Hormesis effects of phosphorus on the viability of Chlorella regularis cells under nitrogen limitation

BackgroundPhosphorus (P) is an essential element of microalgae, which is either required for anabolism or for energy metabolism. When employing a nitrogen limitation strategy to trigger microalgal intracellular lipid accumulation, P supplementation was always simultaneously applied to compensate for the accompanied growth inhibition.ResultsThis study identified that P exerts hormesis effects on microalgae. Slight excess of P (45mgL(-1)) under nitrogen limitation condition stimulated the cell growth of Chlorella regularis and achieved a 10.2% biomass production increase. This also improved mitochondrial activity by 25.0% compared to control (P=5.4mgL(-1)). The lipid productivity reached 354.38mg(Ld)(-1), which increased by 39.3% compared to control. Such an improvement was caused by the intracellularly stored polyphosphate energy pool. However, large excess of P (250mgL(-1)) inhibited the cell growth by 38.8% and mitochondrial activity decreased by 71.3%. C. regularis cells showed obvious poisoning status, such as enlarged size, plasmolysis, deformation of cell walls, and disorganization of organelles. This is probably because the over-accumulated P protonated the amide-N and disrupted membrane permeability.ConclusionsThese results provide new insight into the roles of P in microalgae lipid production: P does not always play a positive role under nitrogen limitation conditions.