multicellularity (Margulis and Dolan 2002). But all that falls way beyond the scope
of this essay. Those readers wishing to go beyond this very brief account of early
microbial life and learn more about the fossil record and how it provides clues as to
clarify the emergence of the main evolutionary lines should refer to publications that
provide a more in-depth summary (e.g., Schopf 2012; Tomescu et al. 2016). For
now, let us go back to microbial colonization of marine siliciclastic sediments, to
then focus on Earth’s oldest ecosystems, microbial mats.
3.3 Microbial Colonization of Siliciclastic Marine
Sediments
Coastal marine environments may fall into either of two broad groups with respect to
their geochemical signature, carbonatic or siliciclastic. In the former ones, chemical
precipitation of minerals and Ca
+2 ions has a preeminent role in biogeochemical
processes (Kazmierczak et al. 2013) producing carbonatic sediments and rocks,
whereas siliciclastic settings are characterized by quartz sediments, and the chemical
precipitation of minerals, while possible, plays a less significant role than physical
processes (Cuadrado 2017). Thus, siliciclastic sediments have imprinted an organizing signature on microbial communities since erosion and weathering of crustal
rocks produced vast amounts of sediments for colonization (Noffke et al. 2002). As
previously mentioned, stromatolites have been the dominant biosedimentary structure in carbonatic settings, whereas in siliciclastic depositional environments,
another type of biogenic structures different from stromatolites has predominated.
These have been termed microbially induced sedimentary structures (MISS) and
result from the interaction of epibenthic cyanobacteria (epi ¼ growing on top of;
benthic ¼ making reference to a sedimentary substrate) with physical agents of
erosion, deposition, transportation, or deformation (Noffke et al. 2001). The remainder of this essay will deal mostly with MISS and their emergent properties.
Similarities and differences between stromatolites and MISS have been critically
analyzed (Noffke and Awramik 2013), on the comparative premise that both
biosedimentary structures have a microbial mat as the principal constructional
unit. Besides the abovementioned contrasting geochemical and sedimentary environments in which either one develops, one of the most striking differences is that
MISS are generally surface phenomena (two-dimensional, only a few mm thick) and
fail to develop into substantial three-dimensional stacked layers of mats, as stromatolites do. The mode of growth of microbial mats will be discussed in depth later.
The association between marine siliciclastic sediments and microbes has been
revisited many times, focusing on fossilized and living MISS in modern settings, by
either applying paleontological or actualistic approaches. For example, 2.9-Ga-old
microbial mat facies have been recognized from South African sandstones, and the
biogenicity of their microscopic textures has been confirmed by comparative microscopic analyses with modern ones, as well as by the light isotopic signature of
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
63
of this essay. Those readers wishing to go beyond this very brief account of early
microbial life and learn more about the fossil record and how it provides clues as to
clarify the emergence of the main evolutionary lines should refer to publications that
provide a more in-depth summary (e.g., Schopf 2012; Tomescu et al. 2016). For
now, let us go back to microbial colonization of marine siliciclastic sediments, to
then focus on Earth’s oldest ecosystems, microbial mats.
3.3 Microbial Colonization of Siliciclastic Marine
Sediments
Coastal marine environments may fall into either of two broad groups with respect to
their geochemical signature, carbonatic or siliciclastic. In the former ones, chemical
precipitation of minerals and Ca
+2 ions has a preeminent role in biogeochemical
processes (Kazmierczak et al. 2013) producing carbonatic sediments and rocks,
whereas siliciclastic settings are characterized by quartz sediments, and the chemical
precipitation of minerals, while possible, plays a less significant role than physical
processes (Cuadrado 2017). Thus, siliciclastic sediments have imprinted an organizing signature on microbial communities since erosion and weathering of crustal
rocks produced vast amounts of sediments for colonization (Noffke et al. 2002). As
previously mentioned, stromatolites have been the dominant biosedimentary structure in carbonatic settings, whereas in siliciclastic depositional environments,
another type of biogenic structures different from stromatolites has predominated.
These have been termed microbially induced sedimentary structures (MISS) and
result from the interaction of epibenthic cyanobacteria (epi ¼ growing on top of;
benthic ¼ making reference to a sedimentary substrate) with physical agents of
erosion, deposition, transportation, or deformation (Noffke et al. 2001). The remainder of this essay will deal mostly with MISS and their emergent properties.
Similarities and differences between stromatolites and MISS have been critically
analyzed (Noffke and Awramik 2013), on the comparative premise that both
biosedimentary structures have a microbial mat as the principal constructional
unit. Besides the abovementioned contrasting geochemical and sedimentary environments in which either one develops, one of the most striking differences is that
MISS are generally surface phenomena (two-dimensional, only a few mm thick) and
fail to develop into substantial three-dimensional stacked layers of mats, as stromatolites do. The mode of growth of microbial mats will be discussed in depth later.
The association between marine siliciclastic sediments and microbes has been
revisited many times, focusing on fossilized and living MISS in modern settings, by
either applying paleontological or actualistic approaches. For example, 2.9-Ga-old
microbial mat facies have been recognized from South African sandstones, and the
biogenicity of their microscopic textures has been confirmed by comparative microscopic analyses with modern ones, as well as by the light isotopic signature of
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
63