This study examined how growth rate affects ethylene production by Mucor hiemalis, a common soil fungus. Using chemostat cultures with controlled glucose availability, researchers found that ethylene production per unit fungal biomass was highest at low growth rates (below 0.04 h⁻¹), which are typical of soil environments. At a growth rate of 0.018 h⁻¹, the fungus produced ethylene approximately 1000 times more efficiently than in standard laboratory batch cultures with higher growth rates. The increased ethylene production resulted from improved conversion of methionine (the ethylene precursor) rather than increased methionine uptake. Unlike other fungi studied, M. hiemalis showed a logarithmic relationship between glucose utilization and growth rate, with notably higher metabolic rates than Penicillium chrysogenum and Aspergillus nidulans. The fungus maintained a filamentous growth form across all tested conditions. These findings emphasize that laboratory culture at high growth rates substantially underestimates microbial product formation in field soils, where organisms grow much more slowly.

Key findings

• Ethylene production by M. hiemalis increased ~1000-fold at low growth rates (0.018 h⁻¹) compared to high-growth batch cultures, suggesting soil predictions require low-growth-rate studies
• M. hiemalis demonstrated logarithmic glucose utilization kinetics, contrasting with the linear relationships observed in other fungi, consistent with its ecological success as a primary saprophytic sugar fungus
• Increased ethylene formation at low growth rates resulted from enhanced methionine-to-ethylene conversion efficiency, not from increased substrate uptake
• M. hiemalis consumed energy substrate and produced carbon dioxide and oxygen at substantially higher rates than P. chrysogenum and A. nidulans at comparable growth rates