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

The present study evaluates the growth response of two strains of filamentous fungi; a Fusarium sp. and Alternaria tenuis, grown on both solid and liquid Czapek Dox medium amended with different concentrations of CdCl₂. Colony extension and the mycelial dry weight of both fungi were significantly inhibited by high concentrations of cadmium. Extended lag phases and low growth rates resulted from cadmium administration. Cadmium drastically affected fungal morphogenesis by the production of stunted sterile thick mycelial filaments of the Fusarium sp. and chains of uncharacterized swellings instead 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 mol Cd (gm dw)^–1 after two days, falling to 29 ± 10 mol Cd (gm dw)^–1 after five days. At 0.5 mmol l^–1 CdCl₂ treatment the maximum cellular cadmium content was 132 ± 14 mol (gm dw)^–1, attained after 3 days, and decreased to 98 ± 9 mol (gm dw)^–1 at the end of the incubation time. There was a simultaneous marked drop in cadmium content and pH of the growth medium during the first few days. The presence of cadmium markedly altered the cellular essential cations; K⁺ and Mg²⁺ being decreased while Na⁺ increased during the growth period. Such findings resulted a reverse pattern of cellular Na⁺/K⁺ ratio for cells grown on cadmium-containing medium in respect to the control treatment. The results 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.