Sorption of toxic metals by fungi and clay minerals

Development of Ni(II) resistant S. cerevisiae and its application: Adsorption study and modeling

The study aims to develop Ni(II) resistant Saccharomyces cerevisiae to decontaminate high Ni(II) concentrations from an aqueous system. Initially, two different microorganisms were taken: Bacillus circulans MTCC 3161, Saccharomyces cerevisiae. For these two strains, the experiments were carried out for successive screening for survival/tolerance, minimum inhibitory concentration (MIC), and biosorption capacity for Ni(II) from an aqueous solution. Ni(II) resistant Saccharomyces cerevisiae AJ208 showed a MIC of 5500 mg/L for Ni(II). Nucleotide sequences of Saccharomyces cerevisiae AJ208 were deposited in the Gene bank. All experiments were conducted to determine the effects of various physical conditions, such as pH, age and volume of inoculum, temperature, and incubation time, the volume of fermentation medium. The characterization of the Saccharomyces cerevisiae AJ208 was carried out using SEM-EDAX, FTIR. The Langmuir isotherm and pseudo-second-order kinetic models are well fitted with the experimental data. The Langmuir maximum adsorption capacity is 170.06 mg/g. The thermodynamic studies showed the mechanism of Ni(II) removal is an endothermic and spontaneous reaction. The experimental data have been analyzed using statistical method (MLR) and Genetic algorithm (GA). This study reports the highest Ni(II) resistant Saccharomyces cerevisiae AJ208 (5000 mg/L) and also the feasibility of Ni(II) removal from 3000 mg/L initial Ni(II) concentration into an aqueous solution, which could be of great interest as a potential reference strain for Ni(II) removal.

Trichovariability in rhizosphere soil samples and their biocontrol potential against downy mildew pathogen in pearl millet

The present work is aimed to examine the genetic variability and the distribution pattern of beneficial Trichoderma spp. isolated from rhizosphere samples and their mode of action in improving the plant health. A total of 131 suspected fungi were isolated from the rhizospheric soil and 91 isolates were confirmed as Trichoderma spp. T. asperellum and T. harzianum were found high in the frequency of occurrence. Genetic diversity analysis using RAPD and ISSR revealed the diverse distribution pattern of Trichoderma spp. indicating their capability to adapt to broad agroclimatic conditions. Analysis of genetic diversity using molecular markers revealed intra-species diversity of isolated Trichoderma spp. The frequency of pearl millet (PM) root colonization by Trichoderma spp. was found to be 100%. However, they showed varied results for indole acetic acid, siderophore, phosphate solubilization, β-1,3-glucanase, chitinase, cellulase, lipase, and protease activity. Downy mildew disease protection studies revealed a strong involvement of Trichoderma spp. in direct suppression of the pathogen (mean 37.41) in the rhizosphere followed by inducing systemic resistance. Our findings highlights the probable distribution and diversity profile of Trichoderma spp. as well as narrate the possible utilization of Trichoderma spp. as microbial fungicides in PM cultivation across different agroclimatic zones of India.

Microbial Interaction with Clay Minerals and Its Environmental and Biotechnological Implications

Clay minerals are very common in nature and highly reactive minerals which are typical products of the weathering of the most abundant silicate minerals on the planet. Over recent decades there has been growing appreciation that the prime involvement of clay minerals in the geochemical cycling of elements and pedosphere genesis should take into account the biogeochemical activity of microorganisms. Microbial intimate interaction with clay minerals, that has taken place on Earth’s surface in a geological time-scale, represents a complex co-evolving system which is challenging to comprehend because of fragmented information and requires coordinated efforts from both clay scientists and microbiologists. This review covers some important aspects of the interactions of clay minerals with microorganisms at the different levels of complexity, starting from organic molecules, individual and aggregated microbial cells, fungal and bacterial symbioses with photosynthetic organisms, pedosphere, up to environmental and biotechnological implications. The review attempts to systematize our current general understanding of the processes of biogeochemical transformation of clay minerals by microorganisms. This paper also highlights some microbiological and biotechnological perspectives of the practical application of clay minerals–microbes interactions not only in microbial bioremediation and biodegradation of pollutants but also in areas related to agronomy and human and animal health.

