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

Phillip E Jardine,Barry H Lomax,Huajie He,Wesley T Fraser,Guillaume Dupont‐Nivet,William D Gosling,Natasha Barbolini,Huasheng Huang,Carina Hoorn,Amber Woutersen,Maxine A.M Beer,Giovanni Bogota‐Angel,Matteo Sciumbata

doi:10.1111/pala.12523

Abstract

AbstractSporopollenin 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.

Highlights

Sporopollenin is a highly resistant biopolymer that forms the outer wall of pollen and spores
Despite the high potential of these methods, there are a series of unresolved issues linked to the feasibility of integrating chemical datasets derived from extant and fossil sporomorphs. (Note that here we use the term ‘fossil’ to denote sporomorphs that have been recovered from the geological record, rather than inferring any fossilization processes per se.) These issues relate mostly to how sporopollenin chemistry changes as a function of diagenesis within natural sedimentary systems, and to the laboratory methods used to isolate sporopollenin from fresh sporomorphs
Our results demonstrate a mix of promising aspects and challenges for sporopollenin chemistry-based palynology

Summary

Introduction

Sporopollenin is a highly resistant biopolymer that forms the outer wall of pollen and spores (sporomorphs). 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 We explore these issues using Fourier transform infrared (FTIR) microspectroscopy data from extant and fossil grass, Nitraria (a steppe plant), and conifer pollen. (Note that here we use the term ‘fossil’ to denote sporomorphs that have been recovered from the geological record, rather than inferring any fossilization processes per se.) These issues relate mostly to how sporopollenin chemistry changes as a function of diagenesis within natural sedimentary systems, and to the laboratory methods used to isolate sporopollenin from fresh sporomorphs Resolving these two issues is essential if fossil material is to be classified or analysed in the context of training sets or calibrations based on extant sporomorphs

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Conclusion