influence of sunlight. Other kinds of non-photosynthetic
microbial mats may exist in other environments, for example,
sulphate-reducing bacteria around deep water hydrothermal
vents (see Sect. 9.7.3).
4.2.3.3 Microbial Mats Forming a Stromatolite
How can stromatolites that can reach one metre or more in
height be formed from mats whose individual thicknesses
vary from a few millimetres to a few centimetres (see Sect.
9.7.3)? Several mechanisms are involved that allow the
vertical growth of microbial mats. Microorganisms forming
the mat layers are embedded in a matrix of gelatinous, sticky
exopolysaccharides. In today’s oxygenic environment,
exopolysaccharides are mainly produced by cyanobacteria
and, because of their adhesive properties, contribute to the
trapping of sedimentary particles in successive layers.
Simultaneously, within the mat, and through the activity of
microorganisms that increase alkalinity, calcium carbonate
precipitates, thus cementing sedimentary particles trapped in
the exopolysaccharide network. To gain access to sunlight,
filamentous cyanobacteria glide through the matrix of
exopolysaccharides and agglutinated sediments to form a
new layer on the surface of the mat. Above this organosedimentary layer, another mat will develop that will give
birth to another organo-sedimentary layer. This repeated
process results in a finely laminated vertical structure.
4.2.3.4 The Stromatolites of the Early Archaean
The oldest known stromatolites occur in ~3.45-Ga-old strata in
the Pilbara (Hofmann et al. 1999; Allwood et al. 2006). These
features apparently formed on a shallow water carbonate platform and exhibit different morphologies related to the different
microenvironments in which they formed: domical forms in
the supra intertidal zone and conical or cuspate forms slightly
in the intertidal zone (Fig. 4.9). These early photosynthesising
microbial mats that formed both vertical and tabular
stromatolites were anaerobic (Westall et al. 2011a).
4.2.3.5 Evolution of the Importance
of Stromatolites Through
the Geological Eras
One of the noticeable characteristics of the earliest
stromatolites is their small size compared to stromatolites
dating from the Later Archaean/Proterozoic (~2.7 Ga to
~542 Ga) to today (Figs. 4.8, 4.9, and 4.10). The difference
in size appears to be related to the appearance of oxygenic
photosynthesisers that could outcompete anaerobic
metabolisms.
From ~ 2.7 Ga, stromatolites reach metric or plurimetric
sizes. For example, stromatolites found at Belingwe in
Zimbabwe (2.7 billion years) are several metres high
(Grassineau et al. 2001).
The late Archaean and Proterozoic stromatolites form
huge reefs, representing the dominant biosignature in the
geological record until the end of the latter eon, about 560
million years ago (My). Their decline, which continued
throughout the Phanerozoic, may be linked to the appearance
of predators and to competition with other species (Fig. 4.11).
4.2.3.6 The Controversy over the Date
of Appearance of Cyanobacteria
To complete this description of ancient and modern
stromatolites, it is worth mentioning a controversy over
whether or not cyanobacteria were among the oldest fossils
of microbial life dating from the Early Archaean. For the
last forty years, cyanobacteria-like structures have been
described from these ancient rocks. However, several recent
studies have shown that, even if the supposed microfossils
contain carbon, they may be abiotic in origin.
The most striking of these controversies occurred
following the work published by J. William Schopf of the
University of California (Schopf 1993). Schopf describes
microfossil remains in rocks dated at 3.465 Ga in the Pilbara
terrane of Western Australia. These remains consist of long
filaments of apparently tens of cells, surrounded by a ‘wall’,
similar to that of extant filamentous cyanobacteria. He thus
identified eleven different species of cyanobacteria. In a
more recent publication, Schopf detected kerogen* in
these same structures (Schopf et al. 2002). These organic
molecules were interpreted as evidence of a bacterial origin.
For this author, these microfossils are therefore the oldest
traces of microbial life on our planet.
However, there are a number of flaws in Schopf’s
reasoning. Garcia-Ruiz et al. (2003) synthesised complex
inorganic structures whose morphology is similar to that of
microfossils described by Schopf (Fig. 4.7), and they consequently questioned some of Schopf’s conclusions as to the
biogenicity of his ‘microfossils’.
In addition, Martin Brasier and colleagues from Oxford
University also questioned the biogenicity of Schopf’s
microfossils on other grounds (Brasier et al. 2002). After
having analysed rock samples taken from the same site,
Brasier and colleagues conclude that the fossils described
by Schopf are artefacts, produced by the reprecipitation of
kerogen in a vein of hydrothermal silica. Although Brasier
does not deny the presence of organic material in the
samples, he believes that its origin is not biological, but
produced by a chemical reaction called Fischer-Tropsch.
This is a catalytic reaction producing paraffinic and olefinic
hydrocarbons (and, to a lesser degree, oxygenated
compounds: alcohols, aldehydes, ketones) from carbon monoxide and dihydrogen (CO + H 2 ):
n CO þ 2n þ 1
ð
ÞH 2 ! C n H 2nþ2 þ H 2 O
However, the products formed have a simple chemical
structure and the subsequent transformation of alkanes into
kerogen remains is still to be explained.
