Geoscience Reference
In-Depth Information
Organic matter analysis: lipids and
macromolecules
From an analytical point of view, bio- and geomole-
cules can be subdivided into two types of organic
substances. The first type consists of relatively
small molecules which dissolve in common organic
solvents and form the lipids. These lipids can be
analysed relatively easily using for example, gas
or liquid chromatography. Examples are archaeal
membrane lipids, higher plant cuticular waxes,
long-chain alkenones from Haptophyta and steroids
such as dinosterol from dinoflagellates, etc. Those
lipids that have a relatively low biodegradability
may fossilize as such and can be applied to recon-
struct past environments and the (early) evolution
of life (e.g. Brocks & Summons 2003).
Apart from this, lipids and lipid ratios can be
used to reconstruct the environment such as for
long chain alkenones (Marlowe 1984; Brassell
et al. 1986; Prahl & Wakeham 1987; Conte et al.
2006) and of archaeal glycerol dibiphytanyl
glycerol tetraether membrane lipids (De Rosa &
Gambacorta 1988; Schouten et al. 2003; Kim
et al. 2008). These biomolecules can also become
diagenetically or thermally modified but, as long
as the resulting products can be reliably related to
their source organisms, they may still provide imp-
ortant clues on past life and environment. For
example, triaromatic dinosteroids are derived from
the thermal modification of the dinoflagellate steroid
dinosterol. Their presence in Palaeozoic sediments
has been used as an argument for a Palaeozoic
rather than Mesozoic origin of the dinoflagellates
(Moldowan & Talyzina 1998; Empt 2004).
The second type of molecules is of macromol-
ecular nature and therefore insoluble in most
solvents. In living organisms, the most abundant
macromolecules are proteins and polysaccharides.
The insoluble organic matter in the sediments, also
termed 'kerogen', is poorly understood despite
being by far the largest organic carbon pool on
Earth (Berner 1989; Vandenbroucke & Largeau
2007) including all the particulate organic matter
we see with the naked eye and through micro-
scopes such as leaf and arthropod cuticles and
palynomorphs.
This kind of material provides considerable
analytical problems with respect to structural eluci-
dation and quantification. Non-destructive methods
provide important structural information on the
atomic level such as nuclear magnetic resonance
(NMR) (Deshmukh et al. 2005) and at the level of
functional groups such as (micro) Fourier transform
infra-red (FTIR) spectroscopy (Marshall et al. 2005;
Versteegh et al. 2007). Destructive methods frag-
ment the macromolecules and these fragments also
provide vital information needed to reconstruct the
original macromolecule. Of these, chemical degra-
dation applies a series of chemical treatments
whereby each successive treatment is able to break
stronger chemical bonds than the previous treatment
(Hunt et al. 1986; Gelin 1996; Blokker et al. 1998).
Pyrolysis breaks down the molecule thermally in an
inert atmosphere (Maters et al. 1977; Nip et al.
1987), however. For the characterization of fossil
macromolecular organic matter and its preservation
pathways, it is essential to combine several of these
techniques. Despite these problems, studying this
material is worthwhile (e.g. Briggs et al. 2000).
The biochemical signal from
terrestrialization
Desiccation management - long-chain
aliphatics
Simple lipids-waxes. Protection against a lack of
water is of prime importance for land organisms.
This can be achieved by resisting desiccation
to keep a positive water balance, for example, by
erecting an evaporative barrier at the organism
surface and/or by developing desiccation tolerance.
It seems reasonable to assume that adaptations to
desiccation developed prior to the terrestrialization
of plants and algae. It might be an important adap-
tation for freshwater species or species that live in
smaller enclosed and coastal habitats to enable
them to resist periods of dryness and allowing
spreading from one watershed to another (e.g. by
wind and animals).
The earliest photosynthetic organisms on land
were probably cyanobacteria possibly colonizing
land as early as 2.6 Ga ago (Watanabe et al. 2000).
They may have been present as single cells or fila-
ments and, with time, colonized a variety of environ-
ments such as tidal zones, soils and desert crusts.
Although emerging much later, this also accounts
for photosynthetic microalgae such as Chlorophyta,
Streptophyta, Bacillariophyta (diatoms), Eustigma-
tales and Dinoflagellata. Generally, these taxa
survive dryness by tolerating dehydration as such,
or by producing resting cysts (e.g. Zygnemataceae).
Mostly they use sugars, lipids or proteins to stabilize
the cell contents upon desiccation (Cardon et al.
2008). To our knowledge, there is no chemical fin-
gerprint known of upon which this strategy may
be reconstructed from the fossil record. Stable
carbon and hydrogen isotopes of lipids can perhaps
help here.
The sister group of the Chlorophyta, the Strepto-
phyta, gave rise to the Embryophytes (land plants),
the only group which developed into macroscopic
organisms with strategies to cope with the uneven
and erratic water supply on land. The very first
