Molecular Decay of Plant Biopolymers - Biopolymers: A Molecular Paleontology Approach - page 12

Yang et al. ( 2005 ) argued that the fi nal pyrolysis products of fossil material refl ect
both original molecular composition and tissue-specifi c degradation, suggesting that
comparative studies of molecular preservation are best performed on an individual
tissue (or organ) within an evolutionarily conserved taxonomic lineage. Indeed, SEM
investigation of Metasequoia from the Miocene of Clarkia and from the decayed sam-
ples here showed that the progressive loss of internal tissue is paralleled by the biomo-
lecular pattern observed using chemical analysis. However, some studies of fossil
leaves of other taxa have shown that this is not always the case; in some instances
morphological preservation may not be a good predictor of chemical preservation
(Logan et al. 1993 ; Mösle et al. 1998 ; Collinson et al. 2000 ).
Investigations of the preservation of plant biopolymers have used high resolution
solid state nuclear magnetic resonance (NMR) spectroscopy and microscopy
(Hedges and Mann 1979 ; Cody and Sághi-Szabó 1999 ; Kelleher et al. 2006 ), and
chemolytic methods based on the products of cutin and lignin following CuO oxida-
tion (Goñi and Hedges 1990 ; Goñi et al. 1993 ; Opsahl and Benner 1995 ).
Polysaccharides generally show a lower preservation potential than lignin (Hedges
et al. 1988 ; Kelleher et al. 2006 ). Lignin has been detected in biopolymeric form in
sediments of Pleistocene age (Hartog et al. 2004 ) and in 0.6 my old lake sediments
(Ishiwatari and Uzaki 1987 ), possibly where fungal degradation is limited (Hedges
and Mann 1979 ; Goñi et al. 1993 ; Benner et al. 1990a , b ).
In a previous study evaluating the relative preservation potential of cutin and
lignin, Goñi and Hedges ( 1990 ) collected whole green litter, fi r, hemlock and cedar
needles, from Dabob Bay (Washington State, USA) and found that cutin acids
accounted for ~3 % of the tissue in green needles, ~4 % in needle litter, 0.5-1.5 %
in sedimentary needles, and about 0.1 % of the organic carbon in bulk sediments.
Analysis of 100 year old needles of the same taxa revealed that ca. 80 % of the
original cutin acids in fresh green needles were lost. Goñi and Hedges ( 1990 )
concluded that cutin was more reactive than lignin and polysaccharides, this reactiv-
ity being facilitated by hydroxyl groups and the presence of double bonds in the
cutin structure.
Opsahl and Benner ( 1995 ) investigated long term (4 year) subaqueous decom-
position in fi ve different vascular plant tissues, including mangrove leaves and
wood ( Avicennia germinans L.), cypress needles and wood ( Taxodium distichum
(L.) Rich.) and smooth cordgrass ( Spartina alternifora Loisel). All tissues were
decomposed under identical conditions and fi nal mass losses were 97, 68, 86, 39
and 93 %, respectively, indicating extensive degradation of both cutin and lignin.
Analysis of the lignin component of herbaceous tissues, using alkaline CuO oxida-
tion, indicated that change in the lignin content was strongly dependent on tissue
type, ranging from 77 % enrichment for smooth cord grass to 6 % depletion for
cypress needles. In contrast, depletion of cutin was extensive (65-99 %) for all
herbaceous tissues.
In the present study, the total C content of Metasequoia leaves decreased by
10 % in the leaves buried in the lake sediment. Lignin and cellulose degraded faster
than cutin as revealed through tracking the pyrolysis products of cuticular compo-
nents vs. lignin and cellulose, and using microscopy to investigate tissue loss in