Optimizing calcium and magnesium in seawater medium with high bicarbonate concentration for efficient growth and self-flocculation harvesting of Chlorella sp

Microalgae cultivation using high bicarbonate concentration in seawater medium triggers CaCO₃ precipitation due to pH elevation, contaminating the photobioreactor and reduce biomass productivity. It is necessary to control Ca²⁺ and Mg²⁺ concentration in appropriate range to allow microalgae rapid growth without precipitation, followed by effective self-flocculation for harvesting. This study optimized this concentration range as 0.5-1.0 mmol L^-1 for Ca²⁺ and 2.5-5.0 mmol L^-1 for Mg²⁺. With this concentration, subsequent pH adjustment to 11 induced > 90 % self-flocculation efficiency. Also, outdoor cultivation in floating photobioreactors utilized treated seawater with optimized Ca²⁺ and Mg²⁺ concentration achieved productivity of 9.36 g m^-2 day^-1, which is 112 % higher than untreated seawater (4.42 g m^-2 day^-1). The optimized process reached the goal of higher biomass productivity without precipitation and efficient harvesting. On the other hand, microalgae-induced precipitation may serve as potential carbon sink, contributing to ocean-negative carbon emissions.