Effect of Nitrogen Limitation on Cell Growth, Lipid Accumulation and Gene Expression in Chlorella sorokiniana

The influence of nitrogen (N) limitation on the cell growth, chlorophyll content, intracellular lipid production and expression levels of two pathway genes in Chlorella sorokiniana was investigated in mixotrophic culture in this study. The maximum biomass concentration of C. sorokiniana cultured in modified BG11 medium containing 3, 6, 9, 12 and 15 mM N were 1.60, 2.21, 2.74, 3.18 and 3.21 g/L of dry cell weight, respectively with maximum specific growth rate of 0.180, 0.198, 0.201, 0.203, 0.206 day-1 during culture with an initial N feed of 3, 6, 9, 12 and 15 mM in the first eight days, respectively. The maximum lipid content was 51 % with 3 mM N. However, the maximum lipid productivity of 0.00884 g L-1 day-1 was achieved with 6 mM N. Expression levels of accD (heteromeric acetyl-CoA carboxylase beta subunit) and accI (homomeric acetyl-CoA carboxylase) genes in C. sorokiniana were studied by real-time PCR. Increased expression levels of accD reflected the increased lipid content in stationary phase. In contrast, expression of the acc1 gene always remained low, showing that the gene might not be critical for lipid accumulation.