Deep sea vents spawn weird, wonderful and unique fauna
Science | Gregory Beatty | October 21, 2021
Bats and spiders have long been popular motifs for Halloween decorations. They’re spooky and weird, so: naturals, right? But Earth harbours animal life far spookier and weirder than that. Two-metre-tall tube worm forests are one example. We didn’t even know about them until 1977! Same with scaly-foot gastropods, hairy hot vent snails, yeti crabs and stalked crinoids.
With Halloween in mind, let’s dive in. Literally, because the ecosystems these animals inhabit are thousands of metres deep on the ocean floor, where the pressure is hundreds of Earth atmospheres and no sunlight penetrates.
Sunlight fuels most life on Earth, both on land and in shallower waters, where plants absorb solar energy and use photosynthesis to grow and reproduce. Herbivores tap into that energy stream and, by generously becoming something’s dinner, pass some on to carnivores. That’s how the ecosystem is sustained.
Deep underwater, photosynthesis isn’t possible. Instead, life survives through chemosynthesis — but only at select hot spots, says Verena Tunnicliffe, Canada Research Chair in Deep Ocean Research at University of Victoria.
“The sea floor is really leaky, it’s not solid rock,” she says. “You can have cold water sinking down, and if it’s at a place where there’s heat underneath the crust, the water starts to heat up.”
Earth’s crust is composed of huge tectonic plates. They’re in constant motion, and volcanic ridges often form when they meet at friction points. These ridges are dotted with hydrothermal vents where magma burbles up.
Temperatures can reach 370 degrees Celsius but the seawater doesn’t boil because of the immense pressure. The water does dissolve out metals and minerals in the surrounding rock, though. Molecular bonds between hydrogen and oxygen break down too, with free hydrogen bonding with sulphur to form hydrogen sulphide.
When an upsurge occurs at a vent, the super-heated fluid hits colder water and the metals, minerals and whatnot precipitate out and form a “chimney”.
Now, there are some definite logistics involved in getting key building blocks of life such as functioning cells and RNA molecules in place — especially on early Earth, with meteors raining down from space, and volcanoes belching deadly toxins into a poisonous atmosphere.
But scientists have done experiments in conditions similar to deep sea vents that show it is possible, and fossil evidence has been found in northern Quebec that suggests that, within a few hundred million years of Earth forming 4.5 billion years ago, chemosynthetic life was emerging.
“The microbes aren’t bacteria, they’re a group called archaea,” says Tunnicliffe. “They were probably the first life on Earth.”
Especially critical for the archaea, says Tunnicliffe, were the dissolved compounds that the vents released — especially hydrogen sulphide, methane and hydrogen.
“If you have energy, and you have some form of carbon, which would be methane in these ancient vents, you can begin to build more complex molecules such as sugar.
“Once you have this milieu of microbes happily surviving on this chemical energy, you’re going to get something that wants to feed on them,” she adds. “So that’s where you get more complex bacteria. But it would’ve taken another three billion years for larger organisms with more complicated cells, which we call eukaryotes, to evolve.”
Today, most life on Earth relies on oxygen as a catalyst to produce energy. But when life was just starting out, there was no free oxygen — it was all bound up in water and other compounds. So where did the oxygen that we rely on to survive come from?
Well, it’s a complicated chemical story. But archaea kickstarted it by producing oxygen as a waste product.
“Oxygen is a poison, even to us, unless we have a protection system against it reacting with everything else going on in our bodies,” says Tunnicliffe. “But about a billion years ago cells evolved that could use oxygen to get energy. It’s a much better way to get energy because oxygen is so reactive. If you can control that reaction, you can start growing like mad and form multi-cellular organisms.”
Most of the animals that evolved at deep sea vents survived by feeding on each other and licking up bacteria and archaea