New highlights of phytolith structure and occluded carbon location: 3-D X-ray microscopy and NanoSIMS results

Abstract

Abstract. Phytoliths contain occluded organic compounds called phytC. Recently, phytC content, nature, origin, paleoenvironmental meaning and impact in the global C cycle have been the subject of increasing debate. Inconsistencies were fed by the scarcity of in situ characterizations of phytC in phytoliths. Here we reconstructed at high spatial resolution the 3-D structure of harvested grass short cell (GSC) phytoliths using 3-D X-ray microscopy. While this technique has been widely used for 3-D reconstruction of biological systems it has never been applied in high-resolution mode to silica particles. Simultaneously, we investigated the location of phytC using nanoscale secondary ion mass spectrometry (NanoSIMS). Our data evidenced that the silica structure contains micrometric internal cavities. These internal cavities were sometimes observed isolated from the outside. Their opening may be an original feature or may result from a beginning of dissolution of silica during the chemical extraction procedure, mimicking the progressive dissolution process that can happen in natural environments. The phytC that may originally occupy the cavities is thus susceptible to rapid oxidation. It was not detected by the NanoSIMS technique. However, another pool of phytC, continuously distributed in and protected by the silica structure, was observed. Its N / C ratio (0.27) is in agreement with the presence of amino acids. These findings constitute a basis to further characterize the origin, occlusion process, nature and accessibility of phytC, as a prerequisite for assessing its significance in the global C cycle.

Interplay between silica deposition and viability during the life span of sorghum silica cells

“…Such structures may explain the difficulty in isolating intact bilobate phytoliths using a harsh extraction method (Watling et al ., ), and the apparent cracks or fragments within the silica structure (Blackman, ). Voids within individual silica cells are interconnected and open to the exterior (Alexandre et al ., ), further supporting this point.…”

Mechanism of silica deposition in sorghum silica cells

“…The condensation of silicic acid at the cell wall may create a Si‐concentration gradient within the leaf, resulting in a flow of silicic acid towards the silica cell. Voids that are left within the silica cell (Laue et al ., ; Alexandre et al ., ; Gallagher et al ., ) may contain cell debris resulting from the constricted protoplasm. In the sblsi1 mutant, where leaf silicifcation machinery is intact but silicic acid availability is limited, mature cells probably undergo programmed cell death and their contents are cleared rendering the cells incapable of accumulating and polymerizing silica.…”

Silicification in Grasses: Variation between Different Cell Types

“…Organic matter with N/C ratio indicative of amino acids is continuously distributed in wheat silica cells (Alexandre et al, 2015), suggesting possible role of proteins in the templating process, similar to diatoms (Kröger et al, 1999). Depending upon the phytolith extraction process, some of nitrogen detected may result from the use of nitric acid in the extraction process or represent acid hydrolyzed proteins occluded within the silica structure (Watling et al, 2011), even without their direct participation in silicification.…”