Freshwater Ferromanganese Stromatolites from Lake Vermilion, Minnesota: Microbial Culturing and Environmental Scanning Electron Microscopy Investigations

Abstract

Lacustrine ferromanganese oxide nodules have been discovered in Lake Vermilion, northern Minnesota, USA. They occur as two morphotypes: laminated discs growing around or on cobbles ("reef-type"), and ovoid nodules on muddy gravel ("oncoidtype"). Nodules have only been found within a depth-range of ~1 to 7 m, and no nodules were found on or in fine mud sediments. Oxide coatings on recent litter allowed estimates of growth rates between 2.3 to 3.5 w m/yr. X-ray diffraction, energy dispersive X-ray spectroscopy, and thin section and environmental scanning electron microscopy of the nodules showed microorganisms, chiefly bacteria but also including cyanobacteria, diatoms, and other protozoans, mineralized by X-ray amorphous ferromanganese oxyhydroxide compounds and forming porous granular layers. ESEM examination also revealed solid-appearing layers, with no microorganism remains. Bacterial culturing experiments were carried out using lake water, sediment, and nodule swabs and pieces as the inocula. All resulted in the growth of bacteria that reduce and/or oxidize iron and manganese, showing that metal oxidizing and reducing bacteria are ubiquitous within the lake habitats. The culturing experiments also demonstrate that the bacteria can cause the formation of solid metal oxides. The combination of culturing and ESEM investigations demonstrate the probable biogenic nature of the Lake Vermilion nodules, which are thus better termed stromatolites and oncoids. However, the solid layers suggest that some abiotic aspects play a role in the genesis of the stromatolites. It is also likely that the development of stromatolites is strongly influenced by environmental factors such as substrate and depth.