Eventually, small focalized increases in mat surface elevation translate into an
increase in the local effective mat surface area available for exchange and decrease
the thickness of the laminar sublayer; this ultimately results in an incremental surface
roughness (as proposed by Tice et al. 2011). A microbial mat’s surface roughness
sets its maximum potential rate of diffusive exchange with the overlying water
column, and one of the components that are exchanged are sediment particles.
With an increase in surface roughness, (3) accretion of sediment particles (through
baffling, trapping, and binding) becomes an emergent property of the
biosedimentary structure (Noffke et al. 2001; Bouougri and Porada 2007). This is
a crucial point at which the sedimentary signature of mats becomes more complex,
beyond that of the underlying sediment. Finally, through the continuous accumulation of microbial EPS and biomass and the development of filamentous structures
(that may include living, interwoven cyanobacteria filaments and the recalcitrant
sheaths enveloping them), mat cohesion increases over time. As a whole, the
resulting mesh of interweaving cyanobacterial filaments together with the
microbially secreted EPS entangle sediment grains more efficiently than does a
diatom biofilm (de Winder et al. 1999; Pan et al. 2013) which translates into
cyanobacterial biofilms displaying a significant increment in the cohesiveness of
sediments (4).
There are salient features of what has been considered “biology-like behavior”
exhibited by microbes which ultimately are indicative of biogenicity in fossils. These
features that are common to microbe-sediment associations include their organization into biofilm-like structures (Hall-Stoodley et al. 2004), a preference for certain
substrates, and a tendency to form clusters and “mats” (Brasier and Wacey 2012).
Such observation is congruent with the abovementioned evolutionary model proposed for Archaean mats (Tice et al. 2011), which translates well to modern
microbial mats, as it has been well-documented in the field. For instance, the in
situ microbial succession, formation, and consolidation of mm-size microbial reticulate structures with specific geometries has been described in Sect. 3.4.1 (Cuadrado
and Pan 2018). Under certain environmental cues even under varying hydrodynamic
energy regimes, these regular microbial arrangements ultimately yield tufts or
“pinnacles” that become regular and stable biosedimentary features over extended
periods of time (months). Their fossil analogues have been described for 3.22-Gaold coastal habitats (Homann et al. 2015). Thus, even if calling these “ontogenetic”
stages of mat and MISS development may be far-fetched until more actualistic
evidence is gathered on this respect, the idea is nonetheless tempting as the regular
observation of the genesis of such structures provides linking references to laboratory and rock record interpretations of MISS.
What follows is an expanded treatment of the four emergent properties (or steps)
that seem to be common features in the formation and consolidation of microbial
mats and MISS.
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
75
increase in the local effective mat surface area available for exchange and decrease
the thickness of the laminar sublayer; this ultimately results in an incremental surface
roughness (as proposed by Tice et al. 2011). A microbial mat’s surface roughness
sets its maximum potential rate of diffusive exchange with the overlying water
column, and one of the components that are exchanged are sediment particles.
With an increase in surface roughness, (3) accretion of sediment particles (through
baffling, trapping, and binding) becomes an emergent property of the
biosedimentary structure (Noffke et al. 2001; Bouougri and Porada 2007). This is
a crucial point at which the sedimentary signature of mats becomes more complex,
beyond that of the underlying sediment. Finally, through the continuous accumulation of microbial EPS and biomass and the development of filamentous structures
(that may include living, interwoven cyanobacteria filaments and the recalcitrant
sheaths enveloping them), mat cohesion increases over time. As a whole, the
resulting mesh of interweaving cyanobacterial filaments together with the
microbially secreted EPS entangle sediment grains more efficiently than does a
diatom biofilm (de Winder et al. 1999; Pan et al. 2013) which translates into
cyanobacterial biofilms displaying a significant increment in the cohesiveness of
sediments (4).
There are salient features of what has been considered “biology-like behavior”
exhibited by microbes which ultimately are indicative of biogenicity in fossils. These
features that are common to microbe-sediment associations include their organization into biofilm-like structures (Hall-Stoodley et al. 2004), a preference for certain
substrates, and a tendency to form clusters and “mats” (Brasier and Wacey 2012).
Such observation is congruent with the abovementioned evolutionary model proposed for Archaean mats (Tice et al. 2011), which translates well to modern
microbial mats, as it has been well-documented in the field. For instance, the in
situ microbial succession, formation, and consolidation of mm-size microbial reticulate structures with specific geometries has been described in Sect. 3.4.1 (Cuadrado
and Pan 2018). Under certain environmental cues even under varying hydrodynamic
energy regimes, these regular microbial arrangements ultimately yield tufts or
“pinnacles” that become regular and stable biosedimentary features over extended
periods of time (months). Their fossil analogues have been described for 3.22-Gaold coastal habitats (Homann et al. 2015). Thus, even if calling these “ontogenetic”
stages of mat and MISS development may be far-fetched until more actualistic
evidence is gathered on this respect, the idea is nonetheless tempting as the regular
observation of the genesis of such structures provides linking references to laboratory and rock record interpretations of MISS.
What follows is an expanded treatment of the four emergent properties (or steps)
that seem to be common features in the formation and consolidation of microbial
mats and MISS.
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
75