Engineering whiting events in culture: A microalgae-driven calcium carbonate and biomass production process at high pH and alkalinity with the marine microalga Nannochloropsis oceanica IMET1

Abstract

Whitings are large scale calcium carbonate precipitation events in oceans and lakes, often during algal blooms. However, at the current ocean pH, formation of calcium carbonate is accompanied by releasing carbon dioxide into the surrounding environment, negating the carbon capture effect of photosynthetic microalgae on short geological time scales. This work demonstrated an ecosystem-inspired biotechnology innovation for carbon capture and storage in a high pH, high alkalinity Nannochloropsis culture through development of a microalgae-driven calcium carbonate and biomass production process. When growing the marine microalga Nannochloropsis oceanica IMET1 in photobioreactors, culture alkalinity more than doubled, increasing from 72.5 mg L^-1 to a maximum level of 159.6 mg L^-1. At the same time, culture pH increased from 7.9 to over 10 and maintained at pH 10.1 for 23 days with concomitant calcium carbonate production. X-ray diffraction analysis revealed the precipitated calcium carbonate was primarily monohydrocalcite. The lab culture was scaled up to a 340-L bioreactor, in which Nannochloropsis ash-free dry weight productivities ranged from 25.1 to 51.4 g m^-2 d^-1 and a maximum monohydrocalcite productivity of 133.4 g m² d^-1 was recorded. In this system, Nannochloropsis biomass contained about 23.9% lipids and the eicosapentaenoic acid content was about 1.8%. Together, these results suggest the microalgae-driven calcium carbonate and biomass production process efficiently captures and stores atmospheric carbon dioxide in the form of calcium carbonate while producing valuable bioproducts with applications in the nutraceutical market. Study of the bacterial communities associated with the Nannochloropsis culture identified four dominant species Maricaulis maris, Marinisubtilis pacificus, Gracilimonas sp., and an uncultured bacterium in the OD1 phylum with interesting features that warrant further investigation.