et al. Red-edge
position of habitable exoplanets around M-dwarfs, Scientific
Reports
(2017). DOI: 10.1038/s41598-017-07948-5

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