Science Advances: Ultraviolet radiation, a sunscreen-like chemical found in fossils, played a role in mass extinction events

The end-Permian mass extinction, 252 million years ago, caused the extinction of about 81% of marine species and 89% of terrestrial species, and the main cause is thought to be environmental changes caused by large-scale volcanic eruptions, but there is still a lack of direct evidence
that environmental factors affect terrestrial ecosystems 。 Liu Feng, a researcher at the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, cooperated with colleagues from the United Kingdom and Germany to study the changes in the content of "sunscreen" in fossil pollen grains during this period, and found direct evidence that ozone layer destruction led to mass extinction of terrestrial organisms at the end of the Permian, and the research results were published online in the internationally renowned journal Science Advances on January 6, 2023 Advances)
。

The mass extinction event at the end of the Permian was the largest extinction event
in the Phanerozoic.
Scientists generally agree that the global increase in greenhouse gases caused by the eruption of the Siberian igneous province at the end of the Permian, global warming and ocean acidification may have been the main triggers
for this extinction.
But there is still much debate
about how these environmental factors affect terrestrial ecosystems.

Some pollenologists have found some malformed spores and pollen in the terrestrial Permian-Triassic transition profile, speculating that these malformed spores or pollen may be caused
by the increase in ultraviolet radiation induced by global ozone layer destruction caused by halogen elements ejected from the large igneous province of Siberia.
However, through the study of the malformed spores and pollen produced in modern plants, it has been shown that the environmental background of the production of these malformed pollen and spores is very complex, drought, air pollution and plant trauma may induce the plant body to produce malformed spores or pollen, so the small number of malformed spores and pollen found in the terrestrial Permian-Triassic transition profile alone does not directly prove that there was an increase in ultraviolet radiation caused by the global ozone layer hole during the end-Permian mass extinction
。

The plant body needs sunlight for photosynthesis and energy for the growth of the plant body, but it is necessary to avoid the destruction of the genetic information in the plant body, especially the plant germ cells (spores and pollen) by the harmful ultraviolet rays in the sunlight
.
In order to adapt to the terrestrial radiation environment, terrestrial plants have evolved some regulatory mechanisms to reduce the damage of ultraviolet rays to plant bodies
.
In the germ cells of the plant body, this mechanism is manifested in the fact that the outer wall of the plant's spores and pollen contains a large number of compounds (coumaric acid and ferulic acid) that are very similar in function to "sunscreen", which can form resonant and stable phenolic radicals to resist oxidation caused by ultraviolet rays, thereby protecting fragile spores and pollen, providing a guarantee
for the spread and reproduction of terrestrial plants.

Modern botanists have observed that plant bodies can automatically adjust the content of "sunscreen" on the outer wall of their germ cells according to the surrounding ultraviolet radiation environment, and these "sunscreens" can be stored in the inert spores and pollen outer walls for a long time
.
Therefore, it is theoretically possible to reverse the intensity
of atmospheric ultraviolet radiation in geological history by measuring the "sunscreen" content in these spores and pollen.

The research team used the infrared spectrum of Fourier changes to quantitatively measure coumaric acid and ferulic acid in 1011 Ali-type pollen produced in the Permian-Triassic transition profile in southern Tibet, and through the big data analysis of the infrared spectrum of these pollen outputs, it was found that the content of coumaric acid and ferulic acid on the outer wall of pollen in the formation during the end-Permian mass extinction was significantly higher than
。 This is a direct testimony to the global increase in ultraviolet radiation during the end-Permian mass extinction
.
At the same time, the research team also analyzed the concentrations of organic carbon isotopes and mercury in the cross-sectional formation, and found that the abnormal fluctuations of these two indicators were basically equivalent to the layers of coumaric acid and ferulic acid in fossil pollen, and the progress confirmed that the increase in global ultraviolet radiation at the end of the Permian may be caused by the intense volcanic activity at that time
.

The increase in the amount of ultraviolet radiation in the air has profound effects
on the entire terrestrial ecosystem.
Because ultraviolet rays not only have a strong killing effect on the germ cells of plants, but also cause damage
to plant mesophyll cells.
In order to resist the destruction of mesophyll cells by ultraviolet rays, the plant body will synthesize a large number of lutein, coumaric acid and ferulic acid in its leaves, and correspondingly reduce the synthesis of chlorophyll, resulting in the weakening of plant photosynthesis, which in turn weakens the absorption capacity of plant bodies for greenhouse gases, further aggravating the increase
of global greenhouse gases caused by volcanic eruptions at the end of the Permian 。 In addition, lutein, coumaric acid and ferulic acid in mesophyll meat are difficult compounds for herbivores and insects to digest and have low nutritional value, so the increase in ultraviolet radiation indirectly affects the terrestrial food chain, which may be the main cause of the mass extinction of terrestrial herbivores and insects at the end of the
Permian.

The research was supported
by the National Natural Science Foundation of China, the Second Tibetan Plateau Scientific Expedition and Research, the Chinese Academy of Sciences Strategic Leading Science and Technology Project (Category B), the German Research Foundation, the Natural Environment Research Council, and the Human Frontier Science Program.

Information on the paper: Liu, F.
*, Peng, H.
P.
, Marshall, E.
A.
J.
, Lomax, B.
H.
, Bomfleur, B.
, Kent, M.
S.
, Fraser, W.
T.
, Jardine, P.
E.
* (2023).
Dying in the Sun: Direct evidence for elevated UV-B radiation at the end-Permian mass extinction.
Science Advances 9, eabo6102.
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Southern Tibetan Qubu profile of late Permian black shale

Pollen fossils (Ali powder) found in Permian-Triassic transitional strata of the Qubu profile

Increased impact of ultraviolet radiation from ozone layer destruction on terrestrial food chains