Researchers at the Scripps Institution of Oceanography and Jacobs School of Engineering at the University of California, San Diego and their colleagues have designed 3D-printed, coral-inspired structures capable of growing dense populations of microscopic algae.

The National Science Foundation-funded work, reported in an open-access paper in the journal Nature Communications, could lead to compact, more efficient bioreactors for producing algae-based biofuels. It could also help researchers better understand the intricate biology of the coral-algae relationship and develop new techniques to repair and restore coral reefs.

Scientists have found that coral-inspired biomaterials could lead to efficient biofuel production. Left. 3D-printed intermediate skeleton design. Right. Final bionic skeleton doped with CNC aggregates. Credit: SIO

In laboratory settings, the printed coral structures were used as platforms to grow two species of microalgae, which rapidly increased to 100 times greater density than in liquid cultures. The species were Marinichlorella kaistiae, which has commercial potential, and Symbiodinium sp., the algae that lives in association with corals in the wild.

Corals are one of the most efficient organisms at using, capturing and converting light to generate energy. And they do so in extreme environments, where light is fluctuating and there’s limited space to grow. Our goal here was to use corals as inspiration to develop more productive techniques for growing microalgae as a form of sustainable energy.

—lead author Daniel Wangpraseurt

To build the coral structures, Wangpraseurt teamed up with UC San Diego nanoengineer Shaochen Chen, who specializes in rapid 3D bioprinting technology capable of reproducing detailed structures that mimic the complex designs and functions of living tissues. The method can print structures with micrometer-scale resolution in just minutes.

Wangpraseurt also worked with researchers at Scripps to measure the photosynthetic activity of the corals in both liquid cultures and in coral models. The scientists looked at how the algae grew on the structures, and at ways to maximize the growth on the “mimic” corals.

The 3D printed corals are built to capture and scatter light more efficiently than natural corals. They consist of cup-shaped, artificial skeletons that support coral-like tissue.

Three-dimensional printing enables complex, multi-material structures that can be designed for specific functions. Coral structures are 3D-bioprinted to grow algae, which serve as biofuels, an alternate energy source.

Our bionic coral increases the photon residence time as light travels through the algal culture. An increase in photon residence time (or photon pathlength) increases the probability of light absorption for a given density of microalgae. As photons travel through the bionic coral, diffuse scattering by the bionic skeleton leads to an increasingly diffuse light field, effectively increasing the scalar irradiance (i.e., the integral of the radiance distribution from all directions around a point) as a function of tissue depth. This photon augmentation strategy leads to a steady increase of scalar irradiance with tissue depth, which counterbalances the exponential light attenuation by photopigment absorption.

Compared to a flat slab of biopolymer (GelMA) with the same microalgal density (5.0 × 106 cells mL⁻¹), the scalar irradiance (for 600 nm light) measured in the photosynthetic layer of the bionic coral is more than 1.5-fold enhanced at 750 µm depth due to the optimized scattering properties of the bionic coral tissue and skeleton. The bionic coral thus mimics the photon pathlength enhancement strategy that natural corals use to avoid algal self-shading.

—Wangpraseurt et al.

Resources

• Wangpraseurt, D., You, S., Azam, F. et al. (2020) “Bionic 3D printed corals.” Nat Commun 11, 1748 doi: 10.1038/s41467-020-15486-4