Although corals may look like plants, they are actually animal structures that host dense populations of algae on their surfaces. Corals provide a host for tiny photosynthetic algae. In return, these algae use light energy to convert carbon dioxide and water into sugars, and enrich corals with the nutrients they need to survive. The external skeleton of coral is an excellent light-scattering material that allows light to reach the algae that inhabit their tissues – even in areas that would ordinarily be shaded from the daylight above. This intricate symbiotic relationship between coral and algae is responsible for maintaining the high diversity of coral reef ecosystems. Whilst coral reefs cover only one per cent of the ocean, they are home to around 25 per cent of all known marine species. These are some of the most productive ecosystems in the world.

Researchers at the University of Cambridge are amongst a team of scientists working to make use of corals’ ability to efficiently collect and use light. They have been looking for ways to effectively mimic these natural strategies whilst overcoming the issue of algal ‘self-shading’, which is currently a limiting factor in this process. Their solution, recently reported in Nature Communications, is to 3D print coral-inspired structures. These structures act as viable habitats for dense populations of microscopic photosynthetic algae, just like natural corals. The 3D printed corals mimic living coral tissue and the underlying skeleton with micron resolution. Within these structures, algae grow 100 times faster than in standard liquid growth medium.
3D printing has become an established technology in recent years, although basic 3D printing techniques can take hours to complete. The Cambridge team has developed an exciting and innovative new 3D printing process using a custom-made 3D bioprinter, which is capable of rapidly reproducing detailed structures in minutes rather than hours. By using a scanning technique similar to ultrasound, natural corals can be imaged to create precise models for the synthetic structures that mimic the intricate and complex designs and functions of living tissues. The team fabricates these synthetic corals from a new ‘bio-ink’ solution. The rapid printing allows immediate encapsulation and survival of algal cells within the structures. In longer processes, these cells would not have survived and the resultant structures would be defunct. The printed coral copies natural coral structures and light-harvesting properties, creating an artificial host-microenvironment for the algae to successfully live within.
3D bioprinting of synthetic corals combines a series of novel techniques – creating a scalable process with many potential applications. This project could have applications in algal biotechnology (including in photobioreactors for the generation of biofuels), coral reef conservation, and in coral-algal symbiosis research, the latter being important for understanding the breakdown of coral-algal symbiosis during coral reef decline. This process could also be used to develop large scale bionic corals for algal growth in dense urban areas or, perhaps, even as life support systems in space.
Greenhouse gas emissions are a serious threat to these creatures and are responsible for coral reef death. Ultimately, 3D printing of synthetic biomaterials could be used to expand research on this topic,  and look for ways of maintaining this vast source of the ocean’s biodiversity.
Featured image: Scanning electron microscope image of the microalgal colonies in the bionic 3D-printed corals. Credit: Daniel Wangpraseurt / University of Cambridge


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