Changes of aliphatic C–H bonds in cyanobacteria during experimental thermal maturation in the presence or absence of silica as evaluated by FTIR microspectroscopy

Motoko Igisu, Tadashi Yokoyama, Yuichiro Ueno, Satoru Nakashima, Mie Shimojima, Hiroyuki Ohta, Shigenori Maruyama

Research output: Contribution to journalArticlepeer-review

12 Citations (Scopus)

Abstract

Aliphatic C–H bonds are one of the major organic signatures detected in Proterozoic organic microfossils, and their origin is a topic of interest. To investigate the influence of the presence of silica on the thermal alteration of aliphatic C–H bonds in prokaryotic cells during diagenesis, cyanobacteria Synechocystis sp. PCC6803 were heated at temperatures of 250–450°C. Changes in the infrared (IR) signals were monitored by micro-Fourier transform infrared (FTIR) spectroscopy. Micro-FTIR shows that absorbances at 2,925 cm−1 band (aliphatic CH2) and 2,960 cm−1 band (aliphatic CH3) decrease during heating, indicating loss of the C–H bonds, which was delayed by the presence of silica. A theoretical approach using solid-state kinetics indicates that the most probable process for the aliphatic C–H decrease is three-dimensional diffusion of alteration products under both non-embedded and silica-embedded conditions. The extrapolation of the experimental results obtained at 250–450°C to lower temperatures implies that the rate constant for CH3 (kCH 3) is similar to or lower than that for CH2 (kCH 2; i.e., CH3 decreases at a similar rate or more slowly than CH2). The peak height ratio of 2,960 cm−1 band (CH3)/2,925 cm−1 band (CH2; R3/2 values) either increased or remained constant during the heating. These results reveal that the presence of silica does affect the decreasing rate of the aliphatic C–H bonds in cyanobacteria during thermal maturation, but that it does not significantly decrease the R3/2 values. Meanwhile, studies of microfossils suggest that the R3/2 values of Proterozoic prokaryotic fossils from the Bitter Springs Group and Gunflint Formation have decreased during fossilization, which is inconsistent with the prediction from our experimental results that R3/2 values did not decrease after silicification. Some process other than thermal degradation, possibly preservation of specific classes of biomolecules with low R3/2 values, might have occurred during fossilization.

Original languageEnglish
Pages (from-to)412-428
Number of pages17
JournalGeobiology
Volume16
Issue number4
DOIs
Publication statusPublished - 1 Jul 2018

Keywords

  • INDIVIDUAL PRECAMBRIAN MICROFOSSILS
  • EARTHS EARLIEST FOSSILS
  • LASER-RAMAN IMAGERY
  • HOT-SPRING SINTER
  • BLUE-GREEN-ALGAE
  • ORGANIC-MATTER
  • EXPERIMENTAL SILICIFICATION
  • SELECTIVE PRESERVATION
  • WESTERN-AUSTRALIA
  • EXPERIMENTAL FOSSILIZATION
  • Carbon/metabolism
  • Hot Temperature
  • Spectroscopy, Fourier Transform Infrared
  • Organic Chemicals/metabolism
  • Fossils
  • Silicon Dioxide/metabolism
  • Hydrogen/metabolism
  • Synechocystis/growth & development

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