Lipid accumulation and membrane fluidity influence mycelial stability and riboflavin production by the riboflavinogenic fungus Eremothecium ashbyii

The plant pathogenic filamentous hemiascomycete fungus Eremothecium ashbyii is a natural overproducer of riboflavin (vitamin B2). Preceding riboflavin overproduction, microdroplets of lipid were observed in the hyphae of E.ashbyii as yellow fluorescent bodies on staining with the lipid specific dye Nile blue. Following this the fungus wasgrown on different substrates-olive oil, sunflower oil and glucose. Lipid accumulation was followed as a time course by gravimetry. The mycelial lipid was fractionated into triglycerides and phospholipids and relative proportions of constituent fatty acids in them was estimated by GC MS during growth and riboflavin overproduction on each substrate. Changes in mycelial morphology were followed as a time course. In parallel, mycelial growth on each medium was converted to protoplasts whose membrane fluidity was monitored by measuring the fluorescence anisotropy using 1,6 - diphenyl - 1,3,5 - hexatriene (DPH). Lipid accumulation and extracellular lipase activity was maximum at 48 h of growth on all growth substrates. Maximum lipase production coincided with maximum lipid accumulation preceding riboflavin overproduction. A GC-MS analysis of fatty acids during growth (48 h) and riboflavin overproduction (96 h) showed the presence of a large percentage of unsaturated fatty acids in triglycerides and phospholipids. Decrease in Octadecadienoic acid (C 18:2) in the triglycerides during the production phase correlated with riboflavin production while mycelial stability correlated to the Octadecenoic acid (C 18:1) content of the phospholipids. Olive oil-grown mycelia showed maximum lipid accumulation, riboflavin production, lipase activity, membrane fluidity, stability and least morphological changes. Maximum riboflavin was obtained on olive oil medium due to a greater decrease in Octadecadienoic acid content in the triglycerides and the stability of olive oil grown mycelia was attributed to the high content of Octadecenoic acid in its phospholipids.