In the ongoing effort to mitigate the increasing human-caused concentration of CO2 in the atmosphere, innovative approaches are emerging. One such approach involves the development of photosynthetic living materials, which offer potential scalable and low-maintenance solution for dual CO2 sequestration. Unlike industrial methods, which often require extreme and energy-intensive conditions, these biological systems operate under ambient conditions using sunlight and commonly available small molecules.

What are photosynthetic living materials?

These materials represent engineered systems where photosynthetic microorganisms, such as cyanobacteria (specifically the strain Cyanobacterium Synechococcus sp. PCC 7002), immobilized in a printable polymer network, specifically a Pluronic F-127 (F127)-based hydrogel. The F127 hydrogel was chosen for its bioinert nature and processability, allowing easy diffusion of small molecules and good light transmission, which is crucial for the photosynthesis of the encapsulated species.

Dual carbon sequestration: Two mechanisms

The key innovation of these living materials is their ability to dually sequester CO2 through two main mechanisms:

• Biomass accumulation: Photosynthetic microorganisms, such as cyanobacteria, have a CO2 concentration mechanism that can accumulate CO2 in the cell body up to 1000 times above ambient levels. This concentrated carbon is then fixed in the form of biomass generated during cell growth, representing a reversible part of CO2 sequestration.
• Microbially induced carbonate precipitation (MICP): In addition to biomass production, certain cyanobacteria can irreversibly fix CO2 in the form of inorganic carbonate precipitates, such as calcium and magnesium carbonates. This process occurs outside the cells, where negatively charged extracellular polysaccharides on the bacterial membrane, together with a suitable extracellular environment (alkaline pH, presence of divalent cations), facilitate the nucleation and formation of insoluble carbonates. These mineral precipitates not only store the fixed carbon in a more stable form, but also mechanically reinforce living materials. Photosynthetic MICP is attractive because it does not require any additional raw materials and does not produce toxic by-products, unlike other MICP methods such as ureolytic MICP, which produces ammonia.

Manufacturing and structural design

Scientists are using digital design and additive manufacturing approaches such as direct ink writing a light-guided additive manufacturing, for structuring photosynthetic living materials. They design 3D lattice structures with strut sizes between 0.15 mm and 0.70 mm to facilitate gas and nutrient transport and light access in printed structures. For example, coral-inspired structures maximize the volume of viable material per unit surface area, thereby increasing the efficiency of CO2 sequestration. These digitally designed structures can be grown for more than a year, continuously producing chlorophyll and performing dual carbon sequestration.

Results and performance

These photosynthetic living materials have demonstrated significant CO2 sequestration capacity. They have been able to bind 2.2 ± 0.9 mg CO2 per gram of hydrogel material in 30 days and impressive 26 ± 7 mg CO2 in 400 days in the form of carbonate precipitates. This long-term cultivation showed that dual carbon sequestration can proceed for more than 30 days, especially in rationally designed 3D structures.

The mechanical properties of the material also improve over time due to the accumulation of carbonate precipitates. The stiffness and elastic modulus (G') of the biogenic samples increased significantly during the 30-day incubation, while they did not change for the abiotic samples. Compared to other CO2 capture initiatives such as the chemical mineralization of recycled concrete aggregates (6.7 mg CO2 per gram), these photosynthetic living materials with 26 ± 7 mg CO2 per gram of material are competitive and even outperform them.

