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Geomycology: metals, actinides and biominerals

Geomycology can be simply defined as 'the scientific study of the roles of fungi in processes of fundamental importance to geology' and the biogeochemical importance of fungi is significant in several key areas. These include nutrient and element cycling, rock and mineral transformations, bioweathering, mycogenic biomineral formation and interactions of fungi with clay minerals and metals. Such processes can occur in aquatic and terrestrial habitats, but it is in the terrestrial environment where fungi probably have the greatest geochemical influence. Of special significance are the mutualistic relationships with phototrophic organisms, lichens (algae, cyanobacteria) and mycorrhizas (plants). Central to many geomycological processes are transformations of metals and minerals, and fungi possess a variety of properties that can effect changes in metal speciation, toxicity and mobility, as well as mineral formation or mineral dissolution or deterioration. Some fungal transformations have beneficial applications in environmental biotechnology, e.g. in metal and radionuclide leaching, recovery, detoxification and bioremediation, and in the production or deposition of biominerals or metallic elements with catalytic or other properties. Metal and mineral transformations may also result in adverse effects when these processes result in spoilage and destruction of natural and synthetic materials, rock and mineral-based building materials (e.g. concrete), acid mine drainage and associated metal pollution, biocorrosion of metals, alloys and related substances, and adverse effects on radionuclide speciation, mobility and containment. The ubiquity and importance of fungi in biosphere processes underlines the importance of geomycology as an interdisciplinary subject area within microbiology and mycology.

Geomycology: biogeochemical transformations of rocks, minerals, metals and radionuclides by fungi, bioweathering and bioremediation

The study of the role that fungi have played and are playing in fundamental geological processes can be termed 'geomycology' and this article seeks to emphasize the fundamental importance of fungi in several key areas. These include organic and inorganic transformations and element cycling, rock and mineral transformations, bioweathering, mycogenic mineral formation, fungal-clay interactions, metal-fungal interactions, and the significance of such processes in the environment and their relevance to areas of environmental biotechnology such as bioremediation. Fungi are intimately involved in biogeochemical transformations at local and global scales, and although such transformations occur in both aquatic and terrestrial habitats, it is the latter environment where fungi probably have the greatest influence. Within terrestrial aerobic ecosystems, fungi may exert an especially profound influence on biogeochemical processes, particularly when considering soil, rock and mineral surfaces, and the plant root-soil interface. The geochemical transformations that take place can influence plant productivity and the mobility of toxic elements and substances, and are therefore of considerable socio-economic relevance, including human health. Of special significance are the mutualistic symbioses, lichens and mycorrhizas. Some of the fungal transformations discussed have beneficial applications in environmental biotechnology, e.g. in metal leaching, recovery and detoxification, and xenobiotic and organic pollutant degradation. They may also result in adverse effects when these processes are associated with the degradation of foodstuffs, natural products, and building materials, including wood, stone and concrete. It is clear that a multidisciplinary approach is essential to understand fully all the phenomena encompassed within geomycology, and it is hoped that this review will serve to catalyse further research, as well as stimulate interest in an area of mycology of global significance.

Mutants of Saccharomyces cerevisiae defective in vacuolar function confirm a role for the vacuole in toxic metal ion detoxification

To directly define vacuolar role(s) in metal detoxification, we have examined the responses of vacuole-deficient mutants of Saccharomyces cerevisiae to several potentially toxic metals known to be mainly detoxified in the cytosol (Cu, Cd) or the vacuole (Co, Mn, Ni, Zn). Three mutants, deficient in targeting of vacuolar proteins, were used with JSR18 delta 1 being devoid of any vacuole-like structure while ScVatB and ScVatC were deficient in specific protein subunits of the V-ATPase. The results obtained show that the absence of a vacuole or a functional vacuolar H(+)-ATPase was associated with increased sensitivity and a largely decreased capacity of the vacuole-deficient strains to accumulate Zn, Mn, Co and Ni, confirming an essential role for the vacuole in detoxification of these metals. In addition, the lack of vacuolar involvement in detoxification of Cu and Cd was confirmed since these metals did not exhibit increased toxicity towards the vacuolar mutants nor were there significant differences in Cu or Cd accumulation between parental and mutant strains.

Transport of small lons and molecules through the plasma membrane of filamentous fungi

Less than 1% of the estimated number of fungal species have been investigated concerning the transport of low-molecular-weight nutrients and metabolites through the plasma membrane. This is surprising if one considers the importance of the processes at the plasma membrane for the cell: this membrane mediates between the cell and its environment. Concentrating on filamentous fungi, in this review emphasis is placed on relating results from biophysical chemistry, membrane transport, fungal physiology, and fungal ecology. Among the treated subjects are the consequences of the small dimension of hyphae, the habitat and membrane transport, the properties of the plasma membrane, the efflux of metabolites, and the regulation of membrane transport. Special attention is given to methodological problems occurring with filamentous fungi. A great part of the presented material relies on work with Neurospora crassa, because for this fungus the most complete picture of plasma membrane transport exists. Following the conviction that we need "concepts instead of experiments", we delineate the lively network of membrane transport systems rather than listing the properties of single transport systems.

Small lipid-soluble cations are not membrane voltage probes for Neurospora or Saccharomyces

Small lipid-soluble cations, such as tetraphenylphosphonium (TPP+) and tetraphenylarsonium (TPA+) are frequently used as probes of membrane voltage (delta psi, or Vm) for small animal cells, organelles, and vesicles. Because much controversy has accompanied corresponding measurements on 'walled' eukaryotic cells (plants, fungi), we studied their transport and relation to Vm in the large-celled fungus Neurospora crassa-where Vm can readily be determined with microelectrodes-as well as in the most commonly used model eukaryotic cell, the yeast Saccharomyces cerevisiae. We found no reasonable conditions under which the distribution of TPP+ or TPA+, between the cytoplasm (i) and extracellular solution (o), can serve to estimate Vm, even roughly, in either of these organisms. When applied at probe concentrations (i.e., < or = 100 microM, which did not depolarize the cells nor deplete ATP), TPP+ stabilized at ratios (i/o) below 30 in both organisms. That would imply apparent Vm values positive to -90 mV, in the face of directly measured Vm values (in Neurospora) negative to -180 mV. When applied at moderate or high concentrations (1-30 mM), TPP+ and TPA+ induced several phases of depolarization and changes of membrane resistance (Rm), as well as depletion of cytoplasmic energy stores. Only the first phase depolarization, occurring within the perfusion-turnover time and accompanied by a nearly proportionate decline of Rm, could have resulted from TPP+ or TPA+ currents per se. And the implied currents were small. Repeated testing, furthermore, greatly reduced the depolarizing effects of these lipid-soluble ions, implicating an active cellular response to decrease membrane permeability.