3.5.4 Consolidation and Preservation of Multilayered
Biosedimentary Structures
To close this essay, it is now fit to recapitulate how sediment microbes have gone
from simple, two-dimensional biofilms to multilayered biosedimentary structures. It
has been hypothesized (Nisbet and Fowler 1999) that modern microbial mats reflect
metabolic developments and pathways acquired over the course of microbial evolution. Hence, the layered structure and the niche segregation among different
microbial guilds with anoxic levels occupied by archaea and bacterial respirers,
fermenters, and green sulfur bacteria and oxic levels harboring aerobic purple sulfur
bacteria and cyanobacteria may reflect how the exposed sediments on Earth
transitioned from being subject to reducing conditions to oxidizing ones. This
necessarily called for a complexation in structure, from simple two-dimensional,
mostly organic biofilms to multilayered biosedimentary structures that become
functional units by themselves and in which the dominant microbes in each layer
exploit specific niches and marked O 2 , H 2 S, and light gradients.
Surface processes are of paramount importance in photosynthetic microbial
communities, particularly with respect to light harvesting and nutrient exchange.
As much as this calls for a disposition of photoautotrophs in the uppermost layers,
the cohesiveness created by trichomes likened to “cyanobacterial sealing” (sensu
Seilacher et al. 1985), the accumulation of organic matter through EPS secretion, and
the accumulation of empty sheaths also set limits for vertical diffusion of O 2 and
create a boundary for obligate prokaryote anaerobes. On top of this metabolic
segregation of microbes, there is an active physical role played by cyanobacterial
trichomes in the accretion of sediment particles ultimately promoting the generation
and consolidation of the three-dimensional biolaminites this essay has focused upon.
Periodic or sporadic physical disruption in the form of tidal or storm currents may
spatially affect surface microbial mats and generate deformation MISS. Hydrodynamic processes may also resupply or redistribute sediment, thus providing new
substrates for colonization. Through the above-described steps which roughly match
the properties of emergent microbial mats, three-dimensional “biolaminites” are
generated over time (Gerdes et al. 1991), which may accumulate as they become
buried, keeping record of microbial textures and MISS. Figure 3.3b provides a
striking graphic synthesis of how the repetition of these processes over time yields
biosedimentary sequences which aid in paleoenvironmental reconstruction.
Due to their biosedimentary nature, MISS are delicate structures that get preserved under exceptional circumstances and are known as Lagerstätten (Martin
1999). One such preservation process is cyanobacterial sealing (Seilacher et al.
1985). Microbial biofilms at the sediment-water interface facilitate the preservation,
inhibit organic decay, and reduce the erosion threshold of sediments. Even while
epibenthic microbial mats are currently restricted to a relatively few coastal environments such as hypersaline siliciclastic basins, their study with the actualistic,
interdisciplinary approach of GeoBiology can shed light into the processes that led
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
81
Biosedimentary Structures
To close this essay, it is now fit to recapitulate how sediment microbes have gone
from simple, two-dimensional biofilms to multilayered biosedimentary structures. It
has been hypothesized (Nisbet and Fowler 1999) that modern microbial mats reflect
metabolic developments and pathways acquired over the course of microbial evolution. Hence, the layered structure and the niche segregation among different
microbial guilds with anoxic levels occupied by archaea and bacterial respirers,
fermenters, and green sulfur bacteria and oxic levels harboring aerobic purple sulfur
bacteria and cyanobacteria may reflect how the exposed sediments on Earth
transitioned from being subject to reducing conditions to oxidizing ones. This
necessarily called for a complexation in structure, from simple two-dimensional,
mostly organic biofilms to multilayered biosedimentary structures that become
functional units by themselves and in which the dominant microbes in each layer
exploit specific niches and marked O 2 , H 2 S, and light gradients.
Surface processes are of paramount importance in photosynthetic microbial
communities, particularly with respect to light harvesting and nutrient exchange.
As much as this calls for a disposition of photoautotrophs in the uppermost layers,
the cohesiveness created by trichomes likened to “cyanobacterial sealing” (sensu
Seilacher et al. 1985), the accumulation of organic matter through EPS secretion, and
the accumulation of empty sheaths also set limits for vertical diffusion of O 2 and
create a boundary for obligate prokaryote anaerobes. On top of this metabolic
segregation of microbes, there is an active physical role played by cyanobacterial
trichomes in the accretion of sediment particles ultimately promoting the generation
and consolidation of the three-dimensional biolaminites this essay has focused upon.
Periodic or sporadic physical disruption in the form of tidal or storm currents may
spatially affect surface microbial mats and generate deformation MISS. Hydrodynamic processes may also resupply or redistribute sediment, thus providing new
substrates for colonization. Through the above-described steps which roughly match
the properties of emergent microbial mats, three-dimensional “biolaminites” are
generated over time (Gerdes et al. 1991), which may accumulate as they become
buried, keeping record of microbial textures and MISS. Figure 3.3b provides a
striking graphic synthesis of how the repetition of these processes over time yields
biosedimentary sequences which aid in paleoenvironmental reconstruction.
Due to their biosedimentary nature, MISS are delicate structures that get preserved under exceptional circumstances and are known as Lagerstätten (Martin
1999). One such preservation process is cyanobacterial sealing (Seilacher et al.
1985). Microbial biofilms at the sediment-water interface facilitate the preservation,
inhibit organic decay, and reduce the erosion threshold of sediments. Even while
epibenthic microbial mats are currently restricted to a relatively few coastal environments such as hypersaline siliciclastic basins, their study with the actualistic,
interdisciplinary approach of GeoBiology can shed light into the processes that led
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
81