Throughout the geologic past, fungi have played an important role in their interactions with the depositional environment, and together with cyanobacteria, are today receiving increasing attention because of their role in shaping the geosphere. The discipline of geomicrobiology involves the study of organisms, their interactions, and the materials that they colonize (Konhauser, 2007). Certain fungi are known to degrade and liquefy coal (Sterflinger, 2000), and they are the only organisms that can completely break down lignin. Fungi in mycorrhizal and lichen symbioses are also involved in weathering rock, a form of biological weathering (Landeweert et al., 2001; Hoffland et al., 2004). Fungi in very cold or desert environments appear to be important as rock colonizers (Selbmann et al., 2005). Fungi are involved in the alteration of several minerals, such as carbonates and silicates; for example, fungi can be involved in both the formation and destruction of carbonate deposits (Sterflinger, 2000). The roles of fungi in geological activities are only beginning to be fully appreciated as the focus in the past has been mostly on bacteria (Gadd, 2007; Perri and Tucker, 2007).
One type of fungal activity seen in the fossil record includes borings in rock. Since there are several groups of organisms, however, that are borers, including cyanobacteria, algae, lichens, sponges, bryozoans, gastropods, and several other invertebrate groups, distinguishing the organism responsible for the boring is often very difficult (Elias and Lee, 1993). In one study on endolithic fungi from Jurassic rocks, scanning electron microscopy was used after resins casts were made of the molds (Gatrall and Golubic, 1970). Based on comparison with extant forms, it was determined that these borings were the result of fungi, although many earlier contributions had regarded such trace fossils as the result of algal borers.
Fungal and algal borers may also be involved in a process termed micritization, in which carbonate particles are reduced in size and combined with chemically precipitated carbonate to produce a finely crystalline form of carbonate, also called calcite mud. Biogenic micritization, which is mediated by organisms, has been noted in rocks dating back to the Paleozoic; in a few cases the filaments of the endolithic borers are preserved in the rocks, but in most cases their presence can only be inferred (Harris et al., 1997). Endolithic filaments are known from the Upper Silurian of Morocco (Barbieri et al., 2004). Although fungi are involved in some of these structures, many are also mediated by various types of microorganisms (cyanobacteria and algae), which produce micrites as byproducts of their metabolism. In some instances these structures demonstrate a faint banding pattern that suggests they may have been produced during different seasons.
Fungi have also been important in the geologic record through their activities in biomineralization—the synthesis of minerals from simple compounds by organisms. For example, the occurrence of needle-fiber calcite in Carboniferous paleosols (fossil soils) has been regarded as evidence of ectomycorrhizal associations in the ecosystem (Wright, 1986). Certain extant vascular plant roots accumulate secondary soil carbonate through biomineralization. Some of these roots have larger diameters because of calcification of cortical parenchyma cells. It is suggested that carbonate biomineralization and acid extrusion in these root cells may represent a mechanism used by plants to acquire soluble nutrients from the rhizosphere, or a method to live in soils with excessive calcium (Kosir, 2006). As paleobotani-cal data are further integrated with sedimentology, it may be possible to document such an adaptive feature in the roots of certain fossil plants.
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