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 350-500mgL(-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 0.06mgL(-1)h(-1) at D=0.0096h(-1), for Chlorella sp. and 0.8 and 0.09mgL(-1)h(-1) at D=0.007h(-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.