Characterizing the Nature and Distribution of Phytolith Organic Matter Using Raman Spectroscopy

Abstract:

Many plants, including grasses and some important human food sources, accumulate and precipitate silica in their cells to form opaline phytoliths. These phytoliths contain small amounts of organic matter (OM) that is trapped during the process of silicification and protected from oxidation. If this OM is derived solely from photosynthesis during the life of the plant, it should preserve an isotopic signature of the atmosphere and the type of photosynthetic pathway [1]. However, radiocarbon dating of this OM gives an older age than expected [2], and studies on modern plants indicate a soil contribution to phytolith OM [3,4]. Thus, a better understanding of the role of phytolith OM and the silica precipitation mechanism is needed. Previous work has suggested that plant silica is associated with compounds such as proteins, lipids, lignin and phenol-carbohydrate complexes [5-7]. It is not known whether these compounds are cellular components passively encapsulated as the cell silicified, polymers actively involved in the precipitation process or random compounds assimilated by the plant and discarded into a "glass wastebasket". Here, we used Raman spectroscopy to map individual phytoliths isolated from Sorghum bicolor plants. We showed that OM in phytoliths is heterogeneously distributed and not related to optical features (i.e. dark spots or holes visible in light microscopy) commonly thought to be the repository for phytolith OM (corroborated by nanoSIMS [8]). The Raman spectra showed peaks at 2970-2960, 2945, and 2906 cm-1, indicative of C-H stretching modes, and further peaks at 1600, 1440, 1410, and 1350 cm-1 consistent with lignins and other OM. These peaks exhibited variability of relative intensities both within and between phytoliths. We will discuss these findings in the context of silica biomineralization in plants, mechanistic implications, and isotopic paleo-reconstructions using phytolith OM.

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8. Alexandre, et al., submitted.