Oleaginous yeast Cryptococcus albidus has demonstrated potential to efficiently convert high strength organic waste streams derived volatile fatty acids (VFA) into lipids. However, the underlying metabolic pathways and factors affecting the oleaginity of C. albidus and other oleaginous microorganism are poorly understood. Relative carbon to nitrogen ratio (COD:N) has long been implicated to play a role in oleaginity and based primarily on empirical evidence, it is assumed the nitrogen can act as a metabolic ‘switch’ to divert carbon away from catabolic tricarboxylic acid cycle (TCA) towards anabolic fatty acid biosynthesis under nitrogen limited growth conditions. In our previous study, the intracellular lipid content increased by 52% w/w (from 19% to 29%) in C. albidus cultures subjected to nitrogen limitation. Here, we report the transcriptome level changes in continuous cultures of C. albidus in response to various degrees of nitrogen limitation to conclusively establish the metabolic effects of nitrogen availability on lipid production. C. albidus was cultured under three different COD:N ratios of 5:1, 25:1 and, 250:1 representing nitrogen in relative excess, limitation and, severe limitation respectively. Differential gene expression data revealed complete suppression of TCA, a major catabolic pathway, under nitrogen limited conditions with all the related genes being downregulated. This downward shift in catabolism was accompanied by induction of lipid biogenesis as suggested by the upregulation of several genes including the ones encoding for enzymes ATP:Citrate lyase (acly) and, Acetyl-CoA carboxylase (acc), which catalyze the first and the rate limiting step in lipid biosynthesis. Upregulation of entire cellular nitrogen metabolism including genes encoding nitrate reductase (nr), nitrite reductase (nit-6) and, several nitrate, nitrite and ammonium transporters (amt, nrt, narK, nrtP, nasA) suggests that C. albidus cells were attempting to utilize any available exogenous nitrogen sources. Additionally, genes encoding amidase, glutamate dehydrogenase and asparginase enzymes were also upregulated under severe nitrogen limitation demonstrating autophagy of amino acids by C. albidus in a possible attempt to scavenge endogenous nitrogen. In conclusion, this study will help elucidate factors driving lipid accumulation in C. albidus and contribute toward bioprocess development and optimization for engineered lipid production from ‘waste’ streams.

Comparative transcriptomic analysis of lipid producing yeast Cryptococcus albidus

253rd National Meeting (2017)

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