# Algae using UV light from auroras for photosynthesis

I was reading some questions on Quora and I found this claim:

The polar regions have a very odd reason for them to have so much oxygen production. You see the Polar aurora run day and night in the UV band. They are about 600 watts per square meter day and night. You don’t see it because you don’t see UV. This hits the sea and causes a bloom of life that makes the biggest fish catches and largest sea animals including whales. This is about equal to 1/2 strength noon sun.

One thing I hate about Quora is how people there don't usually back what they say. In this case I could find nothing to back that claim. So I decided to file it as "probably not true" in my mind for the time being, but that would be nice concept for a fictional world. One of the worlds I am working on could actually benefit from such a feature.

Supposing an alternate Earth where emissions coming from the Sun were much more massive or constant. Think Carrington Event, only constant instead of lasting only a few hours. That would broaden the range of latitudes where auroras happen. Could photosynthesizing life in this world, specially marine algae, evolve to take advantage of the constant auroras?

I don't care how much different or alien the ecosystems of the world would be, that's out of scope. I'm just curious about the prospect of life harvesting energy from auroras.

• Earth-based auroras are very very weak sources of light. Just ask the photographers who made those beautiful pictures. As for those 600 watts per square meter of UV radiation . . . all I can say is that polar explorers are not known for their deeply tanned complexions. The point is that the solar wind just doesn't carry that much energy to begin with. (And I cannot see how a reasonably sized plant could couple with the EM field of a Carrington-type event; a trans-continental telegraph line, yes,surely. But a leaf? Maybe those very long leaves of something like Welwitschia?) – AlexP Aug 20 '19 at 20:16
• The absorption by water of UV wavelengths is near 100%. The algae would need a surface exposed to air otherwise the UV energy will be excessively attenuated. – EDL Aug 21 '19 at 17:06

It might be possible.

We've known for around a century (since at least 1933) that ultraviolet light can inhibit photosynthesis and possibly damage photosynthetic mechanisms inside an organism. Phytoplankton in particular have been recent targets of this sort of research, and it's been shown that their photosynthetic systems are negatively impacted by ultraviolet photons - even more so in the case of shorter wavelengths of light (think $$\sim$$300 nm). Presumably, with a strong, continuous source of ultraviolet radiation, typical photosynthetic organisms like the ones we have on Earth would be quickly killed off.

Now, it's possible that in your world, organisms could adapt to solve the problem of ultraviolet light. As one of those two phytoplankton studies found,

As the cells were grown during long-term exposures to solar radiation, the ratios of repair to UV-related damage increased, indicating their acclimation to UV.

Therefore, if the organisms are exposed to low levels of ultraviolet radiation for millions of years, they will presumably develop a significant degree of tolerance. A way you could make this even simpler would be to replace the Sun with a star that's slightly hotter - say, maybe 7,000 or 8,000 Kelvin, which you might see on an F-type star. That star would emit more ultraviolet light (doing about 2.5 to 7.1 times as much damage to DNA as the Sun does), and so any organisms developing on a planet around it would be more likely to adapt to the high-energy photons, even if the aurora is not the main source of light for photosynthesis. Of course, there's still going to be DNA damage, regardless - a strike against using ultraviolet light from an aurora at all.

Let's talk about auroral brightness. The best source I was able to find was the Australian Government Bureau of Meteorology. They note that in some very rare cases, auroras have emitted fluxes of 1000 kilorayleigh (a unit of photon flux); they describe an aurora like this as

Bright as the full moon. Casts shadows. Very rare.

Of course, the Moon is about 400,000 times dimmer than the Sun, so you would presumably need continuous, intense stellar activity to create an aurora that could come within even an order of magnitude of the Sun's apparent brightness.

Could something like that happen? Perhaps. Some Sun-like stars are known to emit superflares, which release large amounts of energy on timescales of minutes to days. They've been observed in cool F stars, like Omicron Aquilae and 5 Serpentis. It's not out of the question that you could have a hot superflare star that would emit enough energy and ultraviolet light to create the scenario you're looking for. The one problem I can find with superflares is that they only tend to happen once every thousand years or so; perhaps your star could get around this by emitting low-energy superflares more frequently.

• Oh well... A reality check is a reality check. The scenario I wanted is better filed along dantasy and scifi then. Nice answer, thank you :) – Renan Aug 20 '19 at 21:16

They are about 600 watts per square meter day and night

That is a frankly terrifying amount of UV. Forget suntans, think germicidal lamps and epidemic malignant melanomas. The relevant wikipedia article suggests that about 3-5% of sunlight hitting the Earth's surface can be classified as UV (ie. wavelength below 400nm), and we get about $$1kW/m^2$$ of solar irradiance at the equator. This means your 600W of UV (presumably they meant "per square metre") is more than 10 times more UV than you might expect to get on the equator, with clear skies, at midday. Those pale nordic people? They'd be incinerated.

Auroras form quite high up in the atmosphere... at least 80km, and often more. Over 80% of the mass of the atmosphere is found below 20km (including almost all of its ozone) and that's a substantial amount of UV attenuation there. To get 600W at the surface, you're going to need kilowatts of UV at aurora height. I couldn't tell you what sort of particle flux you'd need to generate that much UV, but it sounds like it would ablate the atmosphere in relatively short timescales... certainly shorter than the amount of time you might have to wait for life to evolve.

if the organisms are exposed to low levels of ultraviolet radiation for millions of years, they would presumably develop a significant degree of tolerance

I think after a few million years of being exposed to atmosphere stripping germicidal radiation like this, what they will mostly be needing is a tolerance to vacuum.

This hits the sea and causes a bloom of life that makes the biggest fish catches and largest sea animals including whales

You get lots of whales and fish where you find lots of their food. You find a lot of fish food in the arctic due to upwelling of nutrient-rich water feeding vast phytoplankton blooms.

• Nice point about UV attenuation; I hadn't thought of that. That's a major problem for this scenario. I wonder if you could get around it by having a thinner atmosphere at the poles (although I'm not sure quite how you could make that work. . .). – HDE 226868 Aug 21 '19 at 14:14
• @HDE226868 that would probably require a different composition from our own atmosphere. – Renan Aug 21 '19 at 14:20