Professor of Microbiology University of Patras, Greece
Abstract: The fungus Umbelopsis isabellina has the capacity to metabolize various industrial substrates of low/or zero cost and synthesize high-added value metabolites in significant quantities. In this study, the physiology of U. isabellina growing on mixtures of glycerol (at various concentrations) and glucose (at 40 g L-1) was investigated. The performed kinetics of growth and lipid accumulation revealed that fat-free biomass (xf) production was favored in low glycerol content media (i.e., 5 g L-1 and 10 g L-1) reaching 5.3 g L-1 and intracellular lipid accumulation (L/x%, w/w) maximized at 65.0%, but followed by partial lipid degradation (L/x = 58.0%). On the contrary, when glycerol was added at high initial concentrations (i.e., 80 g L-1 and 100 g L-1) the maximum xf was 4 g L-1 and lipid accumulation reached L/x = 70%, though lipid degradation was not observed. Notably, glycerol consumption occurred only when it was at high initial concentrations in the growth media. Finally, when glycerol was added in the growth medium (at 40 g L-1), upon complete consumption of glucose, limited consumption of glycerol occurred, in parallel with repression of lipid degradation. The obtained results revealed the existence of a complex regulation mechanism of glycerol and glucose co-assimilation, which led to the investigation of the activity of three key-enzymes namely aldolase, glycerol kinase and glycerol dehydrogenase, which are implicated in glycerol and glucose assimilation. The experiments proved the clear preference of the fungus for; (a) glucose over glycerol and (b) storage polysaccharides degradation instead of storage lipid at the late oleaginous phase for maintenance purposes. These new biochemical features will enable the design of appropriate growth media for the co-fermentation of these two substrates by U. isabellina with the aim to maximize lipid accumulation.
