plays a major role in early diagenesis (Stal 2003). Cyanobacteria produce EPS as a
structural cell component, in the form of the external sheaths that encase trichomes
(hence called filaments) (Hoiczyk and Baumeister 1995; Li et al. 2001). On the other
hand, EPS secretion by epipelic (those that live freely on sediment surfaces) diatoms
is associated with their gliding motility through sediment grains (Edgar and PickettHeaps 1984; Hoagland et al. 1993; Poulsen et al. 1999) as a means of structural
protection from predation and potentially offers protection against toxic contaminants (Lawrence et al. 1998). This motility-associated secretion of
exopolysaccharides in diatoms clearly has an energy cost to the cell, but recent
estimates conclude that the cost is almost negligible, representing on average only
0.0001% of the daily net photosynthetic production of a diatom cell (Marques da
Silva et al. 2020).
EPS in sediments also contribute to the precipitation of authigenic minerals (those
minerals which are formed in situ) (Sutherland 2001; Decho 2010; Stal 2010). This
is a crucial first step of early diagenesis, as authigenic minerals conserve the
morphology of tidal flats creating rigid structures, in turn leading to the preservation
of primary sedimentary structures in the geological record (Winsborough 2000).
The development of microbial mats in intertidal and lower-supratidal environments implies regular exposure to air where diel variations in temperature are much
larger than in aquatic environments. Desiccation is a key environmental factor
associated with temperature fluctuations and that elicits a number of ecophysiological responses in sediment microbes, among them being EPS secretion by epipelic
diatoms (McKew et al. 2011). Behavioral responses linked to desiccation such as
vertical migration through sediments also imply the secretion of EPS by epipelic
diatoms (Hoagland et al. 1993; Stal and Brouwer 2005; Perkins et al. 2010).
On the other hand, shallow intertidal environments offer a functional water depth
for sunlight penetration, needed for photosynthesis, but some wavelengths of the
solar radiation spectrum (UV) might be detrimental, and excessive photosynthetically active radiation (PAR) may cause photoinhibition to mat-forming
cyanobacteria (Cartaxana et al. 2013), which typically are adapted to low light
intensities (Stal 2012). Accordingly, microbes in intertidal laminated mats have
developed responses related to the synthesis of photoprotective compounds. For
example, the cyanobacterium Lyngbya aestuarii (Fig. 3.7b) is responsible for synthesis of the passive sunscreening pigment scytonemin (Abed et al. 2008; Balskus
et al. 2011) in response to UV-A radiation. Scytonemin accumulates within the
extracellular sheath or slime (Garcia-Pichel and Castenholz 1991; Decho and
Gutierrez 2017) and is capable of absorbing up to 90% of the radiation. As
scytonemin is only found in epibenthic cyanobacteria (i.e., it is absent from planktonic representatives), it is plausible that its synthetic pathway had evolved in
microbial mat communities (Gao and Garcia-Pichel 2011).
78
J. Pan
structural cell component, in the form of the external sheaths that encase trichomes
(hence called filaments) (Hoiczyk and Baumeister 1995; Li et al. 2001). On the other
hand, EPS secretion by epipelic (those that live freely on sediment surfaces) diatoms
is associated with their gliding motility through sediment grains (Edgar and PickettHeaps 1984; Hoagland et al. 1993; Poulsen et al. 1999) as a means of structural
protection from predation and potentially offers protection against toxic contaminants (Lawrence et al. 1998). This motility-associated secretion of
exopolysaccharides in diatoms clearly has an energy cost to the cell, but recent
estimates conclude that the cost is almost negligible, representing on average only
0.0001% of the daily net photosynthetic production of a diatom cell (Marques da
Silva et al. 2020).
EPS in sediments also contribute to the precipitation of authigenic minerals (those
minerals which are formed in situ) (Sutherland 2001; Decho 2010; Stal 2010). This
is a crucial first step of early diagenesis, as authigenic minerals conserve the
morphology of tidal flats creating rigid structures, in turn leading to the preservation
of primary sedimentary structures in the geological record (Winsborough 2000).
The development of microbial mats in intertidal and lower-supratidal environments implies regular exposure to air where diel variations in temperature are much
larger than in aquatic environments. Desiccation is a key environmental factor
associated with temperature fluctuations and that elicits a number of ecophysiological responses in sediment microbes, among them being EPS secretion by epipelic
diatoms (McKew et al. 2011). Behavioral responses linked to desiccation such as
vertical migration through sediments also imply the secretion of EPS by epipelic
diatoms (Hoagland et al. 1993; Stal and Brouwer 2005; Perkins et al. 2010).
On the other hand, shallow intertidal environments offer a functional water depth
for sunlight penetration, needed for photosynthesis, but some wavelengths of the
solar radiation spectrum (UV) might be detrimental, and excessive photosynthetically active radiation (PAR) may cause photoinhibition to mat-forming
cyanobacteria (Cartaxana et al. 2013), which typically are adapted to low light
intensities (Stal 2012). Accordingly, microbes in intertidal laminated mats have
developed responses related to the synthesis of photoprotective compounds. For
example, the cyanobacterium Lyngbya aestuarii (Fig. 3.7b) is responsible for synthesis of the passive sunscreening pigment scytonemin (Abed et al. 2008; Balskus
et al. 2011) in response to UV-A radiation. Scytonemin accumulates within the
extracellular sheath or slime (Garcia-Pichel and Castenholz 1991; Decho and
Gutierrez 2017) and is capable of absorbing up to 90% of the radiation. As
scytonemin is only found in epibenthic cyanobacteria (i.e., it is absent from planktonic representatives), it is plausible that its synthetic pathway had evolved in
microbial mat communities (Gao and Garcia-Pichel 2011).
78
J. Pan