Strain-dependent carbon partitioning and community-level functional responses during microalgae-based dairy wastewater treatment under CO₂ enrichment

Microalgae-bacteria consortia offer a route for coupling dairy-wastewater treatment with carbon conversion, yet strain-dependent differences in carbon partitioning and microbial functional responses remain insufficiently resolved.

Chlorella vulgaris, Tetradesmus obliquus, and Synechocystis sp. were cultivated in dairy wastewater under air or a one-time 4% CO₂ gas charge followed by closed-loop recirculation. Biomass production, nutrient removal, liquid-solid-gas carbon inventories, biochemical composition, bacterial succession, metagenomic profiles, and entropy-weighted TOPSIS were integrated.

Synechocystis sp. treatment S1 showed the greatest liquid-phase total-carbon drawdown (72.1%), as carbon transferred to the suspended-solid pool, the proportion of liquid inorganic carbon within the instantaneous carbon inventory declined from 54.18% to 8.85%. T. obliquus treatment T1 achieved the highest biomass concentration (0.99 ± 0.01 g·L^-1), NH₄⁺-N removal (84.89%), and comparatively high carbohydrate and lipid accumulation, yielding the highest integrated TOPSIS score. Metagenomic analyses revealed treatment-associated shifts in CBB/TCA-related functions in C. vulgaris communities and late enrichment of glycogen-associated genes in Synechocystis sp. communities. RDA further linked central-carbon gene profiles with TC, TN, TP, EPS, and biochemical pools, while bacterial succession showed marked strain- and gas-condition dependence.

This divergence shows that maximizing biomass recovery and maximizing liquid-carbon drawdown represent different operational objectives rather than a single strain-independent response. The results identify distinct operational advantages among the tested phototrophs: T. obliquus favored biomass and resource recovery, whereas Synechocystis sp. promoted stronger short-term inorganic-carbon conversion. Together, carbon partitioning and community functional patterns indicate that tested-strains identity, carbon availability, nutrient demand, extracellular-carbon exchange, and phycosphere succession jointly shape system performance.