that make up biolaminites range in size from fine silts to fine sands (~ 20–200 μm).
On the other hand, most sedimentary marine bacteria are roughly 1 μm in diameter
(Jumars 1993), and marine archaea are even smaller (Decho and Gutierrez 2017).
Also, although the dominant microeukaryotic phototrophs in mats, which are the
diatoms, have a large size range but rarely exceed 100 μm, it still has been shown
that diatom biofilms have the capacity to retain sediment particles corresponding in
size to clays and very fine silts (Garwood et al. 2015). In stark contrast to these
microbial groups, trichomous cyanobacteria in microbial mats are 100s of μm long
(Pan et al. 2019) and due to their photoautotrophic metabolism regulate their position
within microbial mats to have a preeminent role in light harvesting and other surficial
processes, such as trapping and baffling sediment particles (Noffke 1998; Noffke
et al. 2001). Therefore, while bacteria and archaea may play active roles in surface
adhesion processes and agglutination of minute sediment particles by secretion of
sticky EPS (Dade et al. 1990; Flemming and Wuertz 2019), it is the trichomous
cyanobacteria that exert a more important role in sediment accretion.
In addition to size-related issues, there are architectural features to be considered,
such as the entanglement of cyanobacteria filaments which contribute physically to
the stabilization of tidal flats (Margulis et al. 1980). The distinct trichomous nature of
mat cyanobacteria also has a major impact on sediment accretion and, therefore, the
creation of biosedimentary structures. Figure 3.7 illustrates the morphologies of
some cyanobacteria trichomes, common in modern hypersaline epibenthic microbial
mats. For example, due to its trichome being devoid of enveloping sheaths,
Oscillatoria sp. does not provide mat consistency (García de Lomas et al. 2005),
while the thick filaments of Lyngbya aestuarii present firm, thick sheaths that also
bind sediment particles to the external walls. On the other hand, when it is a
dominant member of the microbial community, Coleofasciculus chthonoplastes,
contributes considerably to mat consistency and forms compact mats (García de
Lomas et al. 2005), owing to the arrangement of the filament, consisting of several
interwoven trichomes encased in a single, thick mucilaginous sheath. This filament
“architecture” allows the free movement of individual trichomes while maintaining a
firm anchorage of the bundle (Stal et al. 1985). On the other hand, Symploca sp. is
made up of a thread of specifically coiled and parallel-oriented filaments, each one
encased in a single, thin sheath; the coiling of filaments provides traction to the mat.
Finally, not to be dismissed is the gliding motility of cyanobacteria, responsible
for trichome aggregation and reticulate formation, as it has been demonstrated in the
early stages of regular reticular patterns that ultimately yield permanent MISS
(Shepard and Sumner 2010; Cuadrado and Pan 2018).
80
J. Pan
On the other hand, most sedimentary marine bacteria are roughly 1 μm in diameter
(Jumars 1993), and marine archaea are even smaller (Decho and Gutierrez 2017).
Also, although the dominant microeukaryotic phototrophs in mats, which are the
diatoms, have a large size range but rarely exceed 100 μm, it still has been shown
that diatom biofilms have the capacity to retain sediment particles corresponding in
size to clays and very fine silts (Garwood et al. 2015). In stark contrast to these
microbial groups, trichomous cyanobacteria in microbial mats are 100s of μm long
(Pan et al. 2019) and due to their photoautotrophic metabolism regulate their position
within microbial mats to have a preeminent role in light harvesting and other surficial
processes, such as trapping and baffling sediment particles (Noffke 1998; Noffke
et al. 2001). Therefore, while bacteria and archaea may play active roles in surface
adhesion processes and agglutination of minute sediment particles by secretion of
sticky EPS (Dade et al. 1990; Flemming and Wuertz 2019), it is the trichomous
cyanobacteria that exert a more important role in sediment accretion.
In addition to size-related issues, there are architectural features to be considered,
such as the entanglement of cyanobacteria filaments which contribute physically to
the stabilization of tidal flats (Margulis et al. 1980). The distinct trichomous nature of
mat cyanobacteria also has a major impact on sediment accretion and, therefore, the
creation of biosedimentary structures. Figure 3.7 illustrates the morphologies of
some cyanobacteria trichomes, common in modern hypersaline epibenthic microbial
mats. For example, due to its trichome being devoid of enveloping sheaths,
Oscillatoria sp. does not provide mat consistency (García de Lomas et al. 2005),
while the thick filaments of Lyngbya aestuarii present firm, thick sheaths that also
bind sediment particles to the external walls. On the other hand, when it is a
dominant member of the microbial community, Coleofasciculus chthonoplastes,
contributes considerably to mat consistency and forms compact mats (García de
Lomas et al. 2005), owing to the arrangement of the filament, consisting of several
interwoven trichomes encased in a single, thick mucilaginous sheath. This filament
“architecture” allows the free movement of individual trichomes while maintaining a
firm anchorage of the bundle (Stal et al. 1985). On the other hand, Symploca sp. is
made up of a thread of specifically coiled and parallel-oriented filaments, each one
encased in a single, thin sheath; the coiling of filaments provides traction to the mat.
Finally, not to be dismissed is the gliding motility of cyanobacteria, responsible
for trichome aggregation and reticulate formation, as it has been demonstrated in the
early stages of regular reticular patterns that ultimately yield permanent MISS
(Shepard and Sumner 2010; Cuadrado and Pan 2018).
80
J. Pan