Effect of acetate and pH on sunflower oil assimilation by Mucor circinelloides f. circinelloides

Mucor circinelloides f. circinelloides CBS 108.16 exhibited poor cell growt h and substrate assimilation (32.5% of extracellular lipids) when grown wit h sunflower oil as sole carbon source. By contrast, in the presence of both sunflower oil (30 g/l) and sodium acetate (10 g/l), nearly complete utiliz ation of both substrates (97.5% of sunflower oil, 100% of sodium acetate) o ccurred and biomass production was increased about five-fold to 23 g dry we ight/l after 72 h. A significantly higher content of gamma-linolenic acid i n the fungal neutral lipid fraction (5.2%) was also observed after 168 h of growth. These different patterns were attributed to the change in pH of th e medium during cell growth in the presence and absence of acetate. In the absence of sodium acetate the pH decreased to 2.2, whereas in its presence it increased to about pH 8.0. During metabolism of sunflower oil in the pre sence of sodium acetate, the percentage of saturated fatty acids in the med ium increased, suggesting a higher specificity of the fungal lipase for uns aturated fatty acids. For growth experiments in a small fermenter in the ab sence of sodium acetate, the gradual pH increase of sodium acetate containi ng medium was mimicked by an identical programme-controlled pH increase. Si milar results as in the presence of sodium acetate were obtained. This obse rvation indicated that the pH increase alone during cultivation was respons ible for the increased sunflower oil utilization, biomass and GLA productio n.