cyanobacteria layer, there is a light-brownish layer (~ 1.5–2.5 mm) with an abundance of purple sulfur bacteria and Beggiatoa; oxic conditions prevail up to this
layer. Purple bacteria are Proteobacteria that contain bacteriochlorophyll a or b, by
virtue of which some are photoautotrophs using CO 2 as the source of carbon; others
may be photoheterotrophs using cyanobacteria-derived organic carbon (Castenholz
2009), but still using light as the source of energy. As it can be seen in Fig. 3.2a,
orange and red wavelengths penetrate down to these layers (Stal et al. 1985). Purple
bacteria have been pointed to as the plausible candidates for early
bacteriochlorophyll-based photosynthetic mats (Nisbet and Fowler 1999).
From 2.5–3.0 mm down to deeper layers, anoxic conditions set the scenario for
the dominance of sulfate-reducing bacteria, whose metabolism is based on SO 4
2À
(or elemental S) replacing O 2 in the oxidation of organic matter derived from
cyanobacterial production, such as accumulated extracellular polymeric substances
(EPS; Section 3.5.2) (Castenholz 2009; Stal 2010). This anoxic layer is appreciated
as a thick, black stratum that may reach several cm in depth. In deeper layers of this
anoxic stratum, a band of green sulfur bacteria may occur, marking the maximum
penetration of the far-red light wavelengths (Fig. 3.2a). Green bacteria have a high
sulfide tolerance and can grow in conditions which are toxic to purple bacteria
(Nisbet and Fowler 1999). They are obligate anaerobic photoautotrophs with bacteriochlorophylls in a single photosystem that use H 2 S as the electron donor
(Castenholz 2009). Finally, methane-producing archaea (not shown) are common
throughout the anoxic zone (Robertson et al. 2009; Cardoso et al. 2019).
Cyanobacteria are dominant from a bioenergetic point of view, as they produce
most of the reduced organic matter, and this opens a niche for recycling anaerobes.
The dominance of cyanobacteria also applies, in ecological terms, to the biomass. A
compilation of bacterial 16S rRNA gene clone libraries generated from modern
hypersaline microbial mats (Des Marais 2010) found that the upper 0–5 mm of these
biosedimentary structures produce as many as 949 clones, with cyanobacteria and
plastids comprising up to 20% of the library. Filamentous cyanobacteria (e.g., the
pioneering Oscillatoria sp. and the dominant Coleofasciculus (Microcoleus)
chthonoplastes occurring in well-established mats; Stal et al. 1985) usually make
up the larger portion of the total biomass of the microbial consortium, even if
diatoms and other prokaryotic groups may contribute a significant proportion (see
Ley et al. 2006); this is due to the trichomous nature of the dominant cyanobacteria
(i.e., organized in linear chains of cells that may be several 100s of μm long). These
filamentous cyanobacteria grow conspicuously in the upper mm in order to find the
optimal light intensity for photosynthesis. A highly coherent structure is produced by
the interwoven cyanobacterial filaments (Fig. 3.3a), providing a dense and coherent
fabric for the binding of sediment particles and giving a remarkable external leathery
appearance to the microbial mat (Figs. 3.4e, 3.5, 3.6c). A striking feature of these
thick mats is their elastic deformation, partly resulting from the cohesiveness of this
cellular lattice (Pan et al. 2019).
Microbial consortia are complex biochemically and biologically diverse systems,
in which the microbial constituents cooperatively exploit every niche (Nisbet and
Fowler 1999; Noffke et al. 2013). As it becomes apparent from the previous
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
67
layer. Purple bacteria are Proteobacteria that contain bacteriochlorophyll a or b, by
virtue of which some are photoautotrophs using CO 2 as the source of carbon; others
may be photoheterotrophs using cyanobacteria-derived organic carbon (Castenholz
2009), but still using light as the source of energy. As it can be seen in Fig. 3.2a,
orange and red wavelengths penetrate down to these layers (Stal et al. 1985). Purple
bacteria have been pointed to as the plausible candidates for early
bacteriochlorophyll-based photosynthetic mats (Nisbet and Fowler 1999).
From 2.5–3.0 mm down to deeper layers, anoxic conditions set the scenario for
the dominance of sulfate-reducing bacteria, whose metabolism is based on SO 4
2À
(or elemental S) replacing O 2 in the oxidation of organic matter derived from
cyanobacterial production, such as accumulated extracellular polymeric substances
(EPS; Section 3.5.2) (Castenholz 2009; Stal 2010). This anoxic layer is appreciated
as a thick, black stratum that may reach several cm in depth. In deeper layers of this
anoxic stratum, a band of green sulfur bacteria may occur, marking the maximum
penetration of the far-red light wavelengths (Fig. 3.2a). Green bacteria have a high
sulfide tolerance and can grow in conditions which are toxic to purple bacteria
(Nisbet and Fowler 1999). They are obligate anaerobic photoautotrophs with bacteriochlorophylls in a single photosystem that use H 2 S as the electron donor
(Castenholz 2009). Finally, methane-producing archaea (not shown) are common
throughout the anoxic zone (Robertson et al. 2009; Cardoso et al. 2019).
Cyanobacteria are dominant from a bioenergetic point of view, as they produce
most of the reduced organic matter, and this opens a niche for recycling anaerobes.
The dominance of cyanobacteria also applies, in ecological terms, to the biomass. A
compilation of bacterial 16S rRNA gene clone libraries generated from modern
hypersaline microbial mats (Des Marais 2010) found that the upper 0–5 mm of these
biosedimentary structures produce as many as 949 clones, with cyanobacteria and
plastids comprising up to 20% of the library. Filamentous cyanobacteria (e.g., the
pioneering Oscillatoria sp. and the dominant Coleofasciculus (Microcoleus)
chthonoplastes occurring in well-established mats; Stal et al. 1985) usually make
up the larger portion of the total biomass of the microbial consortium, even if
diatoms and other prokaryotic groups may contribute a significant proportion (see
Ley et al. 2006); this is due to the trichomous nature of the dominant cyanobacteria
(i.e., organized in linear chains of cells that may be several 100s of μm long). These
filamentous cyanobacteria grow conspicuously in the upper mm in order to find the
optimal light intensity for photosynthesis. A highly coherent structure is produced by
the interwoven cyanobacterial filaments (Fig. 3.3a), providing a dense and coherent
fabric for the binding of sediment particles and giving a remarkable external leathery
appearance to the microbial mat (Figs. 3.4e, 3.5, 3.6c). A striking feature of these
thick mats is their elastic deformation, partly resulting from the cohesiveness of this
cellular lattice (Pan et al. 2019).
Microbial consortia are complex biochemically and biologically diverse systems,
in which the microbial constituents cooperatively exploit every niche (Nisbet and
Fowler 1999; Noffke et al. 2013). As it becomes apparent from the previous
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
67