2012). At peripheral areas where temperatures drop below 45
C, eukaryotic
microalgae may occur. These mats are usually soft, although deposition of SiO 2
and CaCO 3 may harden them.
Acidic hot springs support mats of eukaryotic microalgae of the order Cyanidiales
(a basal clade of Rhodophyta) as all photosynthetic bacteria, including
cyanobacteria, are excluded at pH < 4.0 (Castenholz 2009; Stal 2012). These acidic
conditions are common in hot springs (40–56
C) linked to volcanic activity.
Microbial mats develop in Antarctic and Arctic meltwater ponds and lakes at low
temperatures (4–10
C). These are perennial, slowly accreting cyanobacteriadominated mats that develop in the absence of efficient grazers and lacking potential
eukaryotic microalgal competitors (Castenholz 2009); however, the secretion of
copious amounts of polysaccharides provides protection to less thermotolerant
organisms such as diatoms, flagellates, and ciliates that only thrive seasonally
(Prieto-Barajas et al. 2018). Cyanobacteria in these mats effectively undergo a
dormant phase during winter conditions, and heterotrophic bacteria play a major
role in nutrient cycling.
Terrestrial cyanobacterial mats and crusts can be found in a variety of different
environments, from hot and cold deserts (Castenholz 2009), sand dunes, to the
bottom of drying oxbow lakes (Fayó et al. 2020). A common feature is the occurrence of thickly sheathed cyanobacteria to attenuate the effects of desiccation and
unreliable water supply (Stal 2012).
3.5 Emergent Properties of Sedimentary Microbial Mat
Communities
The morphology of microbialites (stromatolites or MISS) is the resultant of an
overlap of two factors: (1) the intrinsic control owing to the microbial community
(genotype and phenotype) that forms the structure and (2) extrinsic physical factors
such as sedimentology and the effect of hydraulic and sediment dynamics (Noffke
and Awramik 2013). As a resultant microbial communities colonizing siliciclastic
sediments share some common and emergent properties which on the one hand
determine the microbial mat “architecture”, and on the other hand apply to mat
development. According to a model drawing from the study of Archaean examples
(Tice et al. 2011), biosedimentary structures owe their characteristics to the interplay
between cohesion among their components and mat-surface roughness.
The steps leading to the formation and consolidation of a microbial mat are
schematically illustrated in Fig. 3.6. A first step necessary for the development of
microbial communities is (1) the individual adhesion of microbes to substrate
surfaces. As it will be discussed later, this results in the development of microbial
biofilms. As the surface colonization progresses, (2) community-secreted
exopolymers accumulate, contributing to the development of a more cohesive mat
that already departs from the structure formerly presented by a laminar biofilm.
74
J. Pan
C, eukaryotic
microalgae may occur. These mats are usually soft, although deposition of SiO 2
and CaCO 3 may harden them.
Acidic hot springs support mats of eukaryotic microalgae of the order Cyanidiales
(a basal clade of Rhodophyta) as all photosynthetic bacteria, including
cyanobacteria, are excluded at pH < 4.0 (Castenholz 2009; Stal 2012). These acidic
conditions are common in hot springs (40–56
C) linked to volcanic activity.
Microbial mats develop in Antarctic and Arctic meltwater ponds and lakes at low
temperatures (4–10
C). These are perennial, slowly accreting cyanobacteriadominated mats that develop in the absence of efficient grazers and lacking potential
eukaryotic microalgal competitors (Castenholz 2009); however, the secretion of
copious amounts of polysaccharides provides protection to less thermotolerant
organisms such as diatoms, flagellates, and ciliates that only thrive seasonally
(Prieto-Barajas et al. 2018). Cyanobacteria in these mats effectively undergo a
dormant phase during winter conditions, and heterotrophic bacteria play a major
role in nutrient cycling.
Terrestrial cyanobacterial mats and crusts can be found in a variety of different
environments, from hot and cold deserts (Castenholz 2009), sand dunes, to the
bottom of drying oxbow lakes (Fayó et al. 2020). A common feature is the occurrence of thickly sheathed cyanobacteria to attenuate the effects of desiccation and
unreliable water supply (Stal 2012).
3.5 Emergent Properties of Sedimentary Microbial Mat
Communities
The morphology of microbialites (stromatolites or MISS) is the resultant of an
overlap of two factors: (1) the intrinsic control owing to the microbial community
(genotype and phenotype) that forms the structure and (2) extrinsic physical factors
such as sedimentology and the effect of hydraulic and sediment dynamics (Noffke
and Awramik 2013). As a resultant microbial communities colonizing siliciclastic
sediments share some common and emergent properties which on the one hand
determine the microbial mat “architecture”, and on the other hand apply to mat
development. According to a model drawing from the study of Archaean examples
(Tice et al. 2011), biosedimentary structures owe their characteristics to the interplay
between cohesion among their components and mat-surface roughness.
The steps leading to the formation and consolidation of a microbial mat are
schematically illustrated in Fig. 3.6. A first step necessary for the development of
microbial communities is (1) the individual adhesion of microbes to substrate
surfaces. As it will be discussed later, this results in the development of microbial
biofilms. As the surface colonization progresses, (2) community-secreted
exopolymers accumulate, contributing to the development of a more cohesive mat
that already departs from the structure formerly presented by a laminar biofilm.
74
J. Pan