Acclimation of photosynthesis and lipids biosynthesis to prolonged nitrogen and phosphorus limitation in Nannochloropsis gaditana

Microalgae represent a potential sustainable source of molecules and materials and the improvement of the knowledge of their metabolic regulation is essential to maximize their potential. Nutrients deprivation stimulates the accumulation of reserve lipids, triacylglycerols but also inhibits photosynthesis with a negative impact on biomass production and therefore overall productivity. In this work, the seawater microalga Nannochloropsis gaditana was cultivated long term in a semi-continuous system where the concentrations of nitrogen or phosphorus were limiting but still sufficient to sustain growth indefinitely, to highlight the response to a long-term nutrient limitation and distinguishing it from the one to acute short-term stress. N. gaditana cells can acclimate to chronic nutrients limitation maintaining photosynthetic activity while also accumulating lipids. Both nitrogen and phosphorus limitation induced an increase of triacylglycerols content, although not by the induction of the synthesis of fatty acids but rather by modulating the fluxes of reduced carbon molecules toward lipid biosynthesis. Photosynthetic activity was maintained under P limitation while this was strongly affected by nitrogen depletion, where proteins of photosynthetic apparatus were largely reduced in content but still maintained their functionality and were able to achieve half of the biomass productivity with 30% of the nitrogen supply

An increase in the membrane lipids recycling by PDAT overexpression stimulates the accumulation of triacylglycerol in Nannochloropsis gaditana

Oleaginous microalgae represent potential feedstocks for the sustainable production of lipids thanks to their ability to accumulate triacylglycerols (TAGs). TAG accumulation in several algal species is strongly induced under specific conditions such as nutrient deprivation and high light which, however, also negatively impact growth. Genetic modification of lipogenic pathways can potentially enhance TAG accumulation without nega-tively affecting growth, avoiding the trade-off between biomass and lipid productivity.In this study, the phospholipid: diacylglycerol acyltransferase (PDAT), an enzyme involved in membrane lipid recycling, was overexpressed in the seawater alga Nannochloropsis gaditana. PDAT overexpression induced increased TAG content in actively growing algae cultures while no effects were observed in conditions naturally stimulating strong lipid accumulation such as high light and nitrogen starvation. The increase of TAG content was confirmed also in a strain cultivated in industrially relevant conditions even though PDAT overexpression, if too strong, the gene overexpression becomes detrimental for growth in the longer term. Results overall suggest that genetic modulation of the PDAT gene represents a promising strategy to increase microalgae lipid content by minimizing negative effects on biomass productivity