Potential applications and the future

Photosynthetic living materials have great potential for a wide range of applications, including carbon-neutral infrastructure, CO2 mitigation, uses as surface coatings for ecological building materials or as bioreactors in commercial sequestration facilities. Their simple requirements (sunlight and atmospheric CO2) and easy maintenance allow them to be installed in a variety of environments, from urban to rural areas, for long-term and sustainable CO2 sequestration. Future work includes quantitative evaluation of CO2 sequestration through biomass accumulation and the possibility of genetic modification or selection of microorganisms with higher photosynthetic rates to further increase efficiency. Spring

Glossary of key terms

• Abiotic samples: Polymer matrix samples without encapsulated cyanobacteria, which serve as a control in the experiments.
• Biomass accumulation: The increase in the total mass of organic material produced by living organisms, in this case cyanobacteria, during photosynthesis. It represents a reversible form of CO2 sequestration.
• Alizarin red S staining: Chemical staining used to visualize and quantify the accumulation of calcium, and thus carbonate precipitates, in samples.
• Bioink: A printable mixture of polymers and living microorganisms used in 3D bioprinting to produce living materials.
• Biotic samples: Polymer matrix samples containing encapsulated photosynthetic microorganisms, which are the subject of experimental study.
• Carbon concentrating mechanism (CCM): A mechanism in many photosynthetic microorganisms that actively accumulates CO2 in the cell body up to 1000 times above ambient levels.
• Carbon capture and storage (CCS): Industrial technologies for capturing CO2 from large emission sources and storing it, usually underground.
• Cyanobacteria Synechococcus sp. strain PCC 7002: A specific species of photosynthetic microorganisms used in the study for their dual carbon sequestration ability.
• Double CO2 sequestration: Simultaneous CO2 capture through two different mechanisms: biomass formation and microbially induced carbonate precipitation (MICP).
• Energy-dispersive X-ray spectroscopy (EDS): An analytical technique used to determine the elemental composition of a material, often in conjunction with a scanning electron microscope.
• F127 and F127-BUM (Pluronic F-127 and F127-bis urethane methacrylate): Polymeric compounds forming the basis of the hydrogel matrix. F127-BUM is a functional version that can be photocrosslinked for stability.
• Photosynthetic Living Materials (PLM): Engineered materials that contain encapsulated photosynthetic microorganisms in a polymer network, designed to capture CO2 and produce carbon materials.
• Hydrogel: A three-dimensional polymer network that is capable of absorbing large amounts of water, forming a gel-like structure; used to encapsulate microorganisms.
• Microbially induced carbonate precipitation (MICP): A biological process in which microorganisms facilitate the formation of insoluble carbonate minerals, serving as an irreversible carbon sink.
• Modulus of elasticity (G'): A measure of a material's stiffness or resistance to shear deformation, which indicates the mechanical properties of the material.
• Optical density (OD730nm): A measure of the turbidity or absorbance of a liquid culture at a wavelength of 730 nm, used to monitor the concentration of cyanobacterial cells.
• Pericellular carbonate formation: Formation of carbonate precipitates in close proximity to or around cyanobacterial cells.
• Photocrosslinking: A process in which polymers (e.g. F127-BUM) are crosslinked using light, increasing the mechanical stability of the material.
• Rheology: Study of the flow and deformation of matter used to characterize bioink properties such as shear thinning and elastic recovery.
• Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo): An enzyme key to carbon fixation during photosynthesis, which converts CO2 into organic compounds.
• Carbon sequestration: The process of removing and long-term storing atmospheric carbon (usually CO2) in a carbon reservoir.
• Thermogravimetric analysis (TGA): A technique used to measure changes in the mass of a sample as a function of temperature, which is used to confirm the presence of carbonates.
• Hardness: The amount of energy a material absorbs before breaking, which is an indicator of the material's resistance to cracking.
• Ureolytic MICP: A type of MICP that relies on the breakdown of urea to form carbonates; often associated with ammonia production.
• UV-vis spectroscopy: A technique that measures the absorption of light by a sample in the ultraviolet and visible ranges, which is used to evaluate the light transmission of a hydrogel.
• Volumetric 3D printing: A 3D printing method that creates three-dimensional objects by polymerizing an entire volume area at once using light.
• X-ray diffraction (XRD): An analytical technique used to identify crystalline phases in a material, used to confirm the crystalline structure of carbonates.