Direct Air Capture of CO₂ by Microalgae with Buoyant Beads Encapsulating Carbonic Anhydrase

Microalgae cultures have promise as a CO₂ sink for atmospheric carbon and as a sustainable source of food and chemical feedstocks. However, large-scale microalgae cultivation is currently limited by the need to provide carbon dioxide from point sources, as the diffusion of atmospheric CO₂ is too slow. Carbonic anhydrase (CA) is an effective enzyme to facilitate the dissolution of atmospheric CO₂ that could be used to enhance the photosynthetic uptake of this greenhouse gas. Here we investigate a means of retaining CA at the surface of algae ponds to facilitate direct air capture by cross-linking CA with glutaraldehyde (GA) before encapsulation into buoyant calcium alginate beads. Coomassie Blue dyeing and Wilbur–Anderson assays confirmed the successful bonding of CA to the beads. Microscopic images showed the paraffin-embedded alginate framework. The CA–GA beads retain virtually all hydrase activity throughout 10 assay cycles. Compared with a natural growth rate of 22.7 ± 0.5 mg L^–1 day^–1, free CA and CA–GA beads increased the productivity of Nannochloropsis salina to 37 ± 3 mg L^–1 day^–1 and 40 ± 1 mg L^–1 day^–1, respectively. The CA–GA beads further provided a stable growth enhancement for three rounds of microalgae cultivation, confirming that these buoyant beads can be readily recovered and re-used, which is promising for industrial biomass production.