Explore scaled-up 3D models created from CT scans of genuine coral skeletons, then use UV light to reveal fluorescent dots representing microplastic pollution. Discover how researchers use light-based imaging to find tiny plastic particles hidden inside coral tissue and skeletons without adding dyes or damaging the sample.
Have a go
Explore a coral reef built from real science. The model corals are scaled-up 3D prints created from CT scans of genuine coral skeletons, so you can see structures normally too small or too fine to make out with the naked eye. Hidden within them are fluorescent dots standing in for something much less visible in nature: microplastic pollution.
Shine a UV light on the models and the ‘microplastics’ reveal themselves instantly. This represents something researchers are working on for real: finding genuine microplastic particles hidden inside coral tissue and skeletons using light instead of chemicals or dyes.
The activity explains how that detective work is done, what coral is and how it grows, and why plastic pollution has become such a difficult problem to track in the ocean.
What's going on
Finding the invisible: label-free imaging
Microplastics are often tiny, colourless and irregularly shaped, which makes them extremely difficult to spot inside biological tissue, even under a microscope. Traditionally, scientists stain or dye a sample to make plastic particles show up. However, this can damage or destroy the sample, and dyes do not always stick reliably to plastic.
This is where label-free imaging comes in. Instead of adding a dye, stain or chemical marker, these techniques use laser light to interact directly with the molecules already present in a sample. One key method is Coherent Anti-Stokes Raman Scattering (CARS) microscopy, often paired with a complementary technique called two-photon excited fluorescence (TPEF).
CARS works by tuning a laser to the exact vibrational ‘fingerprint’ of a specific chemical bond. This can target common plastics such as polyethylene or polystyrene, so that only those structures light up in the resulting image while the surrounding biological tissue stays comparatively dim.
The result is a detailed, three-dimensional chemical map showing precisely where microplastic particles sit within a sample, whether suspended in soft coral tissue or locked inside the hard skeleton beneath it. This can be done without cutting, staining or damaging the coral.
The biology of coral: animal, architect and ecosystem
Corals are one of the ocean’s great surprises. Despite looking like plants or even rocks, they are animals. A coral colony is made up of thousands of tiny individual animals called polyps, which are genetically identical and connected by a shared layer of living tissue across the colony.
Most reef-building corals get their vivid colour from a partnership with microscopic algae called zooxanthellae, which live inside coral tissue. The algae photosynthesise, sharing nutrients with their coral host in exchange for shelter and access to sunlight. This relationship gives healthy reefs their colour. When corals experience stress, most commonly from unusually warm water, they can expel the algae, leaving the coral’s naturally white skeleton showing through. This is known as coral bleaching.
Beneath the living tissue lies the coral’s skeleton, built from aragonite, a crystal form of calcium carbonate. The coral forms it by extracting calcium and carbonate ions directly from seawater, a process known as biomineralisation.
Corals grow their skeletons in two ways at once: by extending outward or upward to increase the colony’s overall size, and by thickening existing structures to increase their density over time. The balance between these processes, and the pattern in which each species deposits its skeleton, produces the huge diversity of coral shapes found on a reef.
Microplastic pollution in the ocean
Plastic pollution is one of the defining environmental challenges of our time, and a significant portion enters the ocean as microplastics: plastic fragments smaller than 5mm.
These particles come from two main sources. Primary microplastics are manufactured at a small size from the start, including raw pellets or “nurdles” used in plastic manufacturing and synthetic fibres shed during washing. Secondary microplastics are created when larger plastic waste slowly breaks down under sunlight, waves and abrasion.
Estimates of exactly how much plastic enters the ocean each year vary, but commonly cited figures are in the range of 8 to 14 million tonnes annually. Current research suggests scientists can locate only around 1% of the plastic believed to have entered the ocean, with the rest dispersed as particles that are too small, too deep or too well hidden to detect easily.
Coral reefs are directly affected by this pollution. Corals can ingest microplastic particles, mistaking them for food. Particles can also become physically trapped within living tissue and gradually overgrown as the coral skeleton continues to develop around them.
Understanding how and where these particles accumulate is an important step in assessing the long-term impact of plastic pollution on reef ecosystems. Label-free imaging techniques are helping researchers uncover this hidden pollution.