Pattern of cadmium accumulation and essential cations during growth of cadmium-tolerant fungi

The present study evaluates the growth response of two strains of filamento us fungi; a Fusarium sp. and Alternaria tenuis, grown on both solid and liq uid Czapek Dox medium amended with different concentrations of CdCl2. Colon y extension and the mycelial dry weight of both fungi were significantly in hibited by high concentrations of cadmium. Extended lag phases and low grow th rates resulted from cadmium administration. Cadmium drastically affected fungal morphogenesis by the production of stunted sterile thick mycelial f ilaments of the Fusarium sp. and chains of uncharacterized swellings instea d of conidia in A. tenuis. Experiments showed that cadmium accumulation by the Fusarium sp. grown in liquid medium was a concentration dependent, and over the incubation time it displayed a plateau pattern. The cells grown on medium containing 0.25 mmol l(-1) CdCl2 accumulated up to 89 +/- 12 mu mol Cd (gm dw)(-1) after two days, falling to similar to 29 +/- 10 mu mol Cd ( gm dw)(-1) after five days. At 0.5 mmol l(-1) CdCl2 treatment the maximum c ellular cadmium content was similar to 132 +/- 14 mu mol (gm dw)(-1), attai ned after 3 days, and decreased to similar to 98 +/- 9 mu mol (gm dw)(-1) a t the end of the incubation time. There was a simultaneous marked drop in c admium content and pH of the growth medium during the first few days. The p resence of cadmium markedly altered the cellular essential cations; K+ and Mg2+ being decreased while Na+ increased during the growth period. Such fin dings resulted a reverse pattern of cellular Na+/K+ ratio for cells grown o n cadmium-containing medium in respect to the control treatment. The result s are discussed in relation to a further dimension of cadmium effects that might reflect its toxicity, as well as the implication of cadmium extrusion for tolerance during fungal growth.