Repression of reserve lipid turnover in Cunninghamella echinulata and Mortierella isabellina cultivated in multiple-limited media

Abstract

To study patterns of reserve lipid biosynthesis and turnover (degradation) in two oleaginous Zygomycetes, namely Cunninghamella echinulata and Mortierella isabellina under various growth conditions. Fatty acid composition of the reserve lipid of both strains was also studied in all growth steps. Cunninghamella echinulata and Mortierella isabellina were grown in carbon-excess batch cultures. In the investigated strains, accumulation of reserve lipid occurred only when the activity of both NAD(+)-isocitrate dehydrogenase (ICDH) and NADP(+)-ICDH were not detectable in the cell-free extract. Specifically, in C. echinulata, NAD(+)-ICDH activity was detected even after depletion of ammonium nitrogen in the medium, resulting in a delay of the initiation of lipid accumulation period. On the contrary, in M. isabellina, lipid accumulation occurred simultaneously with ammonium nitrogen exhaustion in the growth medium, as the activity of both NAD(+)- and NADP(+)-ICDH were not detectable after nitrogen depletion. In C. echinulata reserve lipid was not degraded after glucose had been exhausted. Supplementations of the medium with Fe(3+), yeast extract or Mg(2+) induced, however, reserve lipid breakdown and formation of lipid-free material. In M. isabellina after glucose exhaustion, notable lipid degradation occurred, accompanied by a significant lipid-free material biosynthesis. Nevertheless, in multiple-limited media, in which Mg(2+) or yeast extract, besides carbon and nitrogen, were limiting nutrients, reserve lipid breakdown was repressed. In both strains, the quantity of gamma-linolenic acid (GLA) in the reserve lipids [varying between 9 and 16% (w/w) in C. echinulata and 1.5-4.5% (w/w) in M. isabellina] was proportional to lipid-free biomass. Lipid accumulation period in Zygomycetes is initiated by the attenuation of ICDH activity in the mycelium while the regulation of ICDH from ammonium nitrogen is strain specific. While a single nitrogen limitation was enough to induce lipid accumulation, however, multiple limitations were needed in order to repress lipid turnover in oleaginous Zygomycetes. As for GLA, its biosynthesis in the mycelium seemed proportional to lipid-free biomass synthesis. Several nutrients are indispensable for functioning the mechanisms involved in the mobilization of reserve lipid in oleaginous moulds. Therefore, reserve lipid turnover in oleaginous moulds could be repressed in multiple-limited media.