Filming coral bleaching at an unprecedented level

Few images capture the impact of climate change quite like a coral reef turned ghostly white. Yet remarkably, it was only in 1998, less than 30 years ago, that the first global mass bleaching event was documented. In the decades since, we've witnessed three more global-scale events, and the sight of once-vibrant reefs losing their colour has become all too common.

But what is actually happening to the coral?

Many recognise what a bleached reef looks like, but the processes taking place inside these tiny animals is far less familiar.

So, for Blue Planet III, we get closer, showing the process of coral bleaching in unprecedented detail.

And that means – going macro.

Macro photography is a filming technique whereby very small subjects are captured and ultimately made to feel much larger than they are in real life.

This technique enables the viewer to feel like they are truly in the world of the miniscule creatures on their screen. If you've ever watched a nature documentary of an ant scurrying along a jungle floor, or a bee up close, buzzing between brightly coloured flowers, then you'll know exactly what macro filming can do and how it can make you feel.

A perfect technique for filming tiny insects, but also for uncovering the secret world of coral polyps and their microscopic algae - zooxanthellae.

Corals are made up of thousands of individual animals called coral polyps which look like mini sea anemones. Most coral polyps are no bigger than a pin head, but even smaller are the symbiotic algae, the zooxanthellae, that live inside them.

Individual zooxanthellae cells are invisible to the naked eye - a typical individual cell measures between 5 and 15 micrometres in diameter. For context, a single human hair is about 70 micrometres wide.

Zooxanthellae may be small, but they are mighty, providing the coral with up to 90% of the energy it needs to stay alive. They do this by producing energy from sunlight and sharing much of that energy with their coral host.

It wouldn't be a stretch to say they are incredibly important little algae – even providing the coral with much of their colour!

But, this partnership is sensitive to something that's becoming ever more problematic…

Heat.

For millions of years, corals and their algae have thrived together in warm tropical seas, finely adapted to a narrow range of temperatures. But as ocean temperatures rise, this remarkable partnership is breaking down.

When waters are too hot for too long, zooxanthellae can turn toxic - producing harmful chemicals that would otherwise damage the coral. So the coral expels them into the surrounding water, turning the coral white. This is what happens across the world's reefs during mass bleaching events, of which the 2023 to 2025 event was the biggest on record, impacting the health of 84% of the world's coral reefs.

Our goal was to film exactly this – the expelling of algae – but in unprecedented detail. To do this required working with corals at a microscopic level, something that can only be done in highly controlled conditions with specialist cameras and lenses. Additionally, corals are protected, and rightly so. So in order to film this, Blue Planet III enlisted the help of scientists from the Australian Institute of Marine Science (AIMS), who keep live corals in their specialist laboratory. It was only with their expertise, specialised tanks and live corals could we ever hope to succeed.

By showcasing coral bleaching at an unprecedented level of detail, we set out to reveal the extraordinary complexity of a phenomenon that can otherwise seem deceptively simple. To show not just what bleaching looks like, but what is happening at the smallest scale and what that means for individual corals, for entire reefs, and for the communities that depend on them.

Because helping people understand the natural world is often the first step towards inspiring them to care.

And when we care, we protect.