ECR feature: Chemical variations in fossil pollen with Florian Muthreich

Florian Muthreich is a PhD student in the Department of Biological Sciences at the University of Bergen, focusing on the application of chemical variations in fossil pollen to understand ecosystem dynamics. Florian’s work focuses on identifying Oak (Quercus) pollen in sediment cores – which is particularly difficult to differentiate with a light microscope – at species level.

Florian in the overgrown remains of an old monastery along the coast in Portugal.

Links: Research Gate | Google Scholar | Twitter | Lab Group

Institution: University of Bergen, Department of Biological Sciences

Current academic life stage: PhD

Research interests: I am interested in understanding the application of chemical variations in fossil pollen to understand ecosystem dynamics in sediment cores.

Current study system: I am currently studying the chemistry of pollen grains with the long term goal to use chemical methods to improve identification of pollen grains in sediment records. For most of my PhD I focused on Oak (Quercus) pollen, because oaks are an integral and important part of European and especially Mediterranean forests. One difficulty is that Quercus pollen is quite hard to differentiate using a light microscope. This means that past distributions of oaks are currently only studied on sub-genus or even genus level and chemical methods have the potential to improve our ability to discern Quercus pollen at the species level.

Recent paper in Journal of Biogeography: Muthreich, F., Zimmermann, B., Birks, H.J.B., Vila-Viçosa, C.M. and Seddon, A.W.R. 2020. Chemical variations in Quercus pollen as a tool for taxonomic identification: implications for long-term ecological and biogeographic research. Journal of Biogeography 47: 1298-1309.

Motivation for the paper: We wanted to explore the variability of pollen chemistry in fresh Quercus pollen using a large dataset sampled from a variety of environmental conditions. Other studies have utilised a large number of different species and showed the potential to differentiate pollen using their chemical composition, but often used a limited number of samples. We therefore aimed to collect pollen from a large number of different trees all over Portugal to see how this affects our ability to differentiate the species.

Key methodologies: We used fourier transformed infrared spectroscopy (FTIR) to record the chemical composition of the pollen. This is a fairly new and exciting method to analyse pollen and other studies showed great potential to separate pollen using their chemical composition. Analyses with FTIR produce large multivariate datasets, because FTIR records the absorbance of the sample over the entire infrared region. Different chemical compounds (lipids, protein, etc) have peaks at specific wavelengths and give differences in relative amounts of these compounds in the sample. Spectral datasets require multivariate methods for data analysis and we opted to use a variant of partial least squares regression (PLSR) to test the classification performance.

(left) Cork oak (Quercus suber) tree in a public park in Lisbon. (right) Mixed forest with cork oak (Quercus suber), kermes oak (Quercus coccifera) and Portuguese oak (Quercus faginea).

Unexpected challenges: We were quite surprised how variable the chemical composition of Querucs pollen was. Previous studies had shown that the lipid component was quite variable with climatic conditions (temperature, precipitation, etc), but we also found considerable variation in the composition of the grain wall of pollen. The pollen grain wall consists of sporopollenin, a polymer largely resistant to chemical degradation, which is the reason why pollen can be recovered from lake and bog sediments and used for past reconstructions. The exact structure of sporopollenin is unknown, but our results suggest that there is quite a bit of variability in sporopollenin chemistry within the same genus.

Major result and contribution to the field: We were able to show that identification using FTIR methods had similar performance to scanning electron microscopy (SEM), which is used to identify Quercus pollen into three sub genus sections. We were able to do the same with our FTIR methods and a relatively simple PLS model. At species level we had some success at separating the different Quercus species (~70% recall). Another result from our study was the amount of variability in the spectra we observed, both within and between species. The variability was not just contained to lipid content of the pollen, but also the sporopollenin content. This suggests a difference in grain wall chemistry between the three sub-genus sections of Quercus, and indicates that sporopollenin chemical composition varies between species.

What are the next steps? The next step for this research is to apply our FTIR methods to actual fossil pollen and see how it performs. There are several challenges to be addressed: i.) we aim to extract fossil pollen without any aggressive chemical treatment. ii.) we expect that most of the fresh components in pollen we relied on in our Quercus study (lipids, etc) are not present in fossil pollen. iii.) FTIR may not be ideal to resolve detailed chemical changes in sporopollenin (we are exploring other IR methods, such as Raman). These challenges are a great motivation for my future research.

If you could study any organism on Earth, what would it be and why?
I feel very fortunate to work on such an exciting project that is ambitious and trying to push the boundaries of pollen identification. Pollen are incredibly diverse and I have explored only a tiny fraction of what pollen has to offer. For now I am quite happy with what I am doing and want to apply our methods to additional pollen types, besides oaks.

Any other little gems you would like to share? Most pollen is yellow, but there are some species that have orange, red or even white coloured pollen. Quercus pollen is bright yellow. Most flavonoids absorb UV light and it is believed that flavonoids developed in pollen to protect the genetic material from the mutagenic UV-radiation. John Flenley, who we dedicated our article to, wrote about this in his article “Why is pollen yellow? And why are there so many species in the tropical rain forest?” (2011) in Journal of Biogeography: 38(5).

Valley in north eastern Portugal with olive trees Olea sp., cork oaks (Quercus suber) and Portuguese oaks (Quercus faginea)

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