Digestate dilution shapes carbohydrate and pigment production during microalgal and cyanobacterial-based biogas upgrading

Microalgae and cyanobacteria offer a promising platform for integrating sustainable technologies aligned with circular and green economy goals. However, current studies often focus on a limited number of genera and overlook how centrate dilution influences metabolite production. This study investigates the potential of the freshwater microalga Parachlorella hussii N9 and the marine cyanobacterium Cyanothece sp. CE4 for photobiological biogas upgrading coupled with nutrient recovery from centrate, assessing the impact of centrate dilution on carbohydrate and pigment content. By varying centrate concentration (5–50 %) in tap or seawater, this research explores how the biogas-to-centrate ratio can be adjusted for biomass production, TN and CO2 abatement, and to target specific metabolites, advancing circular bioeconomy strategies. The microalga exhibited faster growth than the cyanobacterium, achieving the stationary phase in three days, and higher cellular and soluble carbohydrate productivity (up to 237 and 75 mg L−1d−1, respectively). CO₂ abatement (almost complete in all treatments) reached ∼513 ± 28 mg L−1 of culture, while nitrogen removal considering initial centrate concentration ranged between 32 and 250 mg N L−1, but 100 % TN removal was exhibited only with the lower centrate concentrations (5–10 %). These lower concentrations also induced the highest carbohydrate content in biomass (41–44 % dw). In contrast, pigment content increased with higher centrate concentrations: the microalga reached 3.6 % dw of chlorophyll at 50 % centrate, while the cyanobacterium produced up to 0.6 % dw of C-phycocyanin; both strains showed similar carotenoid content (0.4–0.5 % dw). This study highlights the potential of adjusting centrate dilution to target microalgal metabolism for integrated CO₂ capture, nutrient recovery, and bioproduct generation.