Microbial biomodification of clay minerals

Clay minerals are important reactive centers in the soil system. Their interactions with microorganisms are ubiquitous and wide-ranging, affecting growth and function, interactions with other organisms, including plants, biogeochemical processes and the fate of organic and inorganic pollutants. Clay minerals have a large specific surface area and cation exchange capacity (CEC) per unit mass, and are abundant in many soil systems, especially those of agricultural significance. They can adsorb microbial cells, exudates, and enzymes, organic and inorganic chemical species, nutrients, and contaminants, and stabilize soil organic matter. Bacterial modification of clays appears to be primarily due to biochemical mechanisms, while fungi can exhibit both biochemical and biomechanical mechanisms, the latter aided by their exploratory filamentous growth habit. Such interactions between microorganisms and clays regulate many critical environmental processes, such as soil development and transformation, the formation of soil aggregates, and the global cycling of multiple elements. Applications of biomodified clay minerals are of relevance to the fields of both agricultural management and environmental remediation. This review provides an overview of the interactions between bacteria, fungi and clay minerals, considers some important gaps in current knowledge, and indicates perspectives for future research.

In Vitro Bioadsorption of Cd2+ Ions: Adsorption Isotherms, Mechanism, and an Insight to Mycoremediation

The objective of this paper is to establish the significance of the mycoremediation of contaminants such as Cd2+ to achieve sustainable and eco-friendly remediation methods. Industries such as electroplating, paint, leather tanning, etc. release an enormous amount of Cd2+ in wastewater, which can drastically affect our flora and fauna. Herein, we report on the in vitro bioadsorption of Cd2+ ions using fungal isolates obtained from different contaminated industrial sites. The detailed studies revealed that two fungal species, i.e., Trichoderma fasciculatum and Trichoderma longibrachiatum, were found to be most effective against the removal of Cd2+ when screened for Cd2+ tolerance on potato dextrose agar (PDA) in different concentrations. Detailed adsorption studies were conducted by exploring various experimental factors such as incubation time, temperature, pH, inoculum size, and Cd2+ salt concentrations. Based on optimum experimental conditions, T. fasciculatum exhibited approximately 67.10% removal, while T. longibrachiatum shows 76.25% removal of Cd2+ ions at pH 5.0, 120 h incubation time, at 30°C. The inoculum sizes for T. fasciculatum and T. longibrachiatum were 2.5% and 2.0%, respectively. Finally, the morphological changes due to Cd2+ accumulation were examined using scanning electron microscopy (SEM). Further, Fourier transform infrared spectroscopy (FTIR) spectroscopy reveals the presence of various functional groups (-CH, –C=O, NH and –OH), which seem to be responsible for the efficient binding of Cd2+ ions over the fungal surfaces.

How can interspecific interactions in freshwater benthic macroinvertebrates modify trace element availability from sediment?

This study aimed to assess how bioturbation by freshwater benthic macroinvertebrates with different biological traits alone or in combination could modify trace elements (TE) fate between sediment and water, and if water TE concentration and animal TE content impair their body stores. Three macroinvertebrate species were exposed to TE contaminated sediment for 7 days: the omnivorous Echinogammarus berilloni (Amphipoda), the sediment feeding Tubifex tubifex (Oligochaeta) and the filter feeding Pisidium sp. (Bivalvia). Treatments were one without invertebrates (control), two with amphipods or mussels alone, and the combinations amphipod-mussel, and amphipod-mussel-worms. Water TE concentration increased significantly in 2 or 3 species mesocosms, concerning mainly Rare Earth Elements, Cr, U and Pb, known to be associated to the colloidal phase. By contrast, water soluble TE were not affected by animals. For both, amphipods and mussels, TE body content increased with the number of coexisting species. For amphipods, this increase concerned both, soluble and colloid-associated TE, possibly due to intense contact and feeding from sediment and predation on tubificids. TE bioaccumulation in mussel was less important and characterized by soluble TE, with water filtration as most plausible uptake route. Protein, triglyceride and Whole Body Energy Budget increased in amphipods with the number of coexisting species (probably by feeding on mussels' feces and tubificids) whereas triglycerides declined in mussels (presumably filtration was disturbed by amphipods). This study highlights interspecific interactions as key drivers explaining both: TE bioturbation, depending on their water solubility or colloidal association, and the exposure/contamination of species through another species activity.

