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

Abstract

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.

Direct air capture: process technology, techno-economic and socio-political challenges

“…18). 335 The buoyant beads float at the air-water interface of an open pond, ensuring that the enzyme is close to the interface. Encapsulation also protects CA from biodegradation and ensures that it is in a recoverable form.…”

Algae as a source of renewable energy: opportunities, challenges, and recent developments

“…[145][146] Current microalgal production systems that deliver a gaseous CO 2 source nd that CO 2 accounts for about 70% of materialinput costs. 144,147 Eustance et al 148 used membrane carbonation (MC) to deliver bubble-free CO 2 via diffusion through the walls of non-porous hollow-ber membranes placed within outdoor raceway ponds. With the productivity of biomass using MC being similar to that when using traditional sparing (by experimental design), the carbon utilization efficiency (CUE) with membrane carbonation was 3-fold higher than for sparging.…”

Geochemical Negative Emissions Technologies: Part I. Review

“…In environments where the dissolution rate of CO 2 is limiting, CAs could be deployed in solution, immobilized on surfaces or within water-permeable beads (Xu et al, 2021), or produced by organisms engineered to secrete or display it (Zhu et al, 2022). CAs could also be used to catalyze the dissolution of calcite, generating alkalinity (Subhas et al, 2017).…”