Plausible mechanisms for the boring on carbonates by microbial phototrophs

Photosynthetic microbes, particularly cyanobacteria, that bore into carbonates are ancient biological players in various geologic phenomena such as the destruction of biogenic carbonates and coastal limestones, the reworking of carbonate sands and the cementation of microbialites. Their signatures are important tools for paleoenvironmental reconstruction, and they play a significant role in marine aquaculture. In spite of their geologic, environmental and economic importance, the mechanism by which they are able to excavate calcareous and calcophosphatic mineral substrates remains unknown. Excavation by acidulation, commonly thought to be a possible mechanism, constitutes nothing less than an apparent paradox, in that the geochemical consequence of oxygenic photosynthesis should be carbonate precipitation, not dissolution. Three alternative mechanistic models are presented here that may allow cyanobacterial boring to proceed and be still consistent with available evidence, as well as microbiological and geologic/geochemical principles. They are based on either temporal or spatial separation of photosynthesis and respiration, and on the active extrusion of calcium ions through an active cellular uptake and transport process. From the three models, the latter is shown to be most appropriate in describing and explaining the boring phenomenon. Several experimental approaches are discussed that would be appropriate to elucidate the paradox.