Uptake of 75[Se]-selenite by Saccharomyces cerevisiae has been characterized. At a 0.5 mM selenite, ~0.14 nmol Se (106 cells)−1 was rapidly accumulated by the cells at a rate of ~56 pmol.min−1 (106 cells)−1 which was independent of temperature and glucose. This rapid phase was followed by a slower uptake phase which was sensitive to glucose, temperature and metabolic inhibitors [2,4-dinitrophenol (DNP), carbonyl cyanide m-chlorophenyl hydrazone (CCCP), potassium cyanide (KCN) and sodium azide (NaN3)] and therefore presumed to be metabolism-dependent. Two transport systems appeared to be involved in selenite uptake. At the low range of selenite concentrations used (0.025–0.1 mM), a high affinity transport system occurred with apparent Km and Vmax values of 54.0 μM and 3.14 pmol.min−1 (106 cells)−1 respectively. A low affinity system was present at higher concentrations (0.1–1.0 mM) with apparent kinetic parameters of Km=435 μM and Vmax=11.6 pmol.min−1 (106 cells)−1. Elevated sulphate concentrations (up to 2.5 mM) did not affect the accumulation of selenite. However, the transport rate from 0.5 mM selenite was stimulated by sulphite, with the maximum effect occurring at 0.5 mM sulphite. Methionine had a detectable inhibitory action on selenite uptake whereas cystine and cysteine completely inhibited active transport of selenite. Transmission electron microscopy of 5 mM selenite-grown cells revealed the presence of abundant small cytoplasmic vesicles containing electron-dense granules which could represent an intracellular selenium-detoxification mechanism.

This article outlines the important biogeochemical roles that fungi play in the degradation, utilization and transformation of organic and inorganic substrates. Fungal populations are intimately involved in element cycling at local and global scales and such processes have major implications for living organisms, notably plant productivity and human health. While most attention has focussed on carbon cycling, the involvement of fungi in most other elemental cycles is emphasised. Some fungal transformations have applications in environmental biotechnology, e.g. in metal leaching, recovery and detoxification, xenobiotic and organic pollutant degradation, as well as result in adverse effects when these mechanisms are involved in degradation of foodstuffs, natural products and building materials. This account highlights the interdisciplinary approach that is necessary to further understanding of the important roles that fungi play in the biogeochemical cycling of elements, the chemical and biological mechanisms that are involved, and their biotechnological significance.

Responses of Rhizoctonia solani to spatial heterogeneity in sources of carbon, and associated translocation of carbon (C), were studied in a simple microcosm system comprising two discrete domains of agar gels separated on a glass slide and overlain with a porous membrane. Two arrangements of the gel pairs were used, one containing two equally large resources (representing ‘homogeneous’ conditions) and one containing a large and a negligible resource (representing ‘heterogeneous’ conditions). The nutrient sources were a standard mineral salt medium with or without glucose as sole C source. The fungus was inoculated onto one domain and growth responses determined by direct measurement of biomass. Translocation of C was quantified by use of 13C-enriched glucose. This substrate was either added to the agar at the outset, when studying newly developing colonies, or as a pulse into already established colonies. When growing in heterogeneous conditions, the fungus actively translocated C from a glucose-containing domain to sustain growth in the adjacent region lacking such a resource. In homogeneous conditions there was evidence of passive translocation (diffusion), but the fungus preferentially used local resource to maintain growth. Active translocation was only observed in newly growing colonies, whereas passive translocation occurred in both growing and established colonies. When the fungus was pulsed with a 13C-enriched glucose solution after 10 d growth, 2.5 times more 13C was taken up by the fungus grown in heterogeneous than homogeneous conditions, suggesting uptake exceeded local demands. In heterogeneous conditions, the total amount of 13C enriched glucose taken up by the fungus was independent of the location of the enriched glucose in the underlying medium. When the nylon membrane was replaced by Cellophane (an additional C source), degradation of the membrane and an increase in biomass occurred only in the heterogeneous system. The possible implications for these results in soil systems are discussed.

The study of filamentous fungi can be difficult through experimental means alone due to the complexity of their natural growth habitat (e.g. soils) and the microscopic scale of growth (e.g. tip vesicle translocation and hyphal tip extension). Mathematical modelling provides a complimentary, powerful and efficient method of investigation. In this article, earlier mathematical models are briefly reviewed, before an overview of the construction and resultant predictions of a new model for fungal growth and function is given. Model predictions are compared to experimentally obtained data, giving new insight into the complex interaction between the developing mycelium and its environment.