4 For Three Billion Years, Microorganisms Were the Only Inhabitants of the Earth
89
microbial mats may exist in other environments, for example,
sulphate-reducing bacteria around deep water hydrothermal
vents (see Sect. 9.7.3).
4.2.3.3 Microbial Mats Forming a Stromatolite
How can stromatolites that can reach one metre or more in
height be formed from mats whose individual thicknesses
vary from a few millimetres to a few centimetres (see Sect.
9.7.3)? Several mechanisms are involved that allow the
vertical growth of microbial mats. Microorganisms forming
the mat layers are embedded in a matrix of gelatinous, sticky
exopolysaccharides. In today’s oxygenic environment,
exopolysaccharides are mainly produced by cyanobacteria
and, because of their adhesive properties, contribute to the
trapping of sedimentary particles in successive layers.
Simultaneously, within the mat, and through the activity of
microorganisms that increase alkalinity, calcium carbonate
precipitates, thus cementing sedimentary particles trapped in
the exopolysaccharide network. To gain access to sunlight,
filamentous cyanobacteria glide through the matrix of
exopolysaccharides and agglutinated sediments to form a
new layer on the surface of the mat. Above this organosedimentary layer, another mat will develop that will give
birth to another organo-sedimentary layer. This repeated
process results in a finely laminated vertical structure.
4.2.3.4 The Stromatolites of the Early Archaean
The oldest known stromatolites occur in ~3.45-Ga-old strata in
the Pilbara (Hofmann et al. 1999; Allwood et al. 2006). These
features apparently formed on a shallow water carbonate platform and exhibit different morphologies related to the different
microenvironments in which they formed: domical forms in
the supra intertidal zone and conical or cuspate forms slightly
in the intertidal zone (Fig. 4.9). These early photosynthesising
microbial mats that formed both vertical and tabular
stromatolites were anaerobic (Westall et al. 2011a).
4.2.3.5 Evolution of the Importance
of Stromatolites Through
the Geological Eras
One of the noticeable characteristics of the earliest
stromatolites is their small size compared to stromatolites
dating from the Later Archaean/Proterozoic (~2.7 Ga to
~542 Ga) to today (Figs. 4.8, 4.9, and 4.10). The difference
in size appears to be related to the appearance of oxygenic
photosynthesisers that could outcompete anaerobic
metabolisms.
From ~ 2.7 Ga, stromatolites reach metric or plurimetric
sizes. For example, stromatolites found at Belingwe in
Zimbabwe (2.7 billion years) are several metres high
(Grassineau et al. 2001).
The late Archaean and Proterozoic stromatolites form
huge reefs, representing the dominant biosignature in the
geological record until the end of the latter eon, about 560
million years ago (My). Their decline, which continued
throughout the Phanerozoic, may be linked to the appearance
of predators and to competition with other species (Fig. 4.11).
4.2.3.6 The Controversy over the Date
of Appearance of Cyanobacteria
To complete this description of ancient and modern
stromatolites, it is worth mentioning a controversy over
whether or not cyanobacteria were among the oldest fossils
of microbial life dating from the Early Archaean. For the
last forty years, cyanobacteria-like structures have been
described from these ancient rocks. However, several recent
studies have shown that, even if the supposed microfossils
contain carbon, they may be abiotic in origin.
The most striking of these controversies occurred
following the work published by J. William Schopf of the
University of California (Schopf 1993). Schopf describes
microfossil remains in rocks dated at 3.465 Ga in the Pilbara
terrane of Western Australia. These remains consist of long
filaments of apparently tens of cells, surrounded by a ‘wall’,
similar to that of extant filamentous cyanobacteria. He thus
identified eleven different species of cyanobacteria. In a
more recent publication, Schopf detected kerogen* in
these same structures (Schopf et al. 2002). These organic
molecules were interpreted as evidence of a bacterial origin.
For this author, these microfossils are therefore the oldest
traces of microbial life on our planet.
However, there are a number of flaws in Schopf’s
reasoning. Garcia-Ruiz et al. (2003) synthesised complex
inorganic structures whose morphology is similar to that of
microfossils described by Schopf (Fig. 4.7), and they consequently questioned some of Schopf’s conclusions as to the
biogenicity of his ‘microfossils’.
In addition, Martin Brasier and colleagues from Oxford
University also questioned the biogenicity of Schopf’s
microfossils on other grounds (Brasier et al. 2002). After
having analysed rock samples taken from the same site,
Brasier and colleagues conclude that the fossils described
by Schopf are artefacts, produced by the reprecipitation of
kerogen in a vein of hydrothermal silica. Although Brasier
does not deny the presence of organic material in the
samples, he believes that its origin is not biological, but
produced by a chemical reaction called Fischer-Tropsch.
This is a catalytic reaction producing paraffinic and olefinic
hydrocarbons (and, to a lesser degree, oxygenated
compounds: alcohols, aldehydes, ketones) from carbon monoxide and dihydrogen (CO + H 2 ):
n CO þ 2n þ 1
ð
ÞH 2 ! C n H 2nþ2 þ H 2 O
However, the products formed have a simple chemical
structure and the subsequent transformation of alkanes into
kerogen remains is still to be explained.
4 For Three Billion Years, Microorganisms Were the Only Inhabitants of the Earth
89