Biochemical activities in Chlorella sp. and Nannochloropsis salina during lipid and sugar synthesis in a lab-scale open pond simulating reactor

Chlorella sp. and Nannochloropsis salina cultivated in a lab-scale open pond simulating reactor grew well and produced 350–500 mg L−1 of biomass containing approximately 40% and 16% of lipids, respectively, while different trends in storage material (lipid and sugar) synthesis were identified for the two strains. In continuous culture the highest biomass and lipid productivity was respectively 0.7 and 0.06 mg L−1 h−1 at D = 0.0096 h−1, for Chlorella sp. and 0.8 and 0.09 mg L−1 h−1 at D = 0.007 h−1 for N. salina. The major polyunsaturated fatty acid (PUFA) in the lipid of Chlorella sp. was α-linolenic acid, found at a percentage of 23.0%, w/w, while N. salina synthesized eicosapentaenoic acid at a percentage of 27.0%, w/w. Glycolipids plus sphingolipids were predominant and richer in PUFA, compared to neutral lipids and phospholipids.Activities of some key enzymes, such as pyruvate dehydrogenase (PDC), ATP-citrate lyase (ATP:CL), malic enzyme (ME) and NAD-isocitrate dehydrogenase (ICDH), which are implicated in acetyl-CoA and NADPH biosynthesis, were studied in cells grown in batch and continuous modes. PDC involved in the conversion of pyruvate to acetyl-CoA presented a constant activity in all growth phases. The high ATP:CL activity observed in algal cells, combined with low or zero ICDH activity, indicated the algae ability to generate acetyl-CoA from sugar via citrate. However, the lipogenic capacity of the strains under investigation seemed to be restricted by the low ME activity resulting to limited NADPH synthesis.

► Chlorella sp. and Nannochloropsis salina were cultivated in batch and continuous modes.
► Chlorella sp. presented a high lipid accumulation up to 40%.
► In microalgae, polyunsaturated fatty acids are mainly located in glycolipids.
► Algal cells, possessing high ATP:CL activity, can generate acetyl-CoA from citrate.
► Microalgae are able to convert storage sugar to lipid.