What is eating my rocks? A possible novel biological niche in limestone

Abstract. Metamorphic limestones in Namibia and Oman were found to be consumed inside the rock mass by microbiological activity of a thus far unknown nature that created bands of parallel tubules. Tubule of up to 0.5 mm wide and 30 mm long collectively form bands of tens of meters long. These bands formed along fractures in the rock and only surfaced after erosion. In fresh outcrops the tubules are filled with white calcium carbonate with internal structures. This filling is depleted for several metal elements that can be incorporated into biomatter and was deposited by the microorganisms as they grew inside the rock. The rim of the tubules contain1 µm wide growth rings enriched for P and S. Fluorescence microscopy confirmed presence of biological material. The rocks are too old to conserve DNA or protein. We consider this a new niche for life that has so far not been described.