Nannochloropsis oceanica 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, Nannochloropsis oceanica 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 N. oceanica 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, Nannochloropsis oceanica 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 CaCO₃.