There are two types of sunscreen - chemical and physical (also called mineral). Physical sunscreen contains the active ingredients titanium dioxide and/or zinc while chemical sunscreen uses oxybenzone, octinoxate, homosalate, or octocrylene.
However, not all sunscreens are created equal. The active ingredients in chemical sunscreens are known to harm coral reefs. Here's why - when these ingredients enter the ocean via humans they damage corals' ability to defend against bleaching, harm their DNA, and cause developmental issues.
When applied to the skin, chemical sunscreens effectively absorb UV rays and convert them into harmless thermal energy. Unfortunately corals are not affected the same way. Oxybenzone and octinoxate in particular increase the organisms’ vulnerability to UV rays that cause mutations in their DNA, having adverse effects. On the other hand, physical compounds like zinc work by acting as a shield to deflect the same rays. They do not have any known effects on corals or other aquatic species and are just as successful in protecting humans from the sun.
The issue of coral bleaching arises when chemical sunscreens are combined with other problems like ocean acidification and rising sea temperatures. Bleaching occurs when corals no longer host the photosynthetic zooxanthellae algae that give them their beautiful colors. When combined with unfavorable environmental conditions the active ingredients in chemical sunscreens stress the corals, speeding up the bleaching process. The symbiotic relationship between corals and zooxanthellae is necessary for a healthy and biodiverse reef. As corals are the basis of the reef ecosystem, bleaching not only affects them but can actually lead to the death of the entire habitat.
Bottom line: always be sure to check ingredient lists before buying and choose physical sunscreens to protect both coral reefs and yourself!