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The methods described below are those commonly used in the study of living microbial mats. Coverage here is not exhaustive; for some methods not here discussed only literature references are indicated. The material here presented (including appended tabulations of data: Tables 20.5–20.7) is relevant to the studies discussed in Chapter 6.
Preserved and Sectioned Material of Whole Mat (J. D. Farmer)
Syringe cores (30 cc) of mat are subsampled by slicing with a razor blade, and are fixed in a mixture of 3% glutaraldehyde and 1% formalin in pond water immediately following collection. Dehydrated samples are infiltrated with Spurr's Low Viscosity Embedding Medium (Spurr 1969; Polysciences, Inc.) under vacuum for several hours. A two-step graded series, beginning with a 50:50 mixture of resin and 100% ethanol for 2 hours, was found to enhance penetration. The hard cure schedule recommended by Polysciences is followed in order to obtain the hardest embedment for thin sections. Embedments are prepared in plastic “peel-a-way” molds (Polysciences, Inc.) and cured at 70°C for 8 hours.
Sections (20 to 30 µm thick, in order to facilitate comparisons with thin sections of fossil stromatolites) are prepared by hand grinding, following a modification of standard petrographic methods (Nye et al. 1972). Organisms in sections prepared from Spurr's resin can be stained using a 1% solution of toluidine blue in distilled water. A 1% solution of alizarine red in dilute HC1 is an effective counterstain for detecting carbonate.
Modern microbial mats are structurally coherent macroscopic accumulations of microorganisms. Mats are widely distributed on earth. They are found in a surprisingly large number of diverse environments from the equatorial zones to both polar regions. They vary in size from extensive terrestrial and hypersaline mats that cover areas several square kilometers in extent to minute mats only a few square centimeters in area found in small thermal springs. They vary in thickness from massive accumulations measured in meters, such as those in the Persian Gulf and the Red Sea region, to thin films less than a few millimeters in thickness. In addition to being highly varied in size, modern microbial mats are also very diverse in morphology, community structure, and physiological characteristics. What do such mats have in common? Under what conditions do they form? What is the basis of their diversity? What insight do they provide, if any, to the interpretation of the widespread stromatolites of the Proterozoic?
Microbial mats are accretionary cohesive microbial communities which are often laminated and found growing at the sediment-water (occasionally sediment-air) interface. Most mats stabilize unconsolidated sediment. The mats are comprised of the various microorganisms that accumulate along with their metabolic products. The most conspicuous of these products is usually a copious amount of extracellular polysaccharide which helps hold the cells together to form a cohesive structure.
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