Excessive phosphorus enhances Chlorella regularis lipid production under nitrogen starvation stress during glucose heterotrophic cultivation

Recent studies claimed nitrogen starvation in addition to sufficient phosphorus content to be a "lipid trigger" for microalgae autotrophic cultivation. However, the role of phosphorus in heterotrophic cultivation still remains controversial. Here, we investigated the utilization of a series of phosphorus concentrations of up to 818% in BG11 medium to cultivate Chlorella regularis under nitrogen starvation stress during glucose heterotrophic cultivation. Phosphorus positively influenced microalgal cell synthesis, and the lipid content increased with increasing levels of supplied phosphorus. The excess phosphorus was stored in the form of Poly-P, thus providing energy and participating in the synthesis of cellular materials, which enhanced cell growth and lipid accumulation. The biomass production reached 4.53 g/L under nitrogen starvation with excess phosphorus supply, which was comparable to the effect of a nitrogen-sufficient condition (control). The excessive phosphorus level upregulated the genes ROT3, BAS1, CYP735A, FBP, GOT2, and AOC3 associated with growth. Microalgal lipid content reached 42.3% and lipid productivity reached 310.0 mg/(L.d), values that were 2 and 1.3 times higher than those in the control, respectively. Furthermore, the additional phosphorus supply increased the amounts of saturated fatty acids, while shifting fatty acid pathways. We suggest that higher amounts of phosphorus should be supplied to achieve high microalgal lipid productivity under nitrogen starvation stress during glucose heterotrophic cultivation. This is an effective strategy to enhance biodiesel production, simultaneously achieving high biomass production and efficient lipid accumulation.