Suitability of Low-Cost Sugars as Substrates for Lipid Production by the Fungus Thamnidium elegans

Thamnidium elegans CCF-1465 was grown in a submerged shake-flask culture in media in which inexpensive sugars derived from a sugar refinery plant (glucose, fructose, and sucrose) were utilized as carbon sources. Cultures were performed in media favoring the accumulation of reserve lipid (high initial C/N ratios imposed, media presenting constant initial nitrogen and increasing initial sugar concentrations, namely, at 30, 60, and 80 g/L). Although significant quantities of fat were accumulated in all cultures performed, lipid production in terms of both absolute (g/L) and relative (%, wt/wt, in dry matter) values clearly increased with sugar concentration increments into the medium. For a given initial sugar concentration, regardless of the individual sugar utilized, lipid was accumulated in almost equivalent quantities inside the mycelia. Sugars were assimilated with comparable consumption rates, while consumption rate was divided into two distinct phases, especially in the high initial sugar concentration media; in the first growth phase, an almost linear assimilation rate of all individual sugars occurred with an uptake rate of similar to 0.20 g/L.h. Thereafter, (in the lipid accumulation period), a decrease of the sugar uptake rate was observed, that remained linear with a noticeably lower value (similar to 0.09 g/L.h) regardless of the initial sugar concentration or the nature of the sugar used. Maximum lipid quantities of more than 9 g/L, with 70% (w/w) fat in dry microbial mass and conversion yield of more than 20% (w/w) lipid per sugar consumed, were obtained. The nature and concentration of sugar as well as the fermentation time seemed to have some effect upon distribution of cellular fatty acids in the various lipids produced. Microbial lipids principally being composed of the fatty acids oleic and palmitic, therefore constituting perfect starting materials for the production of "2nd generation" biodiesel, were produced regardless of the sugar used as substrate. The medically important gamma-linolenic acid was produced in quantities around 510 mg/L. Phospholipids were the more unsaturated fraction of cellular lipids.