Geoscience Reference
In-Depth Information
Aliphatization and related problems
by aliphatic moieties (van Bergen et al. 1995),
were originally aliphatic or have become so
by aliphatization.
For initially aliphatic biomacromolecules such
as cutin, cutan and algaenan, the post mortem ali-
phatization is intrinsically much more difficult to
detect. The incorporation of free lipids to the natu-
rally resistant algaenan of Botryococcus race A by
oxidative cross-linking has been clearly demon-
strated in coorongite (Gatellier et al. 1993), a
rubbery material derived from the accumulation of
algal remains on the shores of lakes. As Botryococ-
cus free lipids and algaenans were both aliphatic, the
aliphaticity of coorongite was very similar to that of
the algaenan; however, the signature upon pyrolysis
was significantly different (Gatellier et al. 1993).
For Botryococcus braunii race B, the algaenan
walls also incorporate polyacetals of polymethyls-
qualanes. This may provide a clear marker for the
presence of cell walls of this taxon in sediments
(Metzger et al. 2007). However, in this case assess-
ment of the degree of change of the original biopo-
lymer by the post mortem oxidative polymerization
of membrane and other associated free lipids also
remains problematic.
One of the classical examples of the selective
preservation pathway is the algaenan of fossil Tetra-
edron envelopes from the Messel Oil Shale. Apart
from being strikingly well-preserved morphologi-
cally, the chemical fingerprint of these cuticles
upon flash pyrolysis closely resembles its modern
counterpart (Goth et al. 1988). But what difference
would a contribution of aliphatic lipids from the
organism have made? Similarly, oxidative polymer-
ization of aliphatic lipids has been suggested to have
played a role in the formation of the aliphatic algae-
nan of the Ordovician alga Gloeocapsamorpha
prisca (Blokker et al. 2001), but it is difficult
to ascertain to what extent this aliphatic material
corresponds to the original cell walls.
An analogous problem is illustrated on cutin and
cutan. The ether cross-linked cutan of CAM plants
is, chemically speaking, more stable than ester
cross-linked cutin of most other higher plants:
cutin is broken down into its original monomers
upon base hydrolysis while cutan resists this treat-
ment. Since the fossil plant cuticles also survive
base hydrolysis, they have been considered to rep-
resent selectively preserved cutan (Tegelaar et al.
1989c). However, fossil non-hydrolysable cuticles
are known from plants that do not produce cutan
(Gupta et al. 2006b, 2007b; de Leeuw 2007). In
fact, the depositional environment of CAM plants
does not at all favour cutan preservation whereas
several cutin-producing plants occur in or near
excellent preservational environments. Laboratory
experiments using elevated temperature and
pressure have recently demonstrated that, similar
Studies on the macromolecular nature of Palaeozoic
and older acritarchs have shown both aliphatic
and aromatic wall compositions (Kjellstr¨m 1968;
Collinson et al. 1994; Arouri et al. 1999, 2000;
Foster et al. 2002; Dutta et al. 2006). Others have
concentrated on the biomarker lipids associated
with the acritarchs (Moldowan & Talyzina 1998;
Talyzina et al. 2000) and the host sediments (Meng
et al. 2005), and have drawn conclusions on the
biological affinities of the acritarchs. What are the
consequences for the application of organic geo-
chemistry to elucidating the terrestrialization of
life?Considering both the diagenetic and catagenetic
processes, attributing an aromatic or aliphatic con-
tribution to an original biomacromolecular structure
remains problematic. As long as the lipids which
have become incorporated in the macromolecular
matrix post mortem have been derived from the
source organisms themselves, the approach of care-
fully releasing and analysing these lipids seems to
be the more successful approach.
Analogous to the transformation of chitinous bio-
molecules into aliphatic geomolecules (see above),
sporopollenin and other biomacromolecules seem
to transform chemically over time. Whereas fresh
megaspores of Isoetes and Salvinia are purely
aromatic, the fossil material consists of a mixture
of aliphatic and aromatic moieties, again suggesting
addition of long-chain aliphatic compounds (van
Bergen et al. 1993; Boom 2004). Furthermore, the
cyst walls of the recent dinoflagellate Lingulodinium
polyedrum seem to be non-aliphatic (Kokinos et al.
1998) whereas fossil dinoflagellate cysts have been
reported to contain mixed aromatic and aliphatic
moieties (de Leeuw et al. 2006).
An extreme case of aliphatization by conden-
sation of aliphatic lipids has been described for
'dinocasts' from the Eocene of Pakistan: the rela-
tively solid to spongy dinoflagellate-shaped struc-
tures occurring in the sediments are believed to
represent the oxidatively polymerized cell contents
of motile dinoflagellates (Versteegh et al. 2004).
Although addition of aliphatic components modifies
the signature of several aromatic biomacromole-
cules (chitin, sporopollenin) such processes seem
to be absent for fossil lignin. This may result from
the fact that, in most cases, the membrane lipids
are very closely located to the biomacromolecule;
in lignin there are no lipids around. As such, this
may be an indirect and circumstantial piece of evi-
dence for the oxidative polymerization pathway.
It is not only aliphatics which are subject to oxi-
dative polymerization. This process also applies to
the terpenoids in resins, leading to resin hardening
and amber formation. One may wonder to what
extent
the oldest ambers, which are dominated
