When recently re-applying sunscreen after a day spent mostly in the water, I looked enviously at the sea creatures around me that could hang out in the sun all day and not have to repeatedly apply layers of paste to every exposed surface. While many of these creatures have shells that physically protect them, others that live under the surface, as well as many of the marine plants and types of algae, do not. It made me wonder how all those living things don’t get burned to a crisp in the same way that I would without sunscreen. It also made me wonder how ultraviolet light works under the water. If I were submerged in the water rather than on the shore, would I not get sunburned?

First, it helps to understand what Ultraviolet (UV) light is in order to understand its impacts. Only part of the full spectrum of the sun’s light is visible to the human eye. That’s the rainbow of colors that we typically see. Below that is ultraviolet light, which has shorter wavelengths. These are more intense and can thus cause damage to our skin. One benefit of this type of light, however, is that its intense radiation acts as a disinfectant. So, you can use UV light to sterilize things – something we are all very aware of right now. Given that we would rather protect our skin than sterilize it, we typically put on sunscreen that blocks UV light either chemically or physically. If you look at your skin with a physical sunscreen block on it you will notice that it is a little sparkly – that’s because of tiny particles of metals like zinc that reflect the sun’s light.

That is what happens out of water. But what happens underwater? Water absorbs much of the visible light, particularly those with shorter wavelengths like red, orange and yellow. That’s why the ocean looks blue – the longer blue wavelengths penetrate further into the water. Because UV light has a shorter wavelength, it is also absorbed closer to the surface. That means that most of the UV light in the ocean is found only in the surface layer. That’s also where photosynthesis takes place, so tiny phytoplankton that live there are at risk of being sunburned. That’s potentially a big problem since phytoplankton produce the bulk of the Oxygen in the ocean and, without them, the ecosystem wouldn’t function.

But, phytoplankton, along with other marine creatures, have evolved ways to protect themselves against UV radiation. It works much like sunscreen – either chemical blocks or physical blocks. Some have chemicals in their tissues that help to reflect or scatter UV light. Others have arrangements of layers of cells that act like tiny mirrors, bouncing the radiation right back. One marine alga that does this, and is pretty easy to find, is Irish moss. This is the reddish leafy looking algae that is found in tide pools and also under the surface clinging to the edges of rocks. If you look closely, you can see that it looks slightly purple and iridescent in the light. The tips of its fronds are made up of multiple transparent layers. When the light hits these layers, light is reflected away from its delicate growing tips, protecting it from harmful radiation.

As often is the case, we learn our best lessons by emulating solutions that nature has spent much time evolving. Our sunscreen gives us the chemical or physical capacity to reflect UV light that we humans don’t naturally have. And as for whether staying submerged in the water would protect me from getting burned, I’d have to be pretty deep for that to happen – past the photosynthetic or light zone. Down there, not only would there be no UV light, but also no red, yellow, or orange, so my lips would look blue not just from the cold, but from the only remaining wavelengths that make it down deep. Perhaps instead, I’ll opt for another coat of sunscreen.

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