Publisher Summary Many organisms excavate calcified substrates by chemomechanical mechanisms. In marine environments, bacteria, fungi, algae, and invertebrates penetrate calcium carbonate substrates by secreting acids, chelators, or enzymes. These may function either singly or in combination. Boring bacteria, fungi, algae, and sponges penetrate calcium carbonate by chemical dissolution at the cellular level. These cellular mechanisms, particularly those utilized by sponges, are described in this chapter. Several hypotheses are presented for the cytological mode of penetration of calcium carbonate by boring bacteria, fungi, and algae. Bacteria isolated from coral skeletons are capable of digesting chitin in vitro, suggesting that the mode of carbonate breakdown is via the organic matrix of skeletal carbonates. Marine fungi penetrate calcium carbonate by first roughening and pitting the surface and then extending hyphae throughout the substrate. Soil fungi penetrate dead bone (calcium phosphate) by simultaneously dissolving both calcium phosphate and organic matrix. Resorption occurs at the site of contact of the fungal membrane with bone.