3. Mineral-weathering bacteria
Many organisms and microorganisms such as plants, algae,lichens, fungi and bacteria can colonize rock surfaces or soil minerals, especially pores and cracks, but their relative impact on the global mineral-weathering process is difficult to discriminate from purely abiotic processes (Landeweert et al., 2001; Uroz et al., 2009a). Despite this, it is now well established that mineral-weathering can be accelerated and even initiated by microorganisms. The role of fungi in mineral-weathering was highlighted in the last few decades, and more recently, it was evidenced that bacterial communities can impact mineral stability, leading to the release of inorganic nutrients (Calvaruso et al., 2006). To date, a large range of bacterial strains or communities fromdiverse genera have been reported to be able to colonize mineral surfaces and/or to weather minerals, and for some of them a plant growth-promoting effect was demonstrated (Fig. 1). The proven or hypothesized mechanisms used by bacteria to weather minerals include several oxidoreduction and dissolution reactions, as well as the production of weathering agents, such as protons, organic acids and chelating molecules.