Arbuscular mycorrhizal fungi effect growth and photosynthesis of Phragmites australis (Cav.) Trin ex. Steudel under copper stress

Arbuscular mycorrhizal fungi (AMF) is an effective way to remove heavy metals' inhibition on plants, however, few relevant research attempts have been made to determine the contribution of AMF to the physiological and biochemical changes related to the enhanced copper tolerance of Phragmites australis under metal-stressed conditions. In this study, the effects of AMF inoculation on P. australis under different concentrations of copper stress were investigated according to the changes in the parameters related to growth and development, and photosynthetic charateristics. Then, differentially expressed proteins (DEPs) were evaluated by the Isobaric Tag for Relative and Absolute Quantification (iTRAQ) system, which could accurately quantify the DEPs by measuring peak intensities of reporter ions in tandem mass spectrometry (MS/MS) spectra. It was found that AMF inoculation may relieve the photosynthesis inhibition caused by copper stress on P. australis and thus promote growth. Proteomic analysis results showed that under copper stress, the inoculation of R. irregularis resulted in a total of 459 differently-expressed proteins (200 up-regulated and 259 down-regulated) in root buds. In addition, the photosynthetic changes caused by AMF inoculation mainly involve the up-regulated expression of transmembrane protein-pigment complexes CP43 (photosystem II) and FNR (ferredoxin-NADP+ oxidoreductase related to photosynthetic electron transport). These results indicate that AMF could effectively improve the growth and physiological activity of P. australis under copper stress, and thus provides a new direction and instructive evidence for determining the mechanisms by which AMF inoculation enhances the copper tolerance of plants.

Bioremediation Options for Heavy Metal Pollution

BACKGROUND

Rapid industrialization and anthropogenic activities such as the unmanaged use of agro-chemicals, fossil fuel burning and dumping of sewage sludge have caused soils and waterways to be severely contaminated with heavy metals. Heavy metals are non-biodegradable and persist in the environment. Hence, remediation is required to avoid heavy metal leaching or mobilization into environmental segments and to facilitate their extraction.

OBJECTIVES

The present work briefly outlines the environmental occurrence of heavy metals and strategies for using microorganisms for bioremediation processes as reported in the scientific literature.

METHODS

Databases were searched from different libraries, including Google Scholar, Medline and Scopus. Observations across studies were then compared with the standards for discharge of environmental pollutants.

DISCUSSION

Bioremediation employs microorganisms for removing heavy metals. Microorganisms have adopted different mechanisms for bioremediation. These mechanisms are unique in their specific requirements, advantages, and disadvantages, the success of which depends chiefly upon the kind of organisms and the contaminants involved in the process.

CONCLUSIONS

Heavy metal pollution creates environmental stress for human beings, plants, animals and other organisms. A complete understanding of the process and various alternatives for remediation at different steps is needed to ensure effective and economic processes.

COMPETING INTERESTS

The authors declare no competing financial interests.

Instant Attraction: Clay Authigenesis in Fossil Fungal Biofilms

Clay authigenesis associated with the activity of microorganisms is an important process for biofilm preservation and may provide clues to the formation of biominerals on the ancient Earth. Fossilization of fungal biofilms attached to vesicles or cracks in igneous rock, is characterized by fungal-induced clay mineralization and can be tracked in deep rock and deep time, from late Paleoproterozoic (2.4 Ga), to the present. Here we briefly review the current data on clay mineralization by fossil fungal biofilms from oceanic and continental subsurface igneous rock. The aim of this study was to compare the nature of subsurface fungal clays from different igneous settings to evaluate the importance of host rock and ambient redox conditions for clay speciation related to fossil microorganisms. Our study suggests that the most common type of authigenic clay associated with pristine fossil fungal biofilms in both oxic (basaltic) and anoxic (granitic) settings are montmorillonite-like smectites and confirms a significant role of fungal biofilms in the cycling of elements between host rock, ocean and secondary precipitates. The presence of life in the deep subsurface may thus prove more significant than host rock geochemistry in directing the precipitation of authigenic clays in the igneous crust, the extent of which remains to be fully understood.

