Regulation by zinc and adenosine 3′,5′-cyclic monophosphate of growth and citric acid accumulation in Aspergillus niger

The citric acid fermentation by Aspergillus niger is divided into two consecutive phases, growth phase when the cells proliferate but do not accumulate citrate, followed by an accumulating phase when they excrete citrate but do not proliferate. The phase alternative was controlled by the concentration of zinc: high zinc (2 μM) maintained growth phase, and a zinc 'deficiency apparently signaled the transition to the accumulating phase. Cyclic AMP affected the rates of growth and acidogenesis when added to cultures growing at low but not at high zinc: that is, cAMP did not induce the phase transition, zinc deficiency did. Cyclic AMP enhanced growth early in the fermentation, but at later stages the response of the mycelia to cAMP changed, and then cAMP inhibited growth. When citrate eventually began to accumulate cAMP augmented its synthesis. The growth and acidogenic responses were quite specific to cAMP, and were sensitive to concentrations of about 1 μM. Cyclic AMP also either promoted or retarded the appearance of an unidentified yellow pigment. It is proposed that the growth and accumulating phases are distinct differentiated states, at least with respect to cAMP metabolism.