Bacteria living at the bottom of shallow lagoons or lakes are thought to be responsible for the formation of hard, layered limestone structures, so-called stromatolites. Stromatolites occur in some of the oldest rocks on Earth and may thus represent the earliest signature of life. A longstanding controversy is which type of bacteria is responsible for the precipitation of the limestone, the building material of the stromatolites. Some scientists suggest cyanobacteria, which are living at the very surface of the stromatolites and produce oxygen like plants. Others suggest that bacteria below the millimeter thick cyanobacterial layer capable of breathing sulfate in absence of oxygen are responsible for the precipitation of limestone. Patrick Meister of the Max Planck Institute, Bremen, demonstrates by a model calculation that if small amounts of sulfate are consumed, sulfate metabolism promotes limestone dissolution. Only if the available sulfate is almost completely consumed, which is not commonly the case in stromatolites, sulfate metabolism promotes precipitation. If we assume that the ocean was more oversaturated with limestone than the modern ocean during several times in Earth history, he writes, then sulfate metabolism would not have contributed to limestone precipitation at all. Thus, stromatolites may either have formed by other groups than sulfate reducing bacteria, or the bacteria were just passively encrusted by limestone.
Reconstructing magma failure and the degassing network of dome-building eruptions
Yan Lavalle et al., Earth, Ocean and Ecological Sciences, Unive
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Geological Society of America