Development and characterization of nickel accumulating mutants of Aspergillus nidulans

Abstract

Stable mutants of Aspergillus nidulans, resistant to 1 mM Ni were developed by step-by-step repeated culturing of the fungus on the medium containing increasing concentrations of nickel chloride. Characterization of mutants could differentiate them into two categories Ni R I and Ni R II. Each category of mutants exhibited alterations in growth, conidial germination and melanin secretion both in Ni-free and Ni-containing media. Ni R II mutants were little slow in growth with sparse mycelia and conidiation but showed high melanin secretion and higher Ni-uptake in comparison to Ni R I mutant. Studies involving metabolic and translational inhibitors could prove that Niaccumulation was biphasic. The initial energy independent surface accumulation was found to be followed by energy dependent intarcellular uptake. Increase in the concentration of the metal in the medium or the time of exposure did not proportionately increase the metal uptake by the mutants. Ni -uptake followed Michaelis-Menton saturation kinetics, which was enhanced under optimum pH of 6.5-7.5 and reduced complexity of the medium due to free availability of ions. Resistance to Ni was found to be constitutive in Ni R I mutant, and could be induced in Ni R II mutant.

Mechanism to combat cobalt toxicity in cobalt resistant mutants of Aspergillus nidulans

“…The selection by step-by-step culturing of wild type Co sensitive strain riboA 1 , biA 1 , on medium containing increasing concentrations of Co, provided mutants having tolerance to 2.5 mM Co. All the mutants retained this level of resistance, even after several subculturing on Co-free medium. The heritable acquisition of metal resistance by continuous training has been reported for other fungi including A. nidulans 3,8 .…”

“…The wild type as well as the mutant strains, selected in this study were normally grown at 37 0 C for 120 h on either supplemented Czapeck-Dox medium (CDM) or complete medium (CM) and assessed for their growth behaviour 8 . Molar stock solution of cobalt sulphate (CoSO 4.…”

“…Cobalt resistant (Co R ) mutants of A. nidulans were obtained by stepwise selection against increasing concentration of Co on Czapeck-Dox agar medium 3,8 . The process led to the selection of Co R mutants that exhibited good growth and conidiation at 2.5mM of Co.…”

Influence of the Environment on the Morphological and Biochemical Characteristics of Different Aspergillus niger Wild Type Strains

“…It is capable of maintaining a relatively neutral pH by pumping protons out of the cell and by establishing a low proton membrane permeability [2]. Wild type strains are very often used in studies of biosorption and bioaccumulation of different heavy metals and toxic elements [3][4][5][6][7].…”

“…Heavy metals which are accumulated in soils, water and sewage sludge may, at sufficiently, in high concentrations affect the growth, sporulation and metabolism of fungi [6,10,11]. Wide spectrum of heavy metal ions effecting soil microorganisms led us to study their effects on selected strains of A. niger from various localities with different level of old ecological burden.…”