Phototrophs in high iron microbial mats: microstructure of mats in iron-depositing hot springs

Chocolate Pots Hot Springs in Yellowstone National Park are high in ferrous iron, silica and bicarbonate. The springs are contributing to the active d evelopment of an iron formation. The microstructure of photosynthetic micro bial mats in these springs was studied with conventional optical microscopy , confocal laser scanning microscopy and transmission electron microscopy. The dominant mats at the highest temperatures (48-54 degrees C) were compos ed of Synechococcus and Chloroflexus or Pseudanabaena and Mastigocladus. At lower temperatures (36-45 degrees C), a narrow Oscillatoria dominated oliv e green cyanobacterial mats covering most of the iron deposit. Vertically o riented cyanobacterial filaments were abundant in the top 0.5 mm of the mat s. Mineral deposits accumulated beneath this surface layer. The filamentous microstructure and gliding motility may contribute to binding the iron min erals. These activities and heavy mineral encrustation of cyanobacteria may contribute to the growth of the iron deposit. Chocolate Pots Hot Springs p rovide a model for studying the potential role of photosynthetic prokaryote s in the origin of Precambrian iron formations. (C) 2000 Federation of Euro pean Microbiological Societies. Published by Elsevier Science B.V. All righ ts reserved.