3.4 Hypersaline Epibenthic Microbial Mats
Microbial mats may be considered as a type of biofilm (Costerton and Stoodley
2003; Tomescu et al. 2016), and in fact, some authors go as far as stating that
microbial mats “must, by definition, begin as biofilms” (Castenholz 2009). As it will
be further developed in this section, modern epibenthic microbial mats are photosynthetic, stratified consortia of prokaryotes (bacteria and archaea) and
microeukaryotes developed at sediment-water interfaces in shallow, intertidal, and
lower-supratidal marine sediments (Des Marais 2003), and this stratification is an
emergent property of microbial mats that distances them from mere two-dimensional
biofilms. Microbial adhesion and the tendency to form biofilms will be discussed
further in Sect. 3.5.1.
Furthermore, epibenthic microbial mats are complete, self-sufficient ecosystems
that participate in nutrient uptake and recycling and play a paramount role in
sediment biostabilization in present-day shallow sand- and mudflats. As discussed
in the previous section, they have a long fossil record, dating back to at least 3.4 Ga;
in fact, some of the oldest fossils correspond to filamentous cyanobacteria colonizing
coastal siliciclastic sediments in the form of biofilms and microbial mats (Schopf and
Walter 1982; Tice and Lowe 2004; Noffke et al. 2006; Noffke et al. 2013). Modern
representatives of these remarkable micro-ecosystems may well be considered as
present-day models for the study of ancient relationships between the three domains
of life on Earth (bacteria, archaea, and eukaryotes) that have been established since
the Archaean (Bartley 1996; Nisbet and Fowler 1999; Oschmann et al. 2002).
The hypersaline conditions under which most modern coastal microbial mats
develop exclude the presence of metazoan predators; thus, complete biolaminite
sequences are preserved in sediments without major disruptions. Hypersaline conditions also set the scenario for the dominance and spread of microbial mats and
stromatolites throughout most of the Precambrian, when mats were ubiquitous
biosedimentary features in shallow ocean sediments in the absence of metazoan
grazers, predators, and bioturbators (Seilacher 1999; Stal 2012).
A typical microbial mat, unaltered by mechanical disruptions or hydrodynamic
deformation, presents a laminar structure in which different groups of microorganisms alternate their dominance in vertical section, in accordance with factors
governing redox potential, illumination, and sediment physicochemistry (Fig. 3.2).
The description that follows is based on the vertical profile of undisturbed, modern,
hypersaline microbial mats from Paso Seco (Argentina), with which I am most
familiar. Paso Seco is an obliterated tidal channel with an intermittent connection
to the coastal ocean. The dominant sediment type corresponds to fine sand, with
feldspars and quartz grains (Cuadrado et al. 2015).
As it is appreciated in Fig. 3.2b and schematized in Fig. 3.2c, a thin diatom
biofilm (~ 300 μm-thick) colonizes the uppermost sediment layer, underneath which
the epibenthic microbial mat proper, dominated in biomass by filamentous
cyanobacteria, develops. This layer varies in thickness ~ 0.3–1.5 mm. These two
topmost layers are characterized by oxygenic photosynthesis. Below the
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
65
Microbial mats may be considered as a type of biofilm (Costerton and Stoodley
2003; Tomescu et al. 2016), and in fact, some authors go as far as stating that
microbial mats “must, by definition, begin as biofilms” (Castenholz 2009). As it will
be further developed in this section, modern epibenthic microbial mats are photosynthetic, stratified consortia of prokaryotes (bacteria and archaea) and
microeukaryotes developed at sediment-water interfaces in shallow, intertidal, and
lower-supratidal marine sediments (Des Marais 2003), and this stratification is an
emergent property of microbial mats that distances them from mere two-dimensional
biofilms. Microbial adhesion and the tendency to form biofilms will be discussed
further in Sect. 3.5.1.
Furthermore, epibenthic microbial mats are complete, self-sufficient ecosystems
that participate in nutrient uptake and recycling and play a paramount role in
sediment biostabilization in present-day shallow sand- and mudflats. As discussed
in the previous section, they have a long fossil record, dating back to at least 3.4 Ga;
in fact, some of the oldest fossils correspond to filamentous cyanobacteria colonizing
coastal siliciclastic sediments in the form of biofilms and microbial mats (Schopf and
Walter 1982; Tice and Lowe 2004; Noffke et al. 2006; Noffke et al. 2013). Modern
representatives of these remarkable micro-ecosystems may well be considered as
present-day models for the study of ancient relationships between the three domains
of life on Earth (bacteria, archaea, and eukaryotes) that have been established since
the Archaean (Bartley 1996; Nisbet and Fowler 1999; Oschmann et al. 2002).
The hypersaline conditions under which most modern coastal microbial mats
develop exclude the presence of metazoan predators; thus, complete biolaminite
sequences are preserved in sediments without major disruptions. Hypersaline conditions also set the scenario for the dominance and spread of microbial mats and
stromatolites throughout most of the Precambrian, when mats were ubiquitous
biosedimentary features in shallow ocean sediments in the absence of metazoan
grazers, predators, and bioturbators (Seilacher 1999; Stal 2012).
A typical microbial mat, unaltered by mechanical disruptions or hydrodynamic
deformation, presents a laminar structure in which different groups of microorganisms alternate their dominance in vertical section, in accordance with factors
governing redox potential, illumination, and sediment physicochemistry (Fig. 3.2).
The description that follows is based on the vertical profile of undisturbed, modern,
hypersaline microbial mats from Paso Seco (Argentina), with which I am most
familiar. Paso Seco is an obliterated tidal channel with an intermittent connection
to the coastal ocean. The dominant sediment type corresponds to fine sand, with
feldspars and quartz grains (Cuadrado et al. 2015).
As it is appreciated in Fig. 3.2b and schematized in Fig. 3.2c, a thin diatom
biofilm (~ 300 μm-thick) colonizes the uppermost sediment layer, underneath which
the epibenthic microbial mat proper, dominated in biomass by filamentous
cyanobacteria, develops. This layer varies in thickness ~ 0.3–1.5 mm. These two
topmost layers are characterized by oxygenic photosynthesis. Below the
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
65