<i>Nannochloropsis oceanica</i> IMET1 and its bacterial symbionts for carbon capture, utilization, and storage: biomass and calcium carbonate production under high pH and high alkalinity

To combat the increasing levels of carbon dioxide (CO₂) released from the combustion of fossil fuels, microalgae have emerged as a promising strategy for biological carbon capture, utilization, and storage. This study used a marine microalgal strain, <i>Nannochloropsis oceanica</i> IMET1, which thrives in high CO₂ concentrations. A high-pH, high-alkalinity culture was designed for CO₂ capture through algal biomass production as well as permanent sequestration through calcium carbonate (CaCO₃) precipitation. This was accomplished by timed pH elevation and the addition of sodium bicarbonate to cultures of <i>N. oceanica</i> grown at lab scale (1 L) and pilot scale (500 L) with 10% and 5% CO₂, respectively. Our data showed that 0.02 M NaHCO₃ promoted algal growth and that sparging cultures with ambient air after 12 days raised pH and created favorable CaCO₃ formation conditions. At the 1 L scale, we reached 1.52 g L^-1 biomass after 12 days and an extra 9.3% CO₂ was captured in the form of CaCO₃ precipitates. At the 500 L pilot scale, an extra 60% CO₂ was captured (Day 40) with a maximum CO₂ capture rate of 63.2 g m^-2 day^-1 (Day 35). Bacterial communities associated with the microalgae were dominated by two novel Patescibacteria. Functional analysis revealed that genes for several plant growth-promotion traits (PGPTs) were enriched within this group. The microalgal-bacterial coculture system offers advantages for enhanced carbon mitigation through biomass production and simultaneous precipitation of recalcitrant CaCO₃ for long-term CO₂ storage.IMPORTANCECapturing carbon dioxide (CO₂) released from fossil fuel combustion is of the utmost importance as the impacts of climate change continue to worsen. Microalgae can remove CO₂ through their natural photosynthetic pathways and are additionally able to convert CO₂ into a stable, recalcitrant form as calcium carbonate (CaCO₃). We demonstrate that microalgae-based carbon capture systems can be greatly improved with high pH and high alkalinity by providing optimal conditions for carbonate precipitation. Our results with the microalga, <i>Nannochloropsis oceanica</i> strain IMET1, show an extra 9.3% CO₂ captured as CaCO₃ at the 1 L scale and an extra 60% CO₂ captured at the 500 L (pilot) scale. Our optimized system provides a novel approach to capture CO₂ through two mechanisms: (i) as organic carbon within microalgal biomass and (ii) as inorganic carbon stored permanently in the form of CaCO_3.