to microbial mats
Microbial mats may be described as laminated self-sustaining ecosystems.
They may be described as laminated because the pigmentation and
by-products of major functional groups provides mats with a distinct
layered appearance. As light enters a mat it is selectively attenuated
according to how photrophic organisms have evolved to exploit certain
wavelengths. They are self-sustaining because they possess the essential
biocomplexity for carrying out the essential biogeochemical processes
that sustain life. Present-day mats are believed to be descendants
of the first extant biological communities on Earth, stromatolites.
Fossils of major mat constituents, cyanobacteria, have been found
in stromatolitic structures dating to 3.5 billion years ago!
the span of a few millimeters, mats contain an incredible amount
of biological diversity. There exists within mats a wide range of
phototrophs (organisms that carry out photosynthesis), heterotrophs
(organisms that consume organic matter), and chemoautrotrophs (organisms
that provide their own carbon resources using reduced chemical species
such as hydrogen sulfide). (Click here
to see a list of organisms that may be found in mats and the reactions
they mediate.) Steep vertical redox
(Eh) and chemical gradients that
span microns to millimeters characterize mats. A wide array of microorganisms
are oriented within the mat according to each's energetic, nutrient,
and ecological requirements. During the day, the bulk of photosynthetic
CO2 fixation supporting primary production and nutrient cycling
is generated by a diverse assemblage of cyanobacteria that dominates
the upper layers of mats. Poorly illuminated lower regions of mats
are hypoxic (low O2) or anoxic (no O2). This leads to sulfide produced
by sulfate-reducing bacteria accumulating in the lower portions
of the mat. During the day, the top portion may be supersaturated
with respect to O2, but turn anoxic within a few millimeters because
O2 is consumed rapidly. At night, after photosynthesis has ceased
the biogeochemical gradients shift upward in the mat, so that reduced
species such as H2S may reach the surface of the mat!
All the major biogeochemical transformations and organisms that
mediate them may be found in mats. These include photosynthesis,
N2 fixation, denitrification, nitrification, sulfate reduction,
methanogenesis, and sulfide oxidation. Metabolites produced by these
processes may cross-inhibitory. One intriguing aspect about mats
is their ability to complete C,
and other elemental cycles under potentially inhibitory conditions.
For example, sulfate reduction, N2 fixation, and denitrification
all are inhibited by even low pO2, yet they can occur during peak
photosynthetic periods (i.e. peak O2 production). Sulfate reduction
is responsible for a large fraction of the organic matter mineralization
(i.e. conversion to CO2) in some mats. Considerable research has
tried to reconcile the detection of sulfate reduction in oxygenated
layers of the mat. Physiological studies of SRB have identified
several possible mechanisms that explain how SRB cope with O2 and
in close proximity to cyanobacteria. These mechanisms include motility,
the ability to respire O2 -but not grow with O2, and the ability
to utilize a wide range of carbon substrates.
Another possibility may be that metabolically active microbial consortia
create small volumes of space, that are depleted enough in O2, anoxic
microzones, to allow anaerobic processes such as N2 fixation and
sulfate reduction to proceed. (Click
here to learn about microbial consortia). Anoxic microzones
may be formed when biological respiration and chemical consumption
of O2 proceed at a rate faster than it can diffuse into an area.
Therefore, a key factor to regulating anoxic microzones is the diffusion
of O2. An important element to slowing O2 diffusion in mats is extracellular
polymeric substances (EPS). Many organisms exude complex polymers
composed of glucose, galactose, and hyaluronic acid. Because of
the vast amounts of EPS in mats, mats may be viewed as systems embedded
in a semi-solid organic matrix. EPS may serve many functions for
mats and their organisms besides impeding O2 diffusion. EPS can
promote sediment stabilization, provide cohesion to the mat, provide
surfaces and substrates for growth, bind potential toxins, bind
heavy metals, and protect against desiccation or increases in salinity.
Thus, aided by laminae, porewater and EPS barriers, O2 consumption
and respiration can exceed diffusion creating O2-depleted microzones.