Sporopollenin chemistry and its durability in the geological record: an integration of extant and fossil chemical data across the seed plants

Abstract

Sporopollenin is a highly resistant biopolymer that forms the outer wall of pollen and spores (sporomorphs). Recent research into sporopollenin chemistry has opened up a range of new avenues for palynological research, including chemotaxonomic classification of morphologically cryptic taxa. However, there have been limited attempts to directly integrate extant and fossil sporopollenin chemical data. Of particular importance is the impact of sample processing to isolate sporopollenin from fresh sporomorphs, and the extent of chemical changes that occur once sporomorphs enter the geological record. Here, we explore these issues using Fourier transform infrared (FTIR) microspectroscopy data from extant and fossil grass, Nitraria (a steppe plant), and conifer pollen. We show a 98% classification success rate at subfamily level with extant grass pollen, demonstrating a strong taxonomic signature in isolated sporopollenin. However, we also reveal substantial chemical differences between extant and fossil sporopollenin, which can be tied to both early diagenetic changes acting on the sporomorphs and chemical derivates of sample processing. Our results demonstrate that directly integrating extant and late Quaternary chemical data should be tractable as long as comparable sample processing routines are maintained. Consistent differences between extant and deeper time sporomorphs, however, suggests that classifying fossil specimens using extant training sets will be challenging. Further work is therefore required to understand and simulate the effects of diagenetic processes on sporopollenin chemistry.

Dying in the Sun: Direct evidence for elevated UV-B radiation at the end-Permian mass extinction

“…6 and data S1) (28). However, repolymerization of the sporopollenin biopolymer is known to take place in the geologic record, altering the sporopollenin chemical structure and decreasing the prominence of the 1510cm −1 aromatic peak in FTIR spectra (29)(30)(31). Consistent with these previous findings, the FTIR spectra generated from the Alisporites grains studied here are substantially different from those of extant material, with larger peaks centered on 1600 cm −1 (C═C stretching) and 1360 cm −1 (symmetric CH 3 bending) and no clear 1510-cm −1 peak (Fig.…”

“…A challenge with analyzing sporopollenin chemistry in deep time is the repolymerized nature of the material, meaning that we are working with a geopolymer rather than a biopolymer ( 31 , 47 ), and we acknowledge that we are measuring more subtle chemical variations than with extant or subfossil sporomorphs, or fossil sporomorphs from thermally immature locations. Nevertheless, we argue that detecting even subtle chemical variations across the PTB is significant, especially because the inferred increases in UACs are consistent with both empirical evidence from aberrant sporomorphs ( 12 , 13 ) and theoretical expectations from modeling experiments ( 11 ), and we have therefore interpreted these chemical shifts as providing tantalizing direct evidence for changes in UV-B flux and ozone thickness across the PTB.…”

Influence of common palynological extraction treatments on ultraviolet absorbing compounds (UACs) in sub-fossil pollen and spores observed in FTIR spectra

“…It is therefore necessary to use sub-fossil samples to investigate directly the impact of chemical treatment on sporopollenin since this is highly relevant for studies on sub-fossil pollen and spores from bogs, lakes and other deposits. In a later study, Jardine et al (2021) examined the informative "fingerprint" area (1800-1,000 cm −1 ) of the FTIR spectrum in multiple samples from different case studies that included processing using different chemical treatment protocols (including by HCl, KOH and acetolysis); thus, questions remain about whether any of the noted spectral differences might result from the use of chemical treatments, or simply due to species-specific differences in pollen chemistry, or between modern and sub-fossil samples. Jardine et al (2021) raised several perspectives from this study: (1) the extent of chemical impacts on sub-fossil samples is still unclear; (2) chemical changes over short geological timescales have to be considered when explaining such spectral difference; (3) further experiments were needed to disentangle the impacts of different processing reagents.…”

“…One approach is a "physical based" methodology of embedding pollen samples into a soft paraffin matrix medium to avoid scattering caused by the spherical boundary of palynomorph grains (Zimmermann et al, 2016;Diehn et al, 2020). Alternatively, multi-grain measurement using a larger FTIR microscopy aperture size could also diminish spectral artifacts caused by spherical shape of individual grain (Jardine et al, 2021). In contrast to "physical based" methodology, Konevskikh et al (2018) applied mathematical models on individual spectra from biological cells to explain all sample effects including Mie scattering.…”

Interactive effects of changes in UV radiation and climate on terrestrial ecosystems, biogeochemical cycles, and feedbacks to the climate system

“…However, before it can be reliably used it is necessary to ascertain the action spectrum of its response to UV radiation, its rate of degradation, the response time of its synthesis, as well as the consistency of response among species and over time [ 202 ]. In the case of fossilised pollen from Nitraria (a steppe plant) and conifers, chemical signatures have been shown to differ from those of contemporary (extant) pollen in a predictable and consistent manner such that stable relationships can be modelled [ 203 ].…”