Sustainable conversion of carbon dioxide to high-value antioxidant astaxanthin through microbial electrosynthesis-assisted microalgae cultivation

Abstract

Microbial electrosynthesis (MES) effectively converts carbon dioxide (CO2) to short carboxylic acids but has lower efficiency for higher molecular value-added products. This study used a two-stage process integrating MES with microalgae cultivation to convert CO2 into a value-added chemical—astaxanthin, a xanthophyll carotenoid with strong antioxidant activity. In the first stage, MESs featuring platinum-coated anodes and carbonaceous cathodes were employed to bioelectrochemically convert CO2 to volatile fatty acids (VFAs). In the second stage, microalgal cultivation for sustainable astaxanthin production was conducted using a VFA-containing MES effluent. In the MES operation, a newly designed cathode with alternating layers of carbon felt (CF) and reticulated vitreous carbon (RVC) significantly improved hydrodynamics and bacterial attachment. Consequently, the CF/RVC composite cathode produced 7.68 g/L VFAs from CO2, with the highest production rate of 1225.72 mg/L/day and the highest coulombic efficiency of 94.15 % at 2.8 V. Feeding acetate-containing catholyte effluent demonstrated faster growth of microalgae and astaxanthin accumulation compared to autotrophic culture, indicating synergistic integration of MES and microalgae for capturing value from CO2. Among tested microalgae strains, Haematococcus pluvialis most efficiently accumulated astaxanthin up to 12.95 mg/L as cultured with MES effluent. Successful CO2-to-astaxanthin conversion using MES and microalgae can contribute to resolving unprecedented environmental challenges attributed to the excess emission of CO2.