We should carefully protect our skin from harmful UV radiation, especially in summer – we know that. Sunscreens often contain substances that may be detrimental to sensitive ecosystems – we know that too. Octocrylene has not been banned from established products and regions such as Hawaii for nothing. However, up until now, we did not know exactly why coral reefs are damaged by these substances. New insights into the underlying mechanism were published last month: they show what happens to the similarly popular UV blocker oxybenzone when it contaminates certain species of corals and their neighbours.
Cancer prevention, sure – but what about the corals?
Coral reefs around the world are exposed to a variety of damaging influences. A constant rise in water temperature, ocean acidification, and different forms of pollution are the main triggers for the gradual destruction of these ecosystems. While protecting their skin from dangerous UV radiation by vigorously applying sunscreen, beach and ocean enthusiasts unknowingly contribute to the water’s contamination with substances that may be fatal to the biodiversity of coral reefs. In coastal areas that are tourist favourites, the population is in steady decline – and younger specimens barely colonise these reefs.
Along with other substances that are popular ingredients of sunscreen, oxybenzone is often blamed for this. Studies show that where there are particularly high concentrations of this UV blocker in the sea water, the originally diverse coral reefs are only sparsely populated. Little by little, authorities begin to take action: Just like the more widely known octocrylene, oxybenzone has been banned from sunscreens used in several coastal regions. However, the underlying mechanism that transforms the oxybenzone from a harmless and protecting substance for humans into a problematic danger for coral reefs was still largely unclear. This posed an additional difficulty when it came to the development of less harmful alternatives.
Teamwork under water: algae as coral protectors
At the University of Stanford, a team of scientists has been taking a closer look at oxybenzone, and they were now able to bring to light the details of what happens to the substance. To this end, they considered the damaging effects of oxybenzone using the example of Aiptasia sea anemones. Their report now shows that the dimensions of coral damage are heavily influenced by algae, as they live in symbiosis with anemones and corals.
Stony corals and related species are often dependent on a close relationship with single-celled organisms. These algae tend to accumulate within the corals and are essential to their energy supply since they are able to perform photosynthesis. This symbiosis is beneficial to both parties – including in case of water pollution by oxybenzone, as was shown now. Aiptasia specimens were much better at dealing with the detrimental substance when they were populated by algae. Those specimens that had been deprived of symbionts, on the other hand, died within a short time. When exposed to environmental stressors, corals tend to expel the algae that might be useful against oxybenzone contamination. The result: coral bleaching, which has increasingly been observed at natural coral reefs in the past decades.
Critical combination: when oxybenzone and sunlight clash
Apparently, it is not the oxybenzone by itself that is harmful: sea anemones that were treated with the substance without simultaneous exposure to UV radiation showed negligible signs of damage. The same is true for specimens that received a UV treatment with oxybenzone being absent. It is only the combination of both factors that turns out to be fatal to the animals – within 17 days and at a rate of 100 percent.
The anemones metabolize the oxybenzone: they attach glucoside to the molecules and thus produce a conjugate that is phototoxic to themselves. This means that these products become toxic to anemones and corals when exposed to UV radiation. However, the animals are protected by algae if these are present – they absorb some of the glucoside conjugates and thus protect their hosts. In those cases where anemones are not in symbiosis with algae, for example when a coral reef is already damaged by an increase in coral bleaching, the animals are exposed to these new phototoxic products without any protection.
It is therefore especially important that we not only make sure to use sunscreens without substances such as oxybenzone, but also that we make it a priority to reduce environmental stressors that further damage coral reefs.