Inhibition of H+ efflux from Saccharomyces cerevisiae by insoluble metal phosphates and protection by calcium and magnesium: inhibitory effects a result of soluble metal cations?

Ca2+ and SO42- accelerate the reduction of Cr(VI) by Penicillium oxalicum SL2

Some ions in soils may affect the growth and metabolism of microorganisms and subsequently alter the remediation efficiency of Cr(VI). Here, the effects of different Ca²⁺ and SO₄^2- levels on the reduction of Cr(VI) by Penicillium oxalicum SL2 were investigated. The results showed that Cr(VI) reduction by P. oxalicum SL2 in potato dextrose liquid (PDL) medium was accelerated by the presence of exogenous Ca²⁺ and SO₄^2-. The Cr(VI) reduction rates were increased by 52.5% (200 mg L^-1 Ca²⁺ treated) and 55.9% (2000 mg L^-1 SO₄^2- treated), respectively. High concentration of Ca²⁺ in medium resulted in the production of calcium oxalate crystals, which was contributed to the adsorption of chromium. In addition, X-ray absorption near-edge spectroscopy (XANES) analysis showed that P. oxalicum SL2 could reduce the toxicity of Cr(VI) by synthesizing cysteine (Cys) and reduced glutathione (GSH). The decrease of thiol compounds (Cys and GSH) in P. oxalicum SL2 mycelia treated with SO₄^2- proved the alleviation of oxidative stress. In conclusion, exogenous Ca²⁺ could reduce the damage of Cr(VI) to P. oxalicum SL2 by maintaining the integrity of cell wall, and the addition of SO₄^2- alleviated the Cr(VI) toxicity to P. oxalicum SL2, thus accelerating the reduction of Cr(VI).

Cobalt-dependent inhibition of nitrite oxidation in Nitrobacter winogradskyi

Nitrobacter winogradskyi is an abundant, intensively studied autotrophic nitrite-oxidizing bacterium, which is frequently used as a model strain in the two-step nitrification of ammonia (NH₃) to nitrate (NO₃^-) via nitrite (NO₂^-), either in activated sludge, agricultural field studies or more recently in artificial microbial consortia for organic hydroponics. We observed a hitherto unknown cobalt ion-dependent inhibition of cell growth and NO₂^- oxidation activity of N. winogradskyi in a mineral medium, which strongly depended on accompanying Ca²⁺ and Mg²⁺ concentrations. This inhibition was bacteriostatic, but susceptible to natural chelators. l-Histidine effectively restored cell growth and NO₂^- oxidation activity of N. winogradskyi in mineral media containing Co²⁺ with >90% recovery. Our results suggest that Co²⁺ competed with alkaline earth metals during uptake and that its toxicity was significantly reduced by complexation.

Induction of laccase activity in Rhizoctonia solani by antagonistic Pseudomonas fluorescens strains and a range of chemical treatments

Fungi often produce the phenoloxidase enzyme laccase during interactions with other organisms, an observation relevant to the development of biocontrols. By incorporating the laccase substrate 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) into agar, we analyzed laccase induction in the plant-pathogenic fungus Rhizoctonia solani when paired against isolates of the soil bacterium Pseudomonas fluorescens. Substantial induction of R. solani laccase was seen only in pairings with strains of P. fluorescens known to produce antifungal metabolites. To study laccase induction further, a range of chemical treatments was applied to R. solani liquid cultures. p-Anisidine, copper(II), manganese(II), calcium ionophore A23187, lithium chloride, calcium chloride, cyclic AMP (cAMP), caffeine, amphotericin B, paraquat, ethanol, and isopropanol were all found to induce laccase; however, the P. fluorescens metabolite viscosinamide did not do so at the concentrations tested. The stress caused by these treatments was assessed by measuring changes in lipid peroxidation levels and dry weight. The results indicated that the laccase induction seen in pairing plate experiments was most likely due to calcium or heat shock signaling in response to the effects of bacterial metabolites, but that heavy metal and cAMP-driven laccase induction was involved in sclerotization.

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.