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FIGURE 2.1 Model indicating the four zones of activity within a mineral-weathering lichen as depicted by Banfield et al. (1999). In zone 1, photosynthetic members generate carbon and crystalline lichenic acids. In zone 2, there is direct contact among microbes, organic products, and the mineral surface. In zone 3, organic acids act to solubilize rock in the presence of direct rock/organism contact, particularly fungal hyphal penetration into cracks. Zone 4 is characterized by unweathered rock and inorganic chemical reactions.

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