Structure of a microbial mat at Great Sippewissett Marsh, Cape Cod, Massachusetts

The microzonation of phototrophic bacteria in a flat laminated microbial mat at Great Sippewissett Salt Marsh on Cape Cod, Massachusetts, was studied using a combination of scanning and transmission electron microscopy, light microscopy and photosynthetic pigment analysis. Comparison of pigment content and ultrastructural information from electron microscopy of thin sections allowed the authors to determine the major groups of photosynthetic bacteria present. The approximately 1-cm-thick mat is located in sandy intertidal sediments of the marsh and comprised four to five distinctly colored layers. The uppermost brown layer contained Lyngbya, Nostoc, Phormidium (cyanobacteria) and Navicula (diatom) species. An intermediate bluish-green layer was dominated by Oscillatoria species. A central pink layer contained purple sulfur bacteria such as Amoebobacter, Thiocapsa, Chromatium and Thiocystis species. Below this was a distinctive orange layer, formed largely by one species of purple sulfur bacteria, Thiocapsa pfennigii . The lowermost and thinnest layer contained green sulfur bacteria of the genus Prosthecochloris .