‘Geomycology’ can be defined as the impact of fungi on geological processes, including the alteration and weathering of rocks and minerals, the accumulation of metals and their role in nutrient cycling and influence on proliferation of microbial communities in mineral substrates. Although many studies on microbial interactions with minerals have been published in recent years, the main focus of geomicrobiology has been on prokaryotes. Recently, it has become apparent that epi- and endolithic fungi comprise a significant component of the microflora in a wide range of rocks including siliceous types (silica, silicates and aluminosilicates), sandstone, granite, limestone, marble and gypsum. However, to date little is known about their in situ growth patterns or biogeochemical roles in such an environment. The aim of this article is to highlight our recent work on the biogeochemical roles of fungi inhabiting limestone (CaCO3) and dolomite (CaMg(CO3)2) rocks and to emphasise the importance of fungi as agents of geological change.

Metal-contaminated soils often contain a spatially heterogeneous distribution of metal concentrations, and the ability of fungi to colonize such metal-contaminated domains will be influenced by the nutritional resources available. An experimental system based upon tessellated agar tiles was used to study the influence of nutrients upon the ability of soil fungi Trichoderma virens and Clonostachys rosea to colonize spatially discrete toxic metal (copper and cadmium) containing domains. The growth parameters recorded demonstrated a decrease in apparent metal toxicity with increasing concentration of available carbon source. It was shown that maximum hyphal extension rates and the efficacy of carbon substrate utilization of both species decreased with increasing concentration of toxic metals. It was also observed that in the gap between metal-free and metal-containing tiles, the presence of toxic metals led to negative chemotropic reactions and cessation of growth, swelling and lysis of some hyphal tips. Penetration of the hyphae into the metal-containing domain was often followed by the formation of very dense mycelia or mycelial ‘bushes’ representing an associative (constraining, exploitative or ‘phalanx’) growth strategy of the mycelial system. After the fungi entered the toxic metal-containing domains, they often produced long sparsely-branched or branchless explorative hyphae representing a dissociative (expansive, explorative or ‘guerrilla’) growth strategy. Our data therefore demonstrate that fungi efficiently use both ‘phalanx’ and ‘guerrilla’ states of the mycelial system as well as shifts in these growth strategies as a response to toxic metal stress combined with nutritionally-poor conditions.

The effects of different temperatures and pH on the growth and solubilization of insoluble calcium phosphate, cobalt phosphate, manganese phosphate, strontium hydrogen phosphate and zinc phosphate by Rhizoctonia solani on solidified media were assessed. Solubilization of the metal phosphates was monitored by the production of a clear zone around or underneath the fungus. R. solani efficiently solubilized all five metal phosphates except cobalt phosphate when grown on medium at pH 7. Solubilization activity by R. solani decreased with increasing pH on medium containing calcium phosphate but increased on strontium hydrogen phosphate-amended medium. The uptake of metals by the mycelia was unaffected by the pH of the medium or the growth temperature. Small quantities of crystals were produced in the agar when R. solani was grown on calcium phosphate- and strontium hydrogen phosphate-amended media and these were identified as calcium or strontium sulphates respectively: there appeared to be little or no production of insoluble oxalates although a role for oxalate in the overall solubilization process cannot be discounted. These results are discussed in relation to their physiological and environmental significance, and the important roles of fungi in effecting transformations of insoluble metal-containing compounds in the environment.

The objective of this study was to assess the influence of clay minerals on the morphology of mycelial pellets produced under submerged conditions as a prelude to the development of biomineral sorbents for toxic metals. The macro- and microscopic morphology of fungal pellets of melanin-containing microfungi Cladosporium cladosporioides, C. herbarum and Humicola grisea grown in liquid clay-containing medium was studied using scanning electron microscopy. It was found that the inclusion of clay minerals (bentonite, palygorskite and kaolinite) in the liquid medium influenced size, shape and structure of the mycelial pellets produced. In general, a reduction of pellet size, an increase in the length of surface hyphae of the pellets (except for H. grisea), and a reduction in exopolymer production were observed with increasing clay mineral concentrations up to 5% (w/v). For C. herbarum, an increasing concentration of bentonite changed the normal pelleted growth form to diffuse star-like growth, which was also observed for C. cladosporioides in the presence of kaolinite. The presence of bentonite and kaolinite in the medium for H. grisea and palygorskyte for C. cladosporioides led to the formation of numerous bulbous hyphae in pellets. However, bentonite decreased such bulbous growth in C. herbarum compared to control pellets which grew as bulbous mycelium. The dynamics of C. cladosporioides growth and pellet formation in the presence of bentonite (0·5% (w/v)) was investigated. It was found that the clay particles were involved in the formation of pellet structure at all stages of fungal growth. The porosity of a growing pellet (evaluated as the ratio of surface hyphae length to pellet radius) increased during early growth (17–24 h) and then decreased up to the stationary phase. A general model of the structure of a fungal pellet grown in clay-containing medium is proposed. The pellets consist of three main layers: a central core, with densely packed mycelium aggregated with solid clay minerals or a matrix of clay/polysaccharides; a middle layer with looser mycelium mixed with clay mineral flakes; and an outer, or ‘hairy’ zone, with loose hyphae surrounded by clay mineral flakes. The relevance of these studies to fungal growth and morphology, and to the development of biomineral sorbents for metal removal is discussed.

