That unraveling is now well documented. More than 60 percent of the world’s reefs are already degraded by local pressures such as overfishing, coastal development and watershed pollution, and climate change has become the single greatest threat, compounding every one of those stressors. Approximately half of the planet’s tropical coral reefs have been lost in the past three decades, and the trajectory is expected to continue. According to the Intergovernmental Panel on Climate Change, if global temperatures rise 1.5 degrees Celsius above preindustrial levels, 70 to 90 percent of modern reefs may vanish; beyond 2 degrees, more than 99 percent would be destroyed. Because the pace of decline now overwhelms the natural regenerative capacity of reefs, scientists and practitioners have turned to active restoration, deploying coral transplantation, algae removal, larval propagation, acoustic enrichment and, increasingly, artificial reefs.

Artificial reefs are structures deliberately submerged on the seabed to mimic the characteristics of natural reef habitat, allowing benthic species to colonize and thrive. When correctly implemented, particularly in sediment-affected environments, they have demonstrably enhanced local marine resources. They can serve as recruitment surfaces for coral larvae, as culture substrates for colonies raised in nurseries, and, once established colonies reach sexual maturity, as intermediate platforms that seed surrounding areas with dispersing larvae. But a new mini-review published in Blue Biotechnology by Baptiste Ozanam, Pascal Romans and Raphaël Lami of Sorbonne Université and CNRS argues that the fate of this entire restoration strategy hinges on an unglamorous question: what, exactly, should an artificial reef be made of?

The answer is constrained by economics as much as ecology. The median cost of deploying artificial reefs is 3,341,754 US dollars, and restoration budgets are chronically stretched, so materials must be affordable as well as biocompatible, durable and non-polluting. International guidelines, including the OSPAR convention, require that reefs be built from inert materials capable of withstanding harsh marine conditions without releasing contaminants through biological, physical or chemical alteration. Judged against those criteria, the four workhorse materials of modern artificial reefs, concrete, plastics, metals and wood, each fall short in revealing ways.

Concrete remains the most popular choice because it is strong, moldable into any shape, and resembles natural rock, and its surface can be tuned by adding coarse sand to adjust roughness. Yet freshly immersed cement can exhibit a surface pH of 10 to 11, requiring three to twelve months of seawater aging before it approaches ambient levels. During that window, pH-tolerant organisms such as barnacles colonize the structure and can block coral settlement entirely. There is also a deeper paradox: cement production accounts for roughly six percent of global carbon emissions, meaning the dominant restoration material is manufactured by an industry that directly fuels the climate change threatening the very reefs being restored.

Recycled materials carry their own liabilities. Old tires are cheap and readily colonized, but they are unstable, easily shifted by currents in ways that can damage surrounding reefs, and they leach toxic compounds including zinc, copper, formaldehyde and acetone products that suppress marine life; large-scale tire deployment is no longer considered viable. PVC structures persist in modern projects, most famously in the Coral Tree Nursery developed by the Coral Restoration Foundation, a lightweight system of PVC columns and arms anchored with steel rebar that rears corals in situ before transplantation. It is portable, inexpensive and gentle on the seafloor, but its plastic components degrade in harsh marine conditions and can shed microplastics. Those particles are phagostimulants that corals mistake for prey, consuming energy that should nourish them, clogging digestive systems, killing polyps, and raising disease risk dramatically: one landmark study found that contact with plastic debris increases the likelihood of coral disease from 4 percent to 89.1 percent.

Metal offers light weight and the ability to form complex shapes at low cost. Mars Incorporated deployed hexagonal modular ‘spider’ structures made of rebar, coated in coarse beach sand, on degraded reefs in Indonesia at roughly 15 US dollars apiece, providing substrate suitable for coral recruitment and rapid recovery. But oxidation products released from corroding metal feed algal growth that competes with corals, and comparative studies have recorded lower coral colonization on metal plates than on concrete, partly because thin plates disintegrated within 17 months, too quickly for corals to establish. Repurposed decommissioned ships promise large colonizable areas and dive-tourism value, but they can carry PCBs, petroleum products, lead, mercury, zinc, tributyltin, radioactive control dials and asbestos, and decontaminating them inflates budgets substantially. Wood, meanwhile, is cheap, nontoxic and carbon-neutral, and experiments show it hosts the greatest fouling biomass of any common substrate; tree-reef structures built from felled pear trees became biodiversity hotspots within six months. Yet wood degrades in as little as 18 months, chemical treatments that extend its life leach copper, arsenic and chrome, buoyant timber must be ballasted to stay on the seabed, and no wood reef has yet been used in a dedicated coral restoration project.

The review’s most compelling section catalogs the next generation of substrates now emerging from materials labs. Concrete itself is being reinvented: mixes incorporating crushed recycled oyster shells produce highly porous, texturally complex blocks that attract significantly more larval attachment, while blocks made with fly ash and granitic dust cost 25 to 40 percent less than commercial concrete without significant leaching. Artists such as Jason deCaires Taylor already sculpt with low-carbon, pH-neutral concrete. Miniaturization cuts costs further: concrete tetrapod seeding units designed by Chamberland and colleagues cost just one US dollar each, deployment included. Surface engineering adds another layer of control, since coral larvae settle more frequently on red surfaces, prefer microtopography matching their own body size, and respond to biochemical cues from crustose coralline algae, while non-biocidal fouling-release coatings can suppress algal competitors. Composite inorganic substrates made from lime mortar with quartz-rich sand and strontianite release ions that attract Caribbean coral larvae, and porous ceramics, chemically inert and durable, have achieved high settlement of Pocillopora damicornis larvae with tailored pore sizes.

Two technologies may ultimately redefine the field. Three-dimensional printing can produce complex, biomimetic ceramic structures with heterogeneous features that conventional manufacturing cannot achieve, while reducing costs and environmental impacts; in the Red Sea, 3D-printed terracotta reefs monitored for three years accumulated dozens of coral colonies alongside 15 species of sponges and ascidians, and the company Archireef now prints clay reef tiles commercially. Low-voltage mineral deposition, which grows the material known as Biorock through electrochemical accretion, yields structures stronger than concrete on which corals grow up to 3.17 times faster than controls. Biodegradable biosourced reefs, built from potato-waste-derived Solanyl and coir ropes that mimic mussel byssal threads, have already facilitated bivalve reef formation in the Netherlands and Florida and satisfy OSPAR’s requirement that reefs be removable, dissolving gradually as corals take over. The authors caution, however, that none of these innovations is a silver bullet: their costs must be rigorously evaluated against the accessibility constraints of restoration programs, larger-scale deployments are needed to prove scalability, and artificial reefs can only support recovery on limited scales so long as eutrophication, pollution, overfishing and rising global emissions continue unabated. The substrate, in other words, is necessary but not sufficient, and the future of coral restoration will be decided as much by what humanity stops doing to reefs as by what it sinks beneath them.

Subject of Research: Material selection for artificial reefs used in coral reef restoration

Article Title: Choosing the right substrate to restore coral reefs through artificial reef construction: a mini-review

Article References: Choosing the right substrate to restore coral reefs through artificial reef construction: a mini-review. (n.d.). https://doi.org/10.1186/s44315-025-00047-5

Image Credits: AI Generated

DOI: 10.1186/s44315-025-00047-5

Keywords: coral reefs, artificial reefs, reef restoration, substrate materials, 3D printing, Biorock, microplastics, concrete, biodegradable materials, marine conservation, coral larvae settlement, climate change