Enhanced biomass production and harvesting efficiency of Chlamydomonas reinhardtii under high-ammonium conditions by powdered oyster shell.

Chlamydomonas reinhardtii prefers ammonium (NH) as a nitrogen source, but its late-stage growth under high-NH concentrations (0.5 ∼ 1 g/L) is retarded due to medium acidification. In this study, oyster shell powders were shown to increase the tolerance of C. reinhardtii to NH supplementation at 0.7 g/L in TAP medium in 1-L bubble-column bioreactors, resulting in a 22.9 % increase in biomass production, 62.1 % rise in unsaturated fatty acid accumulation, and 19.2 % improvement in harvesting efficiency. Powdered oyster shell mitigated medium acidification (pH 7.2-7.8) and provided dissolved inorganic carbon up to 8.02 × 10 μmol/L, facilitating a 76.3 % NH consumption, release of up to 189 mg/L of Ca, a 42.1 % reduction in ζ-potential and 27.7 % increase in flocculation activity of microalgae cells. This study highlights a promising approach to utilize powdered oyster shell as a liming agent, supplement carbon source, and bio-flocculant for enhancing biomass production and microalgae harvesting in NH-rich environments.