This 1981 study investigated sporulation of Aspergillus niger and Aspergillus ochraceus in a continuous tower fermenter system by manipulating nutrient availability. The researchers tested carbon limitation (using sucrose and starch) and nitrogen limitation (using nitrate) under both gradual and shock conditions. Shock carbon limitation produced no sporulation and caused culture washout, while gradual carbon reduction induced slight sporulation with sub-aerial morphology in A. niger. Gradual nitrogen limitation produced no sporulation despite severe growth restriction. However, shock nitrogen limitation (sudden reduction from 0.21 to 0.02 g N/L) induced heavy sporulation in both organisms within 10-18 hours, with simplified spore structures containing fewer phialides than normal. The tower fermenter proved ideal for controlling fungal morphology and inducing sporulation. Maintenance coefficients were calculated for various substrates, revealing that A. ochraceus required more starch for maintenance than previously reported for any fungus.

Key findings

• Shock nitrogen limitation induced rapid, heavy sporulation in both Aspergillus species within 10-18 hours, while gradual nitrogen limitation produced no sporulation despite restricting growth
• Carbon limitation produced minimal or no sporulation; shock carbon limitation caused culture washout without spore formation
• Sporulation structures in nitrogen-limited continuous culture were simplified compared to sub-aerial forms, with reduced numbers of phialides and abnormal morphologies
• The continuous tower fermenter system effectively controlled organism morphology and provided steady-state conditions superior to batch culture for studying fungal sporulation
• Maintenance energy coefficients for A. ochraceus on starch were considerably higher than any previously published values for filamentous fungi