Protoplast fusion in Aspergillus niger strains accumulating citric acid.

Auxotrophic strains of Aspergillus niger were obtained from citric-acid-producing strains of the fungus after irradiation with UV light. Protoplasts were isolated from young hyphae of the auxotrophic strains after treatment with snail enzyme and than treated with polyethylene glycol (30%, W/V), in a Ca2+ (10 mmol/L) solution. The pH value of the suspension was adjusted to 9.

Effect of aeration and agitation on the biosynthetic activity of diffusely growing Aspergillus niger.

A considerable lowering of aeration demands occurs during diffuse growth of citric acid-producing Aspergillus niger in a submerged cultivation. However, the diffuse culture poses stricter demands on the type of aeration and agitation. The impeller frequency affects considerably the morphology of the producer fungus and the accumulation of citric acid.

Accumulation of 2-oxo acids in mutants of Aspergillus niger requiring lysine.

Mutants of Aspergillus niger 194A and 178 requiring lysine differ from the original prototrophic strain K10 and from each other on the course of accumulation of organic acids. In both mutants less citric acid accumulates during the first phase of cultivation but considerably more 2-oxoglutarate and 2-oxoadipate accumulate than in the original strain. Whereas in the 194A mutant this state remains unchanged also during the second phase of cultivation, in the 178 mutant oxo acids are degraded and citric acid is synthesized intensively.

Variability of Aspergillus niger after treatment with N-emthyl-N'-nitro-N-nitrosoguanidine.

N-Methyl-N'-nitro-N-nitrosoguanidine influences strongly the variability of Aspergillus niger MU 90, particularly on long-term treatment. The number of spores capable of growth decreases with the duration of treatment and the number of morphological and biochemical mutants considerably increases. The highest number of mutants with increased organic acid production was obtained after a mutagenic treatment when the number of surviving spores decreased below 1%.