Ultrastructural and chemical study of modern and fossil sporoderms by Scanning Transmission X-ray Microscopy (STXM)

Abstract

Diverse microscopy techniques have been extensively used in the past to study the ultrastructure of modern and fossil sporoderms for various purposes such as taxonomic identification or study of sporoderm formation and its evolution in different groups of plants. Optical microscopy and Scanning and Transmission Electron Microscopies (SEM and TEM) have been particularly useful in providing structural information at the μm- and nm-scales. In parallel, chemical analyses have been conducted on bulk samples to document variations in sporopollenin composition between major groups and compositional evolution/preservation during coalification processes . Here, we describe a microscopy tool, Scanning Transmission X-ray Microscopy (STXM), which offers valuable capabilities for palynological studies as demonstrated by applications of this technique to fossil spores over the past decade. This microscopy technique, which uses soft X-rays produced by synchrotron radiation sources, provides 25-nm spatial resolution element-based image contrast, and K or L near-edge X-ray absorption fine structure (NEXAFS) spectra at each 25-nm x 25-nm pixel in a STXM image for a number of key elements (e.g., K-edges of C, N, O, F, Na, Mg, Al, and Si and L-edges of K, Ca, Ti-Zn, Ga, Ge, As, Se, Rb, and Sr). STXM can be performed in situ on thin samples with no need for prior chemical extraction or staining. Comparison with conventional imaging techniques such as optical microscopy and TEM shows that STXM is an ideal complement to these microscopies. We present new STXM data obtained on modern Lilium longiflorum Thunb. pollen grains and on well-preserved lycopsid spores, identified as Annalepis zeilleri (Fliche) Grauvogel-Stamm and Duringer, collected from the Lettenkohle Formation of the Wasselonne Quarry (NE France). Using this technique, different biochemical compounds such as sporopollenin or cellulose can be specifically imaged at the 25-nm scale. Additionally, NEXAFS spectra can provide information on elemental speciation, i.e. the types of C-containing functional groups (e.g., aromatic, aliphatic, carboxylic, alcoholic…) and the redox state of iron and manganese. Based on these analyses, it is possible to document the chemical preservation of fossil spores that experienced diagenesis and possibly metamorphism.