ASU tests air-captured carbon in cyanobacteria pond, with durability hurdles ahead

Arizona State University reported on September 21 that researchers used a rotating belt to move carbon dioxide captured from ambient air into water used to grow cyanobacteria. The tests included an outdoor pond in Mesa, Arizona. They show a possible way to supply carbon to a growing culture, but the university calls the system a proof of concept: it has not demonstrated commercial production or permanent removal of the captured carbon.

The distinction matters because cyanobacteria and algae can turn carbon into useful material, but feeding them captured CO₂ is a different outcome from storing that CO₂ for the long term. The researchers observed growth using air-captured carbon; they also found that the capture material lost performance during cultivation and outdoor use. Those findings define both the promise of the pilot and the work still required.

How the pond system works

ASU describes a polymer that takes up CO₂ when dry and releases it when wet. Packets of the material ride a belt that moves through the air and then through the liquid growth medium, repeating the cycle. That arrangement brings carbon into the pond without relying on the conventional method of bubbling concentrated CO₂ through water, where some gas can escape before organisms use it.

In the university's account, the team tested the approach in small flasks, a 12-liter bench system and an 840-liter outdoor raceway pond. The outdoor installation occupied 4.2 square meters at ASU's Arizona Center for Algae Technology and Innovation in Mesa. Researchers reported that cyanobacteria grew using carbon released by the capture material, although those trials do not establish a sustained commercial production rate.

What the material trials exposed

Keeping the capture material effective proved difficult. The preprint reports that substances released by the cyanobacteria coated the resin beads and reduced their CO₂ capture capacity to 25% of its earlier level. Washing restored capacity to 70%, but carbon delivery remained three to four times slower. A cleaning step, therefore, did not return the tested material to its previous performance.

The researchers also examined material used in outdoor cultivation trials spanning more than 300 days of wetting and drying over four seasons. They found substantial bead fracturing and a loss of chemical groups needed for CO₂ capture. ASU says the material must dry faster, resist biological fouling, last longer outdoors and become cheaper. Wim Vermaas, an ASU researcher involved in the work, described the pilot as a proof of concept and said substantial development remains.

The companion system study examined changes to the belt's material containers. Its published abstract says elongated mesh tubes shortened drying and CO₂ reloading time by a factor of 4.3 compared with larger mesh bags. That improvement addresses one operating bottleneck; it does not resolve the durability and fouling observed in the cultivation trials.

Costs are estimates, not operating results

A model in the companion study estimates a cost of $229 per tonne of CO₂ delivered into alkaline solution in a practical scenario based on current results. An aspirational scenario, which assumes better capture material and a longer working life, gives $72 per tonne. Neither figure is an observed cost from a commercial plant, and the lower figure depends on improvements the researchers have yet to demonstrate at that scale.

The same model estimates an additional $110 per tonne to extract, purify and compress captured CO₂ for sequestration. The pond pilot did not carry out those steps. The study also describes a potential energy reduction of up to 87% against thermal or vacuum-swing capture, based on using energy from water evaporation. That comparison is a modeled possibility, not a measurement of the pond system's overall climate benefit.

Carbon use is not permanent removal

ASU points to fuels, chemicals, proteins and pigments as possible products from cultivated algae or cyanobacteria. The pilot did not establish commercial production of those products. It also did not show what would happen to the carbon after cultivation, or demonstrate durable storage. A separate preprint assessment of a possible biofuel refinery assumes future engineering can keep the material performing while avoiding biological fouling; it describes a scenario rather than a facility already operating.

Project Drawdown's broader assessment says direct air capture needs low-carbon energy and durable storage to deliver net removal. It judges the technology expensive and energy-intensive and does not recommend broad deployment as a climate solution. That is Drawdown's view of direct air capture in general, not an assessment or independent replication of ASU's specific pond experiment.

For the ASU team, the immediate result is narrower: air-captured CO₂ reached cultivation medium and supported growth in tests, including work beyond the laboratory. The next engineering questions concern how long the resin can keep working, how to prevent fouling and whether improved equipment can deliver carbon reliably at a cost that makes sense. Whether this route can produce useful material at scale, much less remove atmospheric carbon permanently, remains unproven.