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

Abstract

Characteristics of fungal species tolerant to high levels of metals in natural environment can be amplified by isolation and selection of resistant mutants. Step-by-step culturing led to identification of highly stable Co-resistant (Co(R)) mutants of A. nidulans. Based on two distinct morphological features, Co-resistant mutants were categorized as Co(R)I and Co(R)II. The two mutants varied in their growth behavior and colony morphology that were reflected in supplemented as well as unsupplemented growth media over the generations. As compared to the Co(R)I, Co(R)II mutant exhibited sparse mycelia and conidiation but secreted higher amount of melanin. Co(R) mutants could tolerate up to 2.5mM Co in the medium, however, required a threshold concentration of 0.25mM Co for optimal growth and germination. Absence of Co in the medium caused a stressful situation for the Co(R) mutants and led to the secretion of a white extracellular precipitate found to be a glycoprotein. In response to interactions with Co-ions, Co(R) mutants produced oxalic acid and bioprecipitated Co as Co-oxalate providing scope for metal reclamation as well as oxalic acid extraction. The mutants could help to recover the insoluble Co-oxalate salt from aqueous solutions by entrapping it in their growing mycelial meshwork.