Bacterial Stromatolites: Origin of Laminations

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Abstract
Laminated mats composed of motile filamentous photosynthetic bacteria and nonmotile unicellular blue-green algae occur in a large number of Yellowstone hot springs at temperatures between 55° and 70°C. Field studies indicate that the bacteria are the predominant mat-forming component. Under low light intensities, mats composed exclusively of bacteria can be formed. The bacteria undergo a diurnal migration, moving on top of the algae during the night and becoming mixed again with the algae during the day as a result of algal growth. Thus, the laminations probably arise as a result of differential migration of the bacteria in daily response to reduced light intensities. This response to light is exactly opposite to that previously reported for filamentous stromatolite-forming, blue-green algae, but the net result is the same—formation of a laminated mat.