Among the many interactions between living organisms and the mineral world, the formation and destruction of carbonates stands as one of the most conspicuous and widespread, from microscale calcification in biofilms to the accretion of coralline atolls. Biogenic carbonate precipitation has received the most attention, but its dissolution can also be mediated by organisms, and by microorganisms in particular. Fungi, microalgae and cyanobacteria that actively bore into calcareous substrates have been known for more than a century, and have been leaving fossils and trace fossils since the Precambrian. These boring microorganisms are centrally implicated in a variety of geological phenomena, ranging from the erosive morphogenesis of coastal limestones, the destruction of coral reefs, the reworking of carbonaceous sands and the cementation of stromatolites. But for all their significance, the mechanism by which they can excavate carbonates in a controlled manner remains to be studied. The most common hypothesis as to their action mechanisms has been that they dissolve limestone by excretion of acids. However, we contend that, in the case of photosynthetic organisms like cyanobacteria, their activity constitutes an apparent paradox, since the dissolution of carbonates runs contrary to the well-known geomicrobial effects of oxygenic photosynthetic metabolism, which will tend to make the surrounding medium alkaline and therefore promote calcification, not carbonate dissolution. One can say that boring cyanobacteria are intriguing. We will test three alternative models than can explain cyanobacterial boring and still be consistent with thermodynamic, physiological and mineralogical constraints. Two models are based on the separation of photosynthetic and respiratory activities (either temporally or spatially). The third model is based on localized and directed cellular calcium transport. We will undertake a three-tiered experimental approach using cultivated microorganisms and well characterized mineral substrates that should offer evidence regarding the validity of each of these models. We will use: a) long-term monitoring of the rates of growth and boring with manipulations of various environmental parameters, b) short-term studies of microscale mass transfer in actively boring systems, including the effects of specific inhibitors, using microsensors , and c) advance microscopy studies of active mineral /microbe systems that offer both visual and micro-chemical information: laser scanning confocal microscopy and secondary ion mass spectroscopy (SIMS). The project also entails significant educational and outreach activities. It calls for the development of geomicrobiology materials for High School and K-12 educators, a web page for the general public, and participation in yearly outreach activities.
