TY - JOUR

T1 - Changes of aliphatic C–H bonds in cyanobacteria during experimental thermal maturation in the presence or absence of silica as evaluated by FTIR microspectroscopy

AU - Igisu, Motoko

AU - Yokoyama, Tadashi

AU - Ueno, Yuichiro

AU - Nakashima, Satoru

AU - Shimojima, Mie

AU - Ohta, Hiroyuki

AU - Maruyama, Shigenori

N1 - Publisher Copyright: © 2018 John Wiley & Sons Ltd

PY - 2018/7/1

Y1 - 2018/7/1

N2 - 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.

AB - 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.

KW - INDIVIDUAL PRECAMBRIAN MICROFOSSILS

KW - EARTHS EARLIEST FOSSILS

KW - LASER-RAMAN IMAGERY

KW - HOT-SPRING SINTER

KW - BLUE-GREEN-ALGAE

KW - ORGANIC-MATTER

KW - EXPERIMENTAL SILICIFICATION

KW - SELECTIVE PRESERVATION

KW - WESTERN-AUSTRALIA

KW - EXPERIMENTAL FOSSILIZATION

KW - Carbon/metabolism

KW - Hot Temperature

KW - Spectroscopy, Fourier Transform Infrared

KW - Organic Chemicals/metabolism

KW - Fossils

KW - Silicon Dioxide/metabolism

KW - Hydrogen/metabolism

KW - Synechocystis/growth & development

UR - http://www.scopus.com/inward/record.url?scp=85048509810&partnerID=8YFLogxK

U2 - 10.1111/gbi.12294

DO - 10.1111/gbi.12294

M3 - Article

C2 - 29869829

AN - SCOPUS:85048509810

VL - 16

SP - 412

EP - 428

JO - Geobiology

JF - Geobiology

SN - 1472-4677

IS - 4

ER -