Tolerance to heavy metals in filamentous fungi isolated from contaminated mining soils in the Zanjan Province, Iran

In the present study the population of fungi inhabiting metal contaminated soil samples from lead-zinc mining area was investigated, and their tolerance and biosorption capacity towards Cd, Pb, Zn and Cu were evaluated. Fungal species, including Acremonium persicinum, Penicillium simplicissimum, Seimatosporium pistaciae, Trichoderma harzianum, Alternaria chlamydosporigena and Fusarium verticillioides were isolated. Fungal tolerance was determined by measuring of the "Minimum Inhibitory Concentration", after exposure to increasing concentrations of heavy metal chlorides. Among the test fungi, Trichoderma harzianum was the most tolerant against Cd, Pb and Cu. The Heavy metal content in the fungal biomass was quantified after combustion. The calculated total sorption of heavy metals showed that Acremonium persicinum and Penicillium simplicissimum had the highest accumulation of Zn and Cu, respectively. The data presented in this study should help to use the accumulation potential of some of those fungal species examined for bioremediation of contaminated soils.

Engineering Mycorrhizal Symbioses to Alter Plant Metabolism and Improve Crop Health

Creating sustainable bioeconomies for the 21st century relies on optimizing the use of biological resources to improve agricultural productivity and create new products. Arbuscular mycorrhizae (phylum Glomeromycota) form symbiotic relationships with over 80% of vascular plants. In return for carbon, these fungi improve plant health and tolerance to environmental stress. This symbiosis is over 400 million years old and there are currently over 200 known arbuscular mycorrhizae, with dozens of new species described annually. Metagenomic sequencing of native soil communities, from species-rich meadows to mangroves, suggests biologically diverse habitats support a variety of mycorrhizal species with potential agricultural, medical, and biotechnological applications. This review looks at the effect of mycorrhizae on plant metabolism and how we can harness this symbiosis to improve crop health. I will first describe the mechanisms that underlie this symbiosis and what physiological, metabolic, and environmental factors trigger these plant-fungal relationships. These include mycorrhizal manipulation of host genetic expression, host mitochondrial and plastid proliferation, and increased production of terpenoids and jasmonic acid by the host plant. I will then discuss the effects of mycorrhizae on plant root and foliar secondary metabolism. I subsequently outline how mycorrhizae induce three key benefits in crops: defense against pathogen and herbivore attack, drought resistance, and heavy metal tolerance. I conclude with an overview of current efforts to harness mycorrhizal diversity to improve crop health through customized inoculum. I argue future research should embrace synthetic biology to create mycorrhizal chasses with improved symbiotic abilities and potentially novel functions to improve plant health. As the effects of climate change and anthropogenic disturbance increase, the global diversity of arbuscular mycorrhizal fungi should be monitored and protected to ensure this important agricultural and biotechnological resource for the future.

Native fungi as metal remediators: Silver myco-accumulation from metal contaminated waste-rock dumps (Libiola Mine, Italy)

Metal contamination constitutes a major source of pollution globally. Many recent studies emphasized the need to develop cheap and green technologies for the remediation or reclamation of environmental matrices contaminated by heavy metals. In this context, fungi are versatile organisms that can be exploited for bioremediation activities. In our work, we tested silver (Ag) bioaccumulation capabilities of three microfungal strains (Aspergillus alliaceus Thom & Church, Trichoderma harzianum Rifai, Clonostachys rosea (Link) Schroers, Samuels, Seifert & W. Gams) isolated from a silver polluted site. The aim was to select silver tolerant native strains and test their potential silver uptake. Among the three species tested, T. harzianum was the most efficient strain to tolerate and accumulate silver, showing an uptake capability of 153 mg L^-1 taken at the Ag concentration of 330 mg L^-1. Our study highlights the potential use of native microfungi spontaneously growing in sulphide-rich waste rock dumps, for silver bioaccumulation and bioremediation.