The ability of commercially-available citric, oxalic and gluconic acids to bind Co2+ and Zn2+ was investigated and compared with culture filtrates from Aspergillus niger, a fungus capable of citric, gluconic and oxalic acid production, grown in the presence and absence of cobalt or zinc phosphate. This work demonstrated that citric and oxalic acid and the A. niger culture filtrates can bind Co2+ and Zn2+ and in some cases, the culture filtrates were more efficient than commercial organic acids. Gluconic acid did not bind Co2+ or Zn2+ under the conditions used in this study. The presence of insoluble metal phosphates in the growth medium was found to markedly influence the production of organic acids and, while large concentrations of gluconic acid were produced in the presence of Co3(PO4)2, the culture filtrate was unable to bind Zn2+. The production of oxalic acid by A. niger when grown in the presence of Zn3(PO4)2 led to the precipitation of insoluble zinc oxalate, a phenomenon with implications for metal tolerance and toxicity. The significance of these findings in relation to the environmental mobility of metals and phosphate, and the role of fungi in such transformations are discussed.

The ability of six fungi to solubilize natural-occurring gypsum was tested in vitro. The solubilization process was monitored by the production of a clear zone (halo) around or underneath the growing colony on Czapek–Dox agar containing 0·5% (w/v) gypsum (CaSO4.2H2O). Aspergillus niger, Penicillium bilaii, P. simplicissimum and Paxillus involutus displayed differential solubilization activities depending on the supplemented nitrogen source. Colonies grown on nitrate-containing medium showed the ability to solubilize gypsum, but when ammonium (at equivalent nitrogen) was used there was a significant reduction in solubilization. It was found that liquid cultures of nitrate-grown fungi produced substantial amounts of oxalic acid, whereas in ammonium-containing medium oxalic acid was only detected in small amounts. The production of citric and gluconic acid under these experimental conditions was low in both media, although the involvement of citric acid in gypsum solubilization is possible. Coriolus versicolor and Phanaerochaete chrysosporium did not exhibit any solubilization activity in nitrate- or ammonium-containing medium. Additionally these two fungi excreted small quantities of oxalic acid in both media with no citric acid being produced in liquid medium. Concomitant with the solubilization process and inside the clear solubilized zone, A. niger, Pax. involutus and P. bilaii produced crystals of differing shapes and abundance depending on the fungal strain. No crystals were produced by P. simplicissimum. These crystals were identified as calcium oxalate based on HPLC analysis and energy-dispersive X-ray microanalysis. Abundance of these crystals was found to be correlated with both oxalic acid production and the acidity of the medium. It is concluded that gypsum solubilization was predominantly achieved by both oxalic acid, which was accompanied by formation of calcium oxalate crystals, and citric acid production rather than the acidity of the medium. The results are discussed in relation to the significance of such an activity in agricultural applications, e.g. land reclamation, as well as the possible roles played by these fungi in mineral cycling.

