Reduction of selenium oxyanions by unicellular, polymorphic and filamentous fungi: Cellular location of reduced selenium and implications for tolerance

The ability of several filamentous, polymorphic and unicellular fungi to reduce selenite to elemental selenium on solid medium was examined. Fusarium sp. and Trichoderma reeii were the only filamentous fungi, of those tested, which reduced selenite to elemental selenium on Czapek-Dox agar resulting in a red colouration of colonies. Other organisms (Aspergillus niger, Coriolus versicolor, Mucor SK, and Rhizopus arrhizus) were able to reduce selenite only on malt extract agar. Several fungi were able to grow in the presence of sodium selenite but were apparently unable to reduce selenite to elemental selenium, indicating that other mechanisms of selenite tolerance were employed, such as reduced uptake and/or biomethylation to less toxic, volatile derivatives. Sodium selenate was more toxic to Fusarium sp. than selenite, and the toxicity of both oxyanions was increased in sulphur-free medium, with this effect being more marked for selenate. Scanning electron microscopy of Aspergillus funiculosus and Fusarium sp. incubated with sodium selenite showed the presence of needle-like crystals of elemental selenium on the surfaces of hyphae and conidia, while transmission electron microscopy of A. funiculosus revealed the deposition of electron-dense granules in vacuoles of selenite-treated fungi. Several yeasts were able to grow on MYGP agar containing sodium selenate or sodium selenite at millimolar concentrations. Sone, notably Rhodotorula rubra and Candida lipolytica, and the polymorphic fungus Aureobasidium pullulans were also effective at reducing selenite to elemental selenium, resulting in red-coloured colonies. Schizosaccharomyces pombe was able to grow at selenite concentrations up to 5 mmol L^-1 without any evidence of reduction, again indicating the operation of other tolerance mechanisms.