Deciphering the promotion and inhibition of bicarbonate fertilization on microalgal activity and nutrient uptake from wastewater.

Microalgal bioremediation is a promising alternative for biological wastewater treatment but constrained by low microalgal activities. Here, bicarbonate fertilization was introduced to enhance microalgal wastewater treatment, with systematic investigations of its biphasic dose-dependent effects on microalgal activity and nutrient uptake. The results showed that moderate inorganic carbon (MIC, 0.05 M) group significantly improved the biomass production, NH-N removal, and PO-P removal by 76.0%, 21.3%, and 11.9%, respectively; whereas high inorganic carbon (HIC, 0.1 M) group inhibited them by 11.0%, 4.48%, and 52.7%, respectively, compared with low inorganic carbon (LIC, 0.005 M) group. Mechanistic analyses suggested that LIC group encountered high alkalinity, exacerbated carbon/trace element limitation, and attenuated extracellular polymeric substances (EPS) barriers and antioxidant systems; while HIC group increased salinity stresses, triggered morphological defense, and diminished light harvesting and phycospheric mass transfer, restricting microalgal activity and nutrient uptake. In contrast, MIC group relieved carbon limitation, accelerated photosynthetic electron transfer, and sustained intracellular redox homeostasis, underpinning the highest biomass production and nutrient removal. These findings could facilitate the practical application of bicarbonate fertilization in microalgal wastewater treatment.