The myth of reef-safe sunscreen: how chemicals hurt coral life

These strong, non-biodegradable compounds are hard for wastewater treatment plants to filter. This makes them a significant environmental threat. So, places like Hawaii, Palau, and the US Virgin Islands have banned oxybenzone.

Chemical alternatives in sunscreens, like octinoxate and octocrylene, pose their own risks. When coral reefs bleach, it's not just an aesthetic issue. The real problem lies deep inside their cells. New research shows that these filters harm the microalgae corals need to survive.

The solar-powered tenant and the landlord

Coral reefs are in trouble because of their partnerships. To help them thrive, we must understand these connections. There are photosynthetic microalgae called Symbiodinium that live inside corals.

In tropical waters with few nutrients, these algae use sunlight for energy. This process provides corals with up to 90 per cent of their energy. It also helps create large calcium carbonate reefs.

When pollutants harm the algae, the algae can’t make energy. Then, the coral starves, turns white, or dies. This leaves many marine species without a home. We see the bleaching, but we also need to catch the early signs of cellular stress. If we don't, we'll lose the reef altogether.

A screening tool for marine microbes

To tackle this problem, marine ecotoxicologists are going beyond observing biological responses. They are using lab studies and a technique called flow cytometry.

This technique allows researchers to send thousands of cells through a laser beam. The researchers can count the cells and check their health in milliseconds. The data show that even tiny amounts of octinoxate and octocrylene can be harmful. Octinoxate is more toxic than octocrylene. Both harm algae, but in different ways.

Two chemicals, two triggers of cellular damage

Scientists have found that these compounds hurt cells in two distinct ways.

• Octinoxate is toxic, even in small doses. It disrupts algal metabolism, causing oxidative stress – like 'cellular rusting.' It damages the cell's energy centers and photosynthetic apparatus. The algae have to divert energy from growth to survival. This can lead to a decrease in their population.

• Octocrylene alters cellular membranes. At high doses, it disrupts the cell's electrical system. This disruption causes severe damage to lipids. Flow cytometry shows that stressed algae make more cellulose. This helps them create protective barriers against toxins. This process changes their shape before they die.

Shifting the chemical burden

These findings have significant implications for policy. Current environmental monitoring often overlooks early signs of damage. It focuses on coral bleaching after it has occurred.

Banning oxybenzone might lead to the use of other harmful alternatives. To address this, regulators can use flow cytometry as an early-warning system. This system monitors marine microalgae and helps protect coral reefs from permanent damage.

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