Accumulation and transformation of tellurite (TeO32−) by a species of Fusarium and Penicillium citrinum was examined using both solid and liquid Czapek Dox medium. In liquid medium, tellurite partitioned into soluble and insoluble species, and in tellurite-containing (1 mM) liquid medium at pH 6, approx. 60% of added tellurite precipitated after 48 h. Experiments showed that in liquid medium containing 1 mM sodium tellurite, the Fusarium sp. accumulated a maximum of ∼0·6 μmol Te (mg D.W.)−1 after 48 h. P. citrinum accumulated a much lower amount of tellurium, ∼0·07 μmol (mg D.W.)−1 falling to <0·02 μmol (mg D.W.)−1 after 48 h. Both organisms showed marked differences in the pattern of pH change of the medium, with the pH increasing during growth of the Fusarium sp. in 1 mM tellurite to ∼pH 6·7 after 2 wk. In contrast, the pH decreased during growth of P. citrinum in 1 mM tellurite, to a level of ∼pH 2·7 after 2 wk. On agar medium, test fungi exhibited tolerance to high levels of tellurite (up to 100 mM Na2TeO3) and this was associated with blackening of the growing colonies as well as the surrounding agar. TEM revealed the deposition of large black granules, apparently in vacuoles, which corresponded with the reduction of tellurite to amorphous elemental tellurium. Precipitation of amorphous tellurium on and around the biomass was also observed and confirmed by energy-dispersive X-ray microprobe analysis. In addition to the reductive transformation of tellurite, Fusarium sp. also displayed transformation of tellurite into a volatile form. The production of volatile tellurium by Fusarium sp. occurred over the whole growth period and amounted to an average value of 7·8 μmol Te (∼0·16%) from 51 growth medium with an initial concentration of 1 mM Na2TeO3. Although P. citrinum also transformed tellurite to elemental tellurium, volatilization of tellurium was not detected. It is concluded that different mechanisms of tellurium transformation are species-dependent and can be influenced by physico-chemical changes in the medium, e.g. pH, which can affect tellurium speciation into soluble and insoluble forms and bioaccumulation. In view of the extremely small amounts of Te volatilized by the Fusarium sp., this process cannot be considered to be an important detoxification mechanism.

The effects of varying carbon (glucose), nitrogen ((NH4)2SO4, KNO3) and phosphate (KH2PO4) source on solubilization of insoluble Co3(PO4)2.8H2O, Zn3(PO4)2.2H2O and ZnO by the soil fungus Aspergillus niger were assessed. Solubilization activity was quantified by measuring the clear zones produced around colonies of A. niger growing on solidified mineral salts medium amended with the insoluble metal compounds. Effects of nutrient variation on solubilizing properties were compared using ratios of colony growth rate on the metal compounds (Rm) to control growth rate (Rc) and the rate of extension of the zone of solubilization (Rs) compared to the colony growth rate on the metal compound (Rm), i.e. Rm[ratio ]Rc and Rs[ratio ]Rm. Ratios of solubilization rate to growth rate (Rs[ratio ]Rm) on all the compounds decreased with decreasing glucose concentration; there was no solubilization of ZnO below 60 mm glucose and no solubilization of the metal phosphates below 6 mm glucose. Reducing the concentration of ammonium sulphate in the growth medium decreased Rs[ratio ]Rm but these values were increased when the nitrogen source was nitrate. Reducing the phosphate concentration increased solubilization of Co3(PO4)2 but reduced solubilization of Zn3(PO4)2. These findings demonstrate that manipulation of carbon, nitrogen and phosphate sources in the growth medium, and variation of the form of the nutrient source, can be used to alter the solubilizing ability of A. niger. Whilst, in the natural environment, this response to different nutrient sources allows optimal exploitation of resources, the potential to manipulate nutrients for maximum solubilizing ability may prove beneficial for the optimization of the solubilization of metal compounds with respect to the bioremediation of metal-contaminated wastes and polluted ecosystems. It could also prove useful in other biotechnological applications such as metal recycling and extraction of metals from low-grade ores.

The natural dihydrate form of calcium sulphate, CaSO4.2H2O (gypsum), particularly found in gypsiferous soils and in certain building constructions, was found to be effectively solubilized by both Aspergillus niger and Serpula himantioides. When the fungi were grown on malt extract agar (MEA) medium amended with up to 1% (w/v) of gypsum, solubilization activity was evident by the appearance of a clear zone (haol) around and/or under the growing colonies. The haloes were found subsequently to enclose concentric rings of newly formed crystalline precipitate. The pH profile of the medium underneath the growing mycelium of A. niger was not affected by the presence of gypsum, while the S. himantioides pH profile displayed a slight reduction in pH values when grown on gypsum-containing media compared with the control. Electron microscopy and X-ray microanalysis of the crystals revealed the formation of calcium-containing crystals with the characteristic morphology typical of different forms of calcium oxalate. The complexing activity of fungal-produced organic acids with calcium was suggested to be the prevalent mechanism of solubilization with the production of oxalic acid resulting in precipitation of oxalate. Such activity resulted in the release of available sulphate which increased over the incubation period. The concentrations of sulphate released from 0·5% (w/v) gypsum after growth of A. niger and S. himantioides for 8 d were 14·7±0·7 and 7·4±0·9 μmol cm−3 respectively. These results are discussed in the light of their relevance to land reclamation and plant nutrition, as well as the weathering of building materials containing gypsum.