Phosphorus supply via a fed-batch strategy improves lipid heterotrophic production of Chlorella regularis.

Intracellular phosphorus (P) accumulation can improve microalgal growth and lipid synthesis. However, large excess of P causes cell poisoning. This study utilized a P-fed-batch strategy to investigate its potential to improve the utilization of the excessive P, while avoiding toxic side effects. This strategy contributed to a more complete utilization of the intracellularly stored P, which enhanced the microalgae biomass by 10-15% by upregulating the brassinosteroid growth hormone gene at a P-fed-batch frequency of 2-8. Furthermore, the lipid content increased by 4-16% via upregulation of lipid synthesis-related genes. As a result, the P-fed-batch strategy significantly increased the lipid production by 13-19%. The content of saturated fatty acid increased by ~ 100%, implying improved combustibility and oxidative stability. This is the first study of this P-fed-batch strategy and provides a new concept for the complete utilization of excessive P.

Excessive phosphorus caused inhibition and cell damage during heterotrophic growth of Chlorella regularis.

A high phosphorus concentration is widely accepted as favorable for enhancing both microalgae growth and lipid accumulation; however, excessively high P could be counter-productive. In this study, we investigated the effects of increasing P levels (5.4, 25, 45, 150, and 250 mg-P L-1) on the heterotrophic cultivation of Chlorella regularis. Microalgae growth was inhibited and cells were severely damaged in response to highly excessive P levels (≥150 mg-P L-1). In particular, 250 mg-P L-1 resulted in a ∼40% decrease in cell density and a ∼70% loss of cell viability. Microalgae damage induced by excessive phosphorus included enlarged cell size, deformation of cell walls, and disorganization of organelles. These negative effects were associated with the over-accumulation of polyphosphates within cells, which may further cause binding of P to intracellular components. Although P is an essential nutrient, excessive P lowers cell growth and viability.