The Geomycology of Elemental Cycling and Transformations in the Environment

Geomicrobiology addresses the roles of microorganisms in geological and geochemical processes, and geomycology is a part of this topic focusing on the fungi. Geoactive roles of fungi include organic and inorganic transformations important in nutrient and element cycling, rock and mineral bioweathering, mycogenic biomineral formation, and metal-fungal interactions. Lichens and mycorrhizas are significant geoactive agents. Organic matter decomposition is important for cycling of major biomass-associated elements, e.g., C, H, N, O, P, and S, as well as all other elements found in lower concentrations. Transformations of metals and minerals are central to geomicrobiology, and fungi affect changes in metal speciation, as well as mediate mineral formation or dissolution. Such mechanisms are components of biogeochemical cycles for metals as well as associated elements in biomass, soil, rocks, and minerals, e.g., S, P, and metalloids. Fungi may have the greatest geochemical influence within the terrestrial environment. However, they are also important in the aquatic environment and are significant components of the deep subsurface, extreme environments, and habitats polluted by xenobiotics, metals, and radionuclides. Applications of geomycology include metal and radionuclide bioleaching, biorecovery, detoxification, bioremediation, and the production of biominerals or metal(loid) elements with catalytic or other properties. Adverse effects include biodeterioration of natural and synthetic materials, rock and mineral-based building materials (e.g., concrete), cultural heritage, metals, alloys, and related substances and adverse effects on radionuclide mobility and containment. The ubiquity and importance of fungi in the biosphere underline the importance of geomycology as a conceptual framework encompassing the environmental activities of fungi.

High mobilization of arsenic, metals and rare earth elements in seepage waters driven by respiration of old allochthonous organic carbon

Metal and metalloid mobilization processes within seepage water are of major concern in a range of water reservoir systems. The mobilization process of arsenic and heavy metals within a dam and sediments of a drinking water reservoir was investigated. Principle component analysis (PCA) on time series data of seepage water showed a clear positive correlation of arsenic with iron and DOC (dissolved organic carbon), and a negative correlation with nitrate due to respiratory processes. A relationship of reductive metal and metalloid mobilization with respiration of old carbon was shown. The system is influenced by sediment layers as well as a recent DOC input from degraded ombrotrophic peatbogs in the catchment area. The isotopic composition ((12)C, (13)C and (14)C) of DOC is altered along the path from basin to seepage water, but no significant changes in structural parameters (LC-OCD-OND, FT-IR) could be seen. DIC (dissolved inorganic carbon) in seepage water partly originates from respiratory processes, and a higher relationship of it with sediment carbon than with the DOC inventory of infiltrating water was found. This study revealed the interaction of respiratory processes with metal and metalloid mobilization in sediment water flows. In contrast to the presumption that emerging DOC via respiratory processes mainly controls arsenic and metal mobilization it could be shown that the presence of aged carbon compounds is essential. The findings emphasize the importance of aged organic carbon for DOC, DIC, arsenic and metal turnover.

Bioremoval of diethylketone by the synergistic combination of microorganisms and clays: uptake, removal and kinetic studies

The performance of two bacteria, Arthrobacter viscosus and Streptococcus equisimilis, and the effect of the interaction of these bacteria with four different clays on the retention of diethylketone were investigated in batch experiments. The uptake, the removal percentages and the kinetics of the processes were determined. S. equisimilis, by itself, had the best performance in terms of removal percentage, for all the initial diethylketone concentrations tested: 200, 350 and 700 mg/L. The uptake values are similar for both bacteria. A possible mechanism to explain the removal of diethylketone includes its degradation by bacteria, followed by the adsorption of the intermediates/sub-products by the functional groups present on the cells' surfaces. The assays performed with bacteria and clays indicated that the uptake values are similar despite of the clay used, for the same microorganism and mass of clay, but in general, higher values are reached when S. equisimilis is used, compared to A. viscosus. Kinetic data were described by pseudo-first- and pseudo-second-order models.

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.

Effects of dissolved organic matter from the rhizosphere of the hyperaccumulator Sedum alfredii on sorption of zinc and cadmium by different soils

Pot experiments were conducted to investigate the changes of the dissolved organic matter (DOM) in the rhizosphere of hyperaccumulating ecotype (HE) and non-hyperaccumulating ecotype (NHE) of Sedum alfredii and its effects on Zn and Cd sorption by soils. After planted with HE, soil pH in the rhizosphere reduced by 0.5-0.6 units which is consistent with the increase of DOM. The hydrophilic fractions (51%) in DOM from the rhizosphere of HE (HE-DOM) was much greater than NHE-DOM (35%). In the presence of HE-DOM, Zn and Cd sorption capacity decreased markedly in the following order: calcareous clay loam>neutral clay loam>acidic silty clay. The sorption isotherms could be well described by the Freundlich equation (R(2)>0.95), and the partition coefficient (K) in the presence of HE-DOM was decreased by 30.7-68.8% for Zn and 20.3-59.2% for Cd, as compared to NHE-DOM. An increase in HE-DOM concentration significantly reduced the sorption and increased the desorption of Zn and Cd by three soils. DOM derived from the rhizosphere of the hyperaccumulating ecotype of S. alfredii could significantly reduce metal sorption and increase its mobility through the formation of soluble DOM-metal complexes.

