Geoscience Reference
In-Depth Information
2009). The results obtained by Logan & Thomas
(1987) on different Carboniferous plants were con-
sistent with this idea: guaiacyl oxidation products
were mostly detected from Sigillaria ovata, a plant
which contained woody tissues, while no guaiacyl
units were obtained from Lepidodendron and
Lepidophloios which were mostly non-woody
plants. However, these results should be regarded
with care since monolignols also are building
blocks of lignans (Lewis & Davin 1999). As dis-
cussed above, these latter compounds are wide-
spread among tracheophytes and are also present
in bryophytes (Lewis & Davin 1999; Raven 2000).
Consistently, the three lignin phenols were observed
(although in low amounts) in the oxidation products
of different bryophytes (Logan & Thomas 1985).
A second problem with lignin phenols is that,
during diagenesis, all three units degrade differen-
tly. The general order of resistance is p-coumaryl .
guaiacyl . syringyl (Hedges et al. 1985; Logan &
Thomas 1985; Orem et al. 1996). Among diage-
netic/catagenetic transformations of wood are
demethoxylations which also naturally lead to the
diminution of syringyl and guaiacyl units, favouring
p-coumaryl units in the remaining tissues (Orem
et al. 1996; Hatcher & Clifford 1997). A significant
contribution of p-coumaryl units was obtained by
Logan & Thomas (1987) in the oxidation products
of Carboniferous Sigillaria ovata. This feature
could reflect both the diagenetic increase of these
units due to decarboxylation of lignin units and
the contribution of lignan from bryophytes.
Several alkylphenols were observed in the flash
pyrolysates of the Lower Devonian plants Renalia,
Zosterophyllum and Psilophyton (Ewbank et al.
1996). Alhough these compounds may correspond
to lignin pyrolysis products, in particular after
diagenetic demethoxylation of lignin, their presence
in the pyrolysates of Lower Devonian plants does
not unequivocally attest for the presence of lignin
in these early plants; they could also derive from
the pyrolysis of condensed tannins (Ewbank et al.
1996). Despite its interest, the study of Ewbank
et al. (1996) mostly demonstrated that, since pyrol-
ysis products are poorly characteristic, flash pyrol-
ysis is not suited to molecularly characterize the
material of early land plants.
The development of spores (and pollen) is an
important requisite for terrestrialization since it
enables dispersion of gametophytes through air.
The effective UVB absorbance of aromatic rings
(Pf¨ndel et al. 2006) may play a role in protecting
airborne pollen; variations in the aromatic content
of fossil pollen have been proposed as a UVB
proxy (Rozema et al. 2001, 2002b). However, this
proxy is based on pyrolysis of the (fossil) pollen.
The coumaric and ferulic moieties produced in
this way are probably derived from sporopollenin-
type biopolymers in the pollen wall and not
derived from compounds believed to regulate UV
damage (de Leeuw et al. 2006).
The structure of sporopollenin, the wall polymer
of pollen and spores, has long been a matter of
debate and it seems likely that both aliphatic and
aromatic sporopollenins occur (de Leeuw et al.
2006). Although the structure of the aliphatic spor-
opollenins is unclear, the aromatic sporopollenin
consists of coumaric, ferrulic and sinapic acids
(VII-IX) as building blocks. These are the same
building blocks for lignin but with propyl-acids
in stead of propyl-alcohols (Boom et al. 2005).
Biosynthetically, the lignols are formed from these
carboxylic acids by reduction and it is interesting
to note that, prior to the evolution of lignin syn-
thesis, plants already were able to produce the
biopolymer sporopollenin.
Recently, it has been demonstrated that one of
the key enzymes in the phenylpropanoid pathway
needed to convert the phenylpropanoid acids into
their alcohols, 4-coumarate:CoA ligase (4CL)
(Ferrer et al. 2008) already occurs in the Bryophyte
Physcomitrella patens (Silber et al. 2008). The pres-
ence of a similar enzyme cinnamate:CoA ligase
(ScCCL) in the bacterium Streptomyces coelicolor
(Kaneko et al. 2003) suggests that this part of the
pathway has a much longer history than previously
expected. It would be interesting to know to what
extent the phenylpropanoid pathway had to evolve
in order to arrive at sporopollenin synthesis and
further to lignin biosynthesis.
Another open question is why, later in evolution,
lignin and not sporopollenin became a major
structural element in vascular plants. Although
apparently the earliest land plants already produced
spores, it is not known where in evolution the
synthesis of sporopollenin started. Sporopollenin
has been claimed to be produced by Coleochaete
(Delwiche et al. 1989), a member of the Characeae
which is a sister group of Embryophytes (Waters
2003). However, due to a lack of insight in the
nature of sporopollenin in the past and despite
increasing insight into the nature of acid- (and
acetolysis-) resistant algal walls (e.g. de Leeuw
et al. 2006) this has not yet been resolved. One
method of shedding light on sporopollenin evol-
ution could be a systematic analysis of the structural
diversity (if any) of sporopollenins of primitive land
plants and their closest relatives.
Organic matter transformation
Composition and preservation
An important step in investigating the relation
between fossil organic matter and its source organ-
isms from a chemical point of view is assessing
