Activity of acetyl-CoA carboxylase is not directly linked to accumulation of lipids when Chlorella vulgaris is co-immobilised with Azospirillum brasilense in alginate under autotrophic and heterotrophic conditions

Activity of acetyl-CoA carboxylase (ACCase) and lipid accumulation were assayed in the microalga Chlorella vulgaris co-immobilised in alginate beads with Azospirillum brasilense, under autotrophic and heterotrophic growth conditions, with and without ammonium starvation. ACCase is a key enzyme in de novo fatty acid biosynthesis. Under the two growth conditions, co-immobilisation always enhanced the activity of ACCase and yielded a higher level of lipids when compared with immobilisation of the alga alone. The highest lipid content obtained under autotrophic conditions was with ammonium starvation. Cultivation under heterotrophic conditions without limitation of nitrogen, with or without the presence of bacteria, yielded a higher growth rate and accumulated more lipids than under autotrophic conditions. No correlation was found between total lipids and ACCase activity. Unusually, ammonium starvation significantly reduced lipid accumulation under heterotrophic conditions. Consequently, co-immobilisation, sufficient ammonium and heterotrophic growth conditions were the most significant parameters for lipid accumulation and ACCase activity in C. vulgaris where the two latter parameters are not directly linked.