Metals, minerals and microbes: geomicrobiology and bioremediation

Microbes play key geoactive roles in the biosphere, particularly in the areas of element biotransformations and biogeochemical cycling, metal and mineral transformations, decomposition, bioweathering, and soil and sediment formation. All kinds of microbes, including prokaryotes and eukaryotes and their symbiotic associations with each other and 'higher organisms', can contribute actively to geological phenomena, and central to many such geomicrobial processes are transformations of metals and minerals. Microbes have a variety of properties that can effect changes in metal speciation, toxicity and mobility, as well as mineral formation or mineral dissolution or deterioration. Such mechanisms are important components of natural biogeochemical cycles for metals as well as associated elements in biomass, soil, rocks and minerals, e.g. sulfur and phosphorus, and metalloids, actinides and metal radionuclides. Apart from being important in natural biosphere processes, metal and mineral transformations can have beneficial or detrimental consequences in a human context. Bioremediation is the application of biological systems to the clean-up of organic and inorganic pollution, with bacteria and fungi being the most important organisms for reclamation, immobilization or detoxification of metallic and radionuclide pollutants. Some biominerals or metallic elements deposited by microbes have catalytic and other properties in nanoparticle, crystalline or colloidal forms, and these are relevant to the development of novel biomaterials for technological and antimicrobial purposes. On the negative side, metal and mineral transformations by microbes may 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, all with immense social and economic consequences. The ubiquity and importance of microbes in biosphere processes make geomicrobiology one of the most important concepts within microbiology, and one requiring an interdisciplinary approach to define environmental and applied significance and underpin exploitation in biotechnology.

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.

Mycorrhizoremediation--an enhanced form of phytoremediation

Study of plant roots and the diversity of soil micro biota, such as bacteria, fungi and microfauna associated with them, is important for understanding the ecological complexities between diverse plants, microbes, soil and climates and their role in phytoremediation of contaminated soils. The arbuscular mycorrhizal fungi (AMF) are universal and ubiquitous rhizosphere microflora forming symbiosis with plant roots and acting as biofertilizers, bioprotactants, and biodegraders. In addition to AMF, soils also contain various antagonistic and beneficial bacteria such as root pathogens, plant growth promoting rhizobacteria including free-living and symbiotic N-fixers, and mycorrhiza helping bacteria. Their potential role in phytoremediation of heavy metal (HM) contaminated soils and water is becoming evident although there is need to completely understand the ecological complexities of the plant-microbe-soil interactions and their better exploitation as consortia in remediation strategies employed for contaminated soils. These multitrophic root microbial associations deserve multi-disciplinary investigations using molecular, biochemical, and physiological techniques. Ecosystem restoration of heavy metal contaminated soils practices need to incorporate microbial biotechnology research and development. This review highlights the ecological complexity and diversity of plant-microbe-soil combinations, particularly AM and provides an overview on the recent developments in this area. It also discusses the role AMF play in phytorestoration of HM contaminated soils, i.e. mycorrhizoremediation.

The role of glomalin, a protein produced by arbuscular mycorrhizal fungi, in sequestering potentially toxic elements

Naturally occurring soil organic compounds stabilize potentially toxic elements (PTEs) such as Cu, Cd, Pb, and Mn. The hypothesis of this work was that an insoluble glycoprotein, glomalin, produced in copious amounts on hyphae of arbuscular mycorrhizal fungi (AMF) sequesters PTEs. Glomalin can be extracted from laboratory cultures of AMF and from soils. Three different experiments were conducted. Experiment 1 showed that glomalin extracted from two polluted soils contained 1.6-4.3 mg Cu, 0.02-0.08 mg Cd, and 0.62-1.12 mg Pb/g glomalin. Experiment 2 showed that glomalin from hyphae of an isolate of Gigaspora rosea sequestered up to 28 mg Cu/g in vitro. Experiment 3 tested in vivo differences in Cu sequestration by Cu-tolerant and non-tolerant isolates of Glomus mosseae colonizing sorghum. Plants were fed with nutrient solution containing 0.5, 10 or 20 microM of Cu. Although no differences between isolates were detected, mean values for the 20 microM Cu level were 1.6, 0.4, and 0.3 mg Cu/g for glomalin extracted from hyphae, from sand after removal of hyphae and from hyphae attached to roots, respectively. Glomalin should be considered for biostabilization leading to remediation of polluted soils.

