Nutrient stress triggers sugar-mediated carotenoid production in algal-bacterial interactions.

This study examined the impact of co-culturing Chlorella saccharophila (UTEX247) with Exiguobacterium sp. strain AMK1 on carotenoid production under nitrate-depleted conditions and 3% CO₂ supplementation. The co-culture significantly enhanced the productivity of lutein (238.31 µg.L⁻¹d⁻¹), zeaxanthin (220.72 µg.L⁻¹d⁻¹), violaxanthin (185.42 µg.L⁻¹d⁻¹), and antheraxanthin (84.07 µg.L⁻¹d⁻¹). Compared to nitrate-repleted mono-cultures, these carotenoids increased by 3.54-fold, 4.81-fold, 12.28-fold, and 9.34-fold, respectively. The violaxanthin cycle, activated by CO₂ supplementation, resulted in higher zeaxanthin production, verified through HPLC analysis. Metabolic profiling highlighted a notable rise in sucrose, an algal-specific metabolite, in the co-culture, reflecting enhanced carbon metabolism and carotenoid synthesis. Conversely, trehalose levels were significantly higher in the bacterial mono-culture (297.77 µg.mL⁻¹) than in the co-culture (88.84 µg.mL⁻¹), showing a 1.68-fold reduction as confirmed by GC-MS/MS. This suggests trehalose as a stress marker, with its reduction indicating mutualistic interactions between algal and bacterial. Overall, the co-culture strategy emerges as a promising approach to activate unexpressed pathways, generate novel metabolites, and enhance yields of valuable carotenoids like lutein and zeaxanthin. This aligns with the principles of a circular bioeconomy, leveraging bacterial biofertilizers, valorizing CO₂, and minimizing chemical dependency, thus offering potential for biorefinery applications.