According to Botella, de Ory, Webb, Cantero, and Blandino (2005) during the first phase of growth, the fungal strains use the readily available carbon sources (free sugars) and produce cell wall-degrading enzymes, but when the concentration of free reducing sugars is too low, the microorganisms begin to use these enzymes to obtain other carbon sources, leading to a decline in enzyme activity and lipid yield.