Karyotyping of Candida albicans and Candida glabrata isolates from recurrent vaginal infections by pulsed-field gel electrophoresis

In the present study, 16 women with recurrent vulvovaginal candidiasis (RVVC) due to Candida albicans and Candida (Torulopsis) glabrata were followed for a period of 4 to 12 months, and 36 vaginal isolates were evaluated by pulsed-field gel electrophoresis (PFGE). Eleven women were infected by C. albicans and 5 by C. glabrata. Three electrophoretic karyotypes of C. albicans and 3 of C. glabrata were identified throughout the follow-up. All patients but one was infected with the same karyotype of C. albicans or C. glabrata during the follow-up period. Two different karyotypes of C. glabrata were identified in one patient in the course of 12 months. The results confirmed the diversity of the karyotypes of C. albicans and C. glabrata causing vulvovaginitis, and demonstrated the persistence of colonization with the same strain over different periods of time despite therapy (15/16 women).

Copper-resistant bacteria from industrial effluents and their role in remediation of heavy metals in wastewater

Six copper-resistant bacterial strains were isolated from wastewater of tanneries of Kasur and Rohi Nala. Two strains tolerated copper at 380 mg/L, four up to 400 mg/L. Three strains were identified as members of the genus Salmonella; one strain was identified as Streptococcus pyrogenes, one as Vagococcus fluvialis and the last was identified as Escherichia coli. The pH and temperature optimum for two of them were 7.0 and 30 degrees C, respectively; four strains had corresponding optima at 7.5 and 37 degrees C, respectively. All bacterial isola-tes showed resistance against Ag+ (280-350 mg/L), Co2+ (200-420), CrVI (280-400), Cd2+ (250-350), Hg2+ (110-200), Mn2+ (300-380), Pb2+ (300-400), Sn2+ (480-520) and Zn2+ (300-450). Large-sized plasmids (> 20 kb), were detected in all of the strains. After the isolates were cured of plasmids with ethidium bromide, the efficiency of curing was estimated in the range of 60-90%. Reference strain of E. coli was transformed with the plasmids of the bacterial isolates which grew in Luria-Bertani medium containing 100 mg/L Cu2+. The capability to adsorb and afterwards accumulate Cu2+ inside their cells was assayed by atomic absorption spectrophotometer; all bacterial cells had the ability to adsorb 50-80% of the Cu2+ and accumulate 30-45% Cu2+ inside them after 1 d of incubation.

Breeding of mycoparasitic Trichoderma strains for heavy metal resistance

AIMS

This study was designed to investigate the effects of 10 heavy metals on the in vitro activities of beta-glucosidase, cellobiohydrolase, beta-xylosidase and endoxylanase enzymes for six strains of Trichoderma, and to isolate and characterize heavy metal-resistant mutants.

METHODS AND RESULTS

At a concentration of 1 mmol, only mercury showed significant inhibitory effects on the in vitro enzyme activities; in all other cases, the enzymes remained active. A total of 177 heavy metal-resistant mutants were isolated and tested for cross-resistance to other heavy metals. Some mutants were effective antagonists of Fusarium, Pythium and Rhizoctonia strains, even on media containing the respective heavy metals.

CONCLUSION

Trichoderma strains could be developed as biocontrol agents that are effective against plant pathogenic fungi, even under heavy metal stress.

SIGNIFICANCE AND IMPACT OF THE STUDY

Trichoderma mutants resistant to heavy metals might be of value for use with heavy metal-containing pesticides, as part of an integrated plant protection system.

