Molecular Decay of Plant Biopolymers - Biopolymers: A Molecular Paleontology Approach - page 2

Introduction
Metasequoia Miki ex Hu and Cheng (dawn redwood) was fi rst described as a fossil
(Miki 1941 ) before its endangered living species M. glyptostroboides Hu and Cheng
was discovered in central-southwest China (Hu and Cheng 1948 ; Fulling 1976 ). It
has a long and well documented fossil record, displaying remarkable anatomical
and morphological stasis since the Cretaceous (Chaney 1951 ; Liu et al. 1999 ; Yang
and Jin 2000 ; LePage et al. 2005 ). Some Metasequoia fossil remains are among the
best preserved plant fossil material (Yang et al. 2005 , 2007 ) and its molecular dia-
genesis and preservation can be used to explore the relative preservation potential of
cutin, lignin and cellulose.
Hedges and Weliky ( 1989 ) physically isolated intact needles of fi r, hemlock and
cedar from different horizons of a recent sediment core from a coastal marine bay, and
from nearby trees and forest litter; they demonstrated a greater preservation potential
of lignin than polysaccharides, as observed in other studies (Hedges et al. 1985 ;
Kelleher et al. 2006 ). While observing a rapid degradation of carbohydrates during
initial decay, Kelleher et al. ( 2006 ) also noted the stability of aliphatic components
from leaf cuticle. Some studies have shown, however, that cutin, an important ali-
phatic component of cuticle, has a lower preservation potential than lignin (Goñi and
Hedges 1990 ; Opsahl and Benner 1995 ), in which case fossil leaves should primarily
reveal an aromatic signal (derived from lignin, polysaccharides and their diageneti-
cally altered components) and not an aliphatic one (cuticular components). However,
leaf fossils invariably reveal a composition with a signifi cant aliphatic component,
with or without the presence of biopolymeric lignin and/or polysaccharides.
In order to understand the preservation of the primary structural biopolymers (lig-
nin, cutin, and cellulose), Metasequoia leaf samples which had decayed in the natural
environment over a year or more were analysed chemically and microscopically to
evaluate the preserved biomolecules and their relative preservation potential (Bland
et al. 1998 ; Clifford et al. 1995 ; Eglinton and Hamilton 1967 ; Goñi and Hedges 1992 ;
Haddad et al. 1992 ; Kolattukudy 1980 ; Landais et al. 1989 ; Nip et al. 1986 ) and then
compared to the chemistry of fossils from the Miocene of Clarkia (Idaho State, USA),
and Eocene of Republic (Washington State, USA) and Axel Heiberg (Canadian Arctic
Archipelago). Heating experiments were conducted on modern Metasequoia in gold
capsules to evaluate the transformation of these biopolymers for comparison with the
composition of fossils and sedimentary organic matter.
Natural Decay
A series of leaf samples of Metasequoia glyptostroboides representing a sequence
of various stages of tissue development and decay (referred to hereafter as “decay
series samples”) was collected from the Mt. Auburn Cemetery (42.6 N, 71.2 E),
MA, U.S.A. during fall through winter of 2004. This tree, Mt. Auburn Cemetery
Accession #19510001, was planted at the northern bank of the Willow Pond in 1951