Enhanced activity of ADP glucose pyrophosphorylase and formation of starch induced by Azospirillum brasilense in Chlorella vulgaris

• Azospirillum brasilense increased activity of AGPase in Chlorella vulgaris under heterotrophic conditions.
• This yielded a higher quantity of starch in the microalgae–bacteria interaction.
• Either with sufficient or starvation conditions of nitrogen or phosphorus, C. vulgaris responded similarly to A. brasilense.
• Using either d-glucose or sodium acetate as carbon sources yields similar effects.
• Under conditions of starvation of nitrogen or phosphorus, A. brasilense reduces nutrient stress for C. vulgaris and increases its cell density.

ADP-glucose pyrophosphorylase (AGPase) regulates starch biosynthesis in higher plants and microalgae. This study measured the effect of the bacterium Azospirillum brasilense on AGPase activity in the freshwater microalga Chlorella vulgaris and formation of starch. This was done by immobilizing both microorganisms in alginate beads, either replete with or deprived of nitrogen or phosphorus and all under heterotrophic conditions, using d-glucose or Na-acetate as the carbon source. AGPase activity during the first 72 h of incubation was higher in C. vulgaris when immobilized with A. brasilense. This happened simultaneously with higher starch accumulation and higher carbon uptake by the microalgae. Either carbon source had similar effects on enzyme activity and starch accumulation. Starvation either by N or P had the same pattern on AGPase activity and starch accumulation. Under replete conditions, the population of C. vulgaris immobilized alone was higher than when immobilized together, but under starvation conditions A. brasilense induced a larger population of C. vulgaris. In summary, adding A. brasilense enhanced AGPase activity, starch formation, and mitigation of stress in C. vulgaris.