Oleaginous yeast Cryptococcus curvatus for biofuel production: Ammonia’s effect

Culturing oleaginous yeast with organic residues can provide cheap feedstock for biodiesel production, but the high content of nitrogen in some feedstocks (such as food residues) can lead to inhibition to cell growth and lipid synthesis, and this may be caused by ammonia. Thus, the inhibitory effect of ammonia on the growth of yeast Cryptococcus curvatus and its possible mechanism were evaluated in this study. The biomass production, lipid contents, affected metabolic pathways, and activities of the related pathway enzymes were investigated. The results showed that the yeast biomass production on acetate decreased from 5.9 kg m−3 to 2.0 kg m−3 when the initial ammonia concentration (in nitrogen equivalent terms) increased from 131 g m−3 to 3140 g m−3. Ammonia significantly inhibited the yeast growth with volatile fatty acids (VFA) as carbon sources, but not in the culture with pyruvate generating substances (PGS) as carbon sources. However, the fatty acid synthesis was suppressed by ammonia in cultures with either VFA or PGS. The study on mechanism of ammonia inhibition suggested it might include an inhibition on the acyl-CoA synthetase activity, since a 61% decrease in the enzyme activity was found when 3140 g m−3 of ammonia (in nitrogen equivalent terms) was added. This study firstly reported the effect of ammonia with high concentrations to oleaginous yeast growth and lipid accumulation, and provided preliminary evidences on its possible mechanism of acyl-CoA synthetase inhibition.

► Study effects of ammonia on biomass and lipid production of Cryptococcus curvatus.
► Ammonia at high concentrations showed inhibitory effects.
► The inhibition of ammonia was highly dependent on substrates of the culture medium.
► The active compound causing the inhibition appears to be the free ammonia (NH3).
► Mechanism of ammonia inhibition may include a suppression on acyl-CoA synthetase.