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How Life Harvests Light Even in Total Blackness

Should we be hunting a whole new sort of alien life?

by Michael Byrne
Apr 27 2013, 11:27pm
via NOAA

One fun thought is that the Earth gives off its own light. I'd qualify that with "... in a way," but really, while the origin of that light might be wildly different, it's still good ol' fashioned photons radiating through space across a range of frequencies. It's not visible light but, then again, most light in the universe isn't. The Earth's particular light is the result of our planet having a hot core--barely a dim ember compared to the furnace of a star like our sun. You can see it manifested on our planet's surface via volcanic activity or geothermal vents deep undersea. Black light.

Giant tube worms, Riftia pachyptila, live by chemosynthesis

Life congregating around those vents is old news. Many miles below the surface, where the sun might as well exist in another dimension, otherworldly species graze upon mats of bacteria forming what might as well be colonies of aliens. Crucially, this bacteria doesn't survive in the more usual ways of breaking down organic material to generate the energy needed to live (eating stuff, in other words) or converting sunlight into energy (photosynthesis, like plants).

Meanwhile, deep-ocean bacteria rely on chemosynthesis, which is another autotrophic way of living—like photosynthesis, but one that uses inorganic compounds as an energy source rather than sunlight (because, you know, there is none). This is a crucial piece of chemical evolution, the long, weird process by which inorganic stuff on pre-life Earth assembled itself into self-replicating, organic stuff. This process is thought to have begun in those deep-sea geothermal vents, spewing the heat and materials needed for chemosynthesis and, eventually, the sorts of life we're more familiar with.

Those deep-ocean bacteria have one other mechanism, however, and it's just a super-refined version of the the one that makes most of the Earth go 'round: photosynthesis. It's supremely odd, but, despite existing in locations on Earth that are basically other planets in their remoteness and distance from actual visible light, some living things can trade green for black. This is the "new" news, and we'll come back to it in just a moment.

So it's pretty nice that all happened, leaving us here to pontificate on just how inevitable the whole us being here really is. And it gets us thinking how that, if it was inevitable here on Earth, it might as well be inevitable on other planets—or moons, like Jupiter’s Europa--with inhospitable atmospheres but deep-sea geothermal vents and plenty of inorganic junk. Most everything suggests that we need to get underwater (and under-ice) on Europa as soon as possible; right now, that mythologized frozen-on-the-surface sphere is our best chance at finding something alive beyond Earth, even if it’s methane-chomping bacteria.

Deep-sea vent colony/Nicole Rager Fuller, NSF

There’s another life-option, however, and it also has to do with those geothermal vents. While they kick out plenty of chemicals, they also kick out light in very small doses in the form of infrared thermal radiation. It’s not much radiation, but it’s enough to support at least one known form of life, the green sulphur bacterium, which harvests photons (light particles) from a form of deep-sea vent known as a black smoker (which sounds like it should be a Half-Life 2 creature, I know). This discovery came in 2005, and it remains a very big deal for how we understand life in its most basic forms.

Life around abyssal (read: way, way deep) geothermal vents is extremely difficult and expensive to study, placing a frustrating boundary on what we can know about dark photosynthetic bacteria. But a team of researchers at Cuba’s Central University posted a paper on the arVix preprint server last week using a mathematical model to predict the life that might exist in this way. In effect, they've put some important quantitative contstraints on the life that can exist in this way. It’s not quite the same as scraping it off a rock, but a study worth sharing nonetheless.  

The bad news is that the photosynthetic potential is “rather low” for the life we currently know on Earth to survive off this radiation. But it remains possible for photosynthetic creatures to exist in total blackness, as long as they’re very, very efficient, using photons at a wavelength up to 1300 nanometers. Chlorophyll, the molecule used to absorb sunlight by plants and more usual photosynthetic bacteria, can only handle light in the 450 to 650 nm range. That’s a huge leap, of course, but, if nature can find a way … it can probably find another way.

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