substrate. The fluidized and oxidized underlying sandy sediment (pointed by an
arrow in Fig. 3.4e) lies in the cavity created by the mat fold in stark contrast to the
otherwise anoxic sediment underlying an undisturbed mat. As shown by Fig. 3.4f,
mat folds may become embedded and preserved in biolaminite sequences and have
been cited for paleoenvironments (Hagadorn and McDowell 2012).
Finally, the MISS shown in Fig. 3.5 corresponds to an unusually big fragment of
microbial mat that has been detached, transported, and flipped on its edges by a
unidirectional tidal current. These remarkable MISS (that make the person in the
picture look like as if he were deploying “canvasses” over the sedimentary plain) are
a type of mat deformation structures that most likely originated from a tear in the mat
and its interaction with strong hydrodynamic forces. The high shear stress acting on
the wet rims of a tear in the mat channels the incoming tidal water through the tear
generating an ever bigger rip; on the other hand, the penetrating water liquefies the
underlying sand making the surficial mat prone to lose its anchorage to the substrate.
Once detached, the mat gets transported a few meters over as a piece of rug. As a
by-product of the strong hydrodynamic forces, other MISS such as folds and flippedover edges may also be formed, in what probably represents the first step of the
formation of roll-up structures (Cuadrado et al. 2015).
3.4.2 Microbial Mats in Other Environments
The previous exposition referred to microbial mats in hypersaline environments,
which occur in modern settings and also correspond to most fossil representatives.
However, it is worth pointing out that present-day microbial mats occur in an array
of environments and conditions. I will summarize these for you later in this section
of my essay. The microbial diversity is not as high as in hypersaline microbial mats,
nor do they always present the characteristic layered structure previously described.
A common feature of these environments is the occurrence (at least seasonally) of
extreme conditions in either one or several environmental parameters.
Coastal and estuarine sandflats with large tidal fluctuations provide an excellent
habitat for the development of cyanobacterial mats (Stal 2012) under environmental
conditions that preclude the existence of abundant predators, without being as
extreme as in hypersaline microbial mats. Coastal mats have a large number of
microeukaryotic representatives, primarily diatoms (Prieto-Barajas et al. 2018).
Geothermal alkaline spring mats develop at high temperature (> 70
C) in
combination with abundant H 2 S. These mats share similar features with the
hypersaline mats, such as biolaminations (Castenholz 2009) and the presence of
oxygenic (cyanobacteria) and anoxygenic photoautotrophs. The anoxygenic
phototroph Chloroflexus is a dominant member of the microbial community, often
forming a reddish to orange layer that overlays a cyanobacterial mat, as anoxygenic
photosynthetic activity scavenges the sulfide that otherwise would be toxic to most
cyanobacteria (Stal 2012). Species of Synechococcus are particularly abundant
among the cyanobacteria and usually form conical microbialites (Bosak et al.
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
73
arrow in Fig. 3.4e) lies in the cavity created by the mat fold in stark contrast to the
otherwise anoxic sediment underlying an undisturbed mat. As shown by Fig. 3.4f,
mat folds may become embedded and preserved in biolaminite sequences and have
been cited for paleoenvironments (Hagadorn and McDowell 2012).
Finally, the MISS shown in Fig. 3.5 corresponds to an unusually big fragment of
microbial mat that has been detached, transported, and flipped on its edges by a
unidirectional tidal current. These remarkable MISS (that make the person in the
picture look like as if he were deploying “canvasses” over the sedimentary plain) are
a type of mat deformation structures that most likely originated from a tear in the mat
and its interaction with strong hydrodynamic forces. The high shear stress acting on
the wet rims of a tear in the mat channels the incoming tidal water through the tear
generating an ever bigger rip; on the other hand, the penetrating water liquefies the
underlying sand making the surficial mat prone to lose its anchorage to the substrate.
Once detached, the mat gets transported a few meters over as a piece of rug. As a
by-product of the strong hydrodynamic forces, other MISS such as folds and flippedover edges may also be formed, in what probably represents the first step of the
formation of roll-up structures (Cuadrado et al. 2015).
3.4.2 Microbial Mats in Other Environments
The previous exposition referred to microbial mats in hypersaline environments,
which occur in modern settings and also correspond to most fossil representatives.
However, it is worth pointing out that present-day microbial mats occur in an array
of environments and conditions. I will summarize these for you later in this section
of my essay. The microbial diversity is not as high as in hypersaline microbial mats,
nor do they always present the characteristic layered structure previously described.
A common feature of these environments is the occurrence (at least seasonally) of
extreme conditions in either one or several environmental parameters.
Coastal and estuarine sandflats with large tidal fluctuations provide an excellent
habitat for the development of cyanobacterial mats (Stal 2012) under environmental
conditions that preclude the existence of abundant predators, without being as
extreme as in hypersaline microbial mats. Coastal mats have a large number of
microeukaryotic representatives, primarily diatoms (Prieto-Barajas et al. 2018).
Geothermal alkaline spring mats develop at high temperature (> 70
C) in
combination with abundant H 2 S. These mats share similar features with the
hypersaline mats, such as biolaminations (Castenholz 2009) and the presence of
oxygenic (cyanobacteria) and anoxygenic photoautotrophs. The anoxygenic
phototroph Chloroflexus is a dominant member of the microbial community, often
forming a reddish to orange layer that overlays a cyanobacterial mat, as anoxygenic
photosynthetic activity scavenges the sulfide that otherwise would be toxic to most
cyanobacteria (Stal 2012). Species of Synechococcus are particularly abundant
among the cyanobacteria and usually form conical microbialites (Bosak et al.
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
73