Dual carbon sequestration with photosynthetic living materials

[Geoengineering News]

https://www.nature.com/articles/s41467-025-58761-y

Authors

Dalia Dranseike, Yifan Cui, Andrea S. Ling, Felix Donat, Stéphane Bernhard, Margherita Bernero, Akhil Areeckal, Marco Lazic, Xiao-Hua Qin, John S. Oakey, Benjamin Dillenburger, André R. Studart & Mark W. Tibbitt 

23 April 2025

Abstract

Natural ecosystems efficiently sequester CO2 but containing and controlling living systems remains challenging. Here, we engineer a photosynthetic living material for dual CO2 sequestration that leverages biomass production and insoluble carbonate formation via microbially induced carbonate precipitation (MICP). To achieve this, we immobilize photosynthetic microorganisms within a printable polymeric network. Digital design and fabrication of the living structures ensure sufficient light access and nutrient supply to encapsulated cyanobacteria, enabling long-term culture for over a year. We showcase that photosynthetic living materials are able to sequester 2.2 ± 0.9 mg of CO2 per gram of hydrogel material over 30 days and 26 ± 7 mg of CO2 over 400 days. These findings highlight the potential of photosynthetic living materials for scalable, low-maintenance carbon sequestration with applications in carbon-neutral infrastructure and CO2 mitigation.

Source: Nature Communications

[Greg Rau]

(Again) I don’t follow how bio carbonate precipitation in (simulated) seawater qualifies as CO2 sequestration. At scale the scheme would proceed as Ca(HCO3)2aq  CaCO3 + CO2 + H2O, ie it’s a CO2 and acidity generator consuming already well-sequester alkaline SW bicarbonate and releasing CO2*. Instead, the authors experimentally fed the bugs CaCl2 – care to guess what the  CO2 emissions footprint of this material is? Ditto for the hydrogel substrates used? The authors claim that the conditions simulate what would happen in seawater, yet the  DIC and TA were roughly 1/3 to 1/2 of those of seawater, while at an acidity more than 30X that of SW (pH = 6.5).   As far as I can tell the CO2 sequestration achieved by biomass sequestration isn’t even reported.  How has “dual carbon sequestration” been demonstrated here? What am I missing?

*https://onlinelibrary.wiley.com/doi/10.1111/raq.12954

https://iopscience.iop.org/article/10.1088/1748-9326/ad502f

Greg

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[Greg Rau]

Mark,

Thanks for getting back. I don't disagree that you can sequester CO2 as CaCO3 in a purely synthetic medium when phytoplankton draw down CO2, elevate pH and CO3-- and thus supersaturate the medium with CaCO3. The problem here is that you are consuming alkalinity and Ca++ and thus you need a source of these to keep going. If you are using a synthetic source of alkalinity/Ca (not SW Ca(HCO3)2aq), you have to consider the CO2 footprint of such sources not to mention the $$ if this is to be relevant scale. Glad you are aware of these concerns.

Very unfortunate that a reviewer insisted you describe your media as "simulated seawater" given the large chemical differences and the false hope that dual C sequestration might be possible with SW. Without this SW option I don't see how this could scale, but good luck in trying.

Regards,

Greg

On Mon, Apr 28, 2025 at 6:37 AM Tibbitt Mark <mtib...@ethz.ch> wrote:

Dear Greg,

Thank you for your interest in our work and for the questions.

Regarding the bicarbonate precipitation and its ability to serve as a means of CO2 sequestration: as demonstrated in this work we envision the materials to be used in more controlled settings than a full marine environment. In such settings like the closed container experiments (Figure S30), the DIC in the medium (simulated seawater) is in direct equilibrium with atmospheric CO2. In this closed environment, we did not further supplement bicarbonate ions in the growth medium. Therefore, atmospheric CO2 dissolves in the medium to form HCO3-. In this context and based on the citation you list, 0.6 mol of CO2 is released when 2 mols of atmospheric CO2 are dissolved as bicarbonate ions and sequestered as carbonate minerals. Thus, the system should still serve as a net carbon sink. This can be seen experimentally in Figure S30 where during the day cycles (lights on) the system pulled carbon out of the ‘atmosphere’ of the closed containers. Further, the reason for using this strain of Cyanobacteria (or other similar species) is their natural ability to use carbonic anhydrase efficiently to hydrate CO2 making atmospheric carbon sequestration more favorable. That said, we understand that in a full marine environment with potential mineral leaching this balance could be shifted and would certainly need to be considered in any real-world implementation.

Regarding the DIC, TA, and pH: we used BG11-ASNIII as the medium for the Cyanobacteria in this work, which is an artificial seawater solution, as this is a favorable growth medium for the species PCC 7002 that is commonly used in the literature. Again, we in no way intended to replicate a full marine environment. The use of the term ’simulated seawater’ was to appease one of the Reviewers.

Regarding the dual carbon sequestration: we hope that the data included in the manuscript are convincing that the Cyanobacteria grow and proliferate over time. We had originally quantified the extent of CO2 sequestration via biomass, and the value was roughly 1/10th of what was observed to have been sequestered via mineralization. However, one of the Reviewers requested that this quantification be removed as it was based on an indirect measurement. Thus, all quantitive numbers in the manuscript come from the mineral phase alone, but the dual nature is still present given the growth of the biomass in the system.

Regarding the use of CaCl2 and the hydrogel substrate: we are fully aware that these come with substantial CO2 emission footprints. We also use LED-based lights for the culture and an incubator, which also incur CO2 footprints. This was an initial lab-based demonstration of a concept. On-going work is exploring a full life cycle analysis of the process to identify known and potentially unknown pinch points in the system as well as to identify how we might be able to implement this or a similar concept in real-world applications, noting that the source of the minerals in the medium and the hydrogel substrate are important considerations.

We thank you again for your interest and time and hope that this addresses your main questions.

Kind regards,

Mark

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Greg H. Rau, Ph.D.

Senior Research Scientist
Institute of Marine Sciences
Univer. California, Santa Cruz
https://www.researchgate.net/profile/Greg_Rau
Co-founder and manager, the Carbon Dioxide Removal Google group
Co-founder and Senior Scientist, Planetary Technologies, Inc.
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