Aspergillus niger absorbs copper and zinc from swine wastewater

Wastewater from swine confined-housing operations contains elevated levels of copper and zinc due to their abundance in feed. These metals may accumulate to phytotoxic levels in some agricultural soils of North Carolina due to land application of treated swine effluent. We evaluated fungi for their ability to remove these metals from wastewater and found Aspergillus niger best suited for this purpose. A. niger was able to grow on plates amended with copper at a level five times that inhibitory to the growth of Saccharomyes cerevisiae. We also found evidence for internal absorption as the mechanism used by A. niger to detoxify its environment of copper, a property of the fungus that has not been previously exploited for metal bioremediation. In this report, we show that A. niger is capable of removing 91% of the copper and 70% of the zinc from treated swine effluent.

Fungal production of citric and oxalic acid: importance in metal speciation, physiology and biogeochemical processes

The production of organic acids by fungi has profound implications for metal speciation, physiology and biogeochemical cycles. Biosynthesis of oxalic acid from glucose occurs by hydrolysis of oxaloacetate to oxalate and acetate catalysed by cytosolic oxaloacetase, whereas on citric acid, oxalate production occurs by means of glyoxylate oxidation. Citric acid is an intermediate in the tricarboxylic acid cycle, with metals greatly influencing biosynthesis: growth limiting concentrations of Mn, Fe and Zn are important for high yields. The metal-complexing properties of these organic acids assist both essential metal and anionic (e.g. phosphate) nutrition of fungi, other microbes and plants, and determine metal speciation and mobility in the environment, including transfer between terrestrial and aquatic habitats, biocorrosion and weathering. Metal solubilization processes are also of potential for metal recovery and reclamation from contaminated solid wastes, soils and low-grade ores. Such 'heterotrophic leaching' can occur by several mechanisms but organic acids occupy a central position in the overall process, supplying both protons and a metal-complexing organic acid anion. Most simple metal oxalates [except those of alkali metals, Fe(III) and Al] are sparingly soluble and precipitate as crystalline or amorphous solids. Calcium oxalate is the most important manifestation of this in the environment and, in a variety of crystalline structures, is ubiquitously associated with free-living, plant symbiotic and pathogenic fungi. The main forms are the monohydrate (whewellite) and the dihydrate (weddelite) and their formation is of significance in biomineralization, since they affect nutritional heterogeneity in soil, especially Ca, P, K and Al cycling. The formation of insoluble toxic metal oxalates, e.g. of Cu, may confer tolerance and ensure survival in contaminated environments. In semi-arid environments, calcium oxalate formation is important in the formation and alteration of terrestrial subsurface limestones. Oxalate also plays an important role in lignocellulose degradation and plant pathogenesis, affecting activities of key enzymes and metal oxido-reduction reactions, therefore underpinning one of the most fundamental roles of fungi in carbon cycling in the natural environment. This review discusses the physiology and chemistry of citric and oxalic acid production in fungi, the intimate association of these acids and processes with metal speciation, physiology and mobility, and their importance and involvement in key fungal-mediated processes, including lignocellulose degradation, plant pathogenesis and metal biogeochemistry.

Quantification of Fungal Antigens in Soil with a Monoclonal Antibody-Based ELISA: Analysis and Reduction of Soil-Specific Bias

ABSTRACT This paper describes methods to improve the use of immunoassays for quantification of soilborne fungal antigens. Calibration curves, prepared by diluting known quantities of an antigen into soil extracts and into soil, were described by a four-parameter logistic curve from which two principal criteria, the lower detection limit and the horizontal locational parameter, were used to summarize the sensitivity and bias of an immuno-assay. We identify two sources of bias, retention of the antigen in soil due to bonding and interference of soluble soil components in plate-trapped-antigen, enzyme-linked immunosorbent assays. Using a monoclonal antibody that recognizes a putative catechol oxidase secreted by hyphae of Rhizoctonia solani, we show that bias due to retention of the antigen in soil is substantially greater than bias due to interference. Three soils were compared: a sand, a clay, and a loam. The degree of retention varied with soil type, with more than a 1,000-fold reduction in sensitivity in the clay soil. Addition of CuSO(4) to the extraction solution and optimizing the volume of extractant reduced the bias and increased the sensitivity of the assay for all three soils. Possible mechanisms for the effect due to Cu(2+) and the implications for the design and use of calibration curves for assays involving quantification of fungal antigens in soil are discussed.