Introduction to microbial mats

What are 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!
Within 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, S, N, 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.