How Pollen Geochemistry Can Be Used To Reconstruct UV-B Radiation In The Past

Ultraviolet radiation (UV-B; 280 to 315 nm) is electromagnetic radiation originating from the sun, which reaches the Earth’s surface. UV-B radiation can cause chemical reactions in living organisms, and too much exposure can be damaging.

The Ozone layer is the Earth’s defensive mechanism against UV-B, and this acts to reduce the amount of UV-B reaching the surface. Indeed, it is believed that the formation of the Ozone layer made life on Earth possible. However, not all UV-B is absorbed by the Ozone layer, and so all living organisms on Earth are exposed to some UV-B radiation.

Plants can be damaged by too much UV-B exposure, where effects may alter DNA, stunt plant growth, and alter biodiversity. To protect themselves, plants have developed several defensive mechanisms, one of which includes the production of UV-B absorbing compounds (UACs) in plant tissues, including pollen. These compounds (p-coumaric acid and ferulic acid) absorb incoming UV-B radiation, preventing it from reaching the sensitive inner plant tissue. Effectively, plants have developed an in-built sunblock.
Plants increase the amount of UACs in their tissues in response to higher levels of UV-B exposure, in the same way that humans may apply more sunblock with a higher SPF value when UV-B levels are high.

Pollen produced by plants and trees for reproduction is made of an extremely resistant substance known as sporopollenin, which contains high concentrations of UACs. Pollen is so resistant that it can survive in harsh environments for millions of years.

Pollen has traditionally been used to reconstruct past vegetation and climate in different regions by identifying the different pollen types found in sedimentary archives, such as a peat bog. During the development period for pollen in plants (which varies depending on species), the chemical composition (or geochemistry) of the pollen will be altered by climate and environmental influences, including the level of UV-B exposure. Increased UV-B exposure results in higher quantities of UACs in pollen, which is preserved within the sporopollenin.

We can measure the amount of UACs in pollen, and use the relationship between UACs and UV-B exposure in order to reconstruct UV-B levels. Pollen geochemistry therefore acts as a proxy record for UV-B reaching the Earth’s surface on long-term timescales where instrumental records are not available.

In our paper, we focused our study on the pollen of modern Atlas cedar trees (Cedrus atlantica), which are native to Morocco and Algeria, in order to develop a modern calibration dataset for historic UV-B reconstruction. We collected pollen samples from cedar trees growing across Morocco, and additionally from those growing in botanical gardens in Europe and the USA.

We analysed the pollen geochemistry and found that the amount UACs in Atlas cedar pollen increased in response to increasing UV-B levels during the summer months (June to August). Atlas cedar pollinates in the Autumn (mid-September to early October), and its pollen develops during the summer months.
Interestingly, although UACs in pollen increased to increasing UV-B levels in both groups (native trees and botanical garden trees), the response in the botanical garden samples was “out of line” with the response in native pollen samples. The result is only significant when you consider the different groups separately. In some cases, the amount of UACs in the pollen of botanical garden samples was higher than the amount of UACs found in native samples, even though they experienced lower levels of UV-B exposure.

This is an important finding, as it suggests that there may be other influences on UAC production in pollen, particularly in trees growing outside their native environment.

We speculate that there may be a heredity component of UAC production in pollen, passed down from parent trees to their offspring. As pollen is largely composed of UACs, they will exist within the pollen before UV-B exposure; the “baseline” UAC level. Trees which grow in botanical gardens and originate from seed collected from trees in their native environments, effectively have a higher baseline abundance of UACs than is needed for protection against UV-B exposure in their new environment.

Traditionally, modern calibration datasets are established for proxy records by substituting space for time. This means we use the different environmental conditions found over a wide area (or environmental gradient), in place of different environmental conditions which may have occurred over long time-scales. Our findings suggest this approach may not be valid for pollen UV-B proxies if non-native samples are included in the calibration dataset. Consequently, the modern calibration dataset should only be established using pollen samples growing in their native environment.

Our study shows that the pollen from Atlas cedar trees can be used as a proxy for UV-B levels in North Africa, which can now be used to determine historic UV-B levels over long-term timescales from fossil pollen found in sedimentary archives. Because UV-B levels may influence atmospheric climate conditions, affect plant health and the distribution of the species; an historic UV-B reconstruction may provide new insights which will help us understand these interactions and how plants respond – and this may have implications for plants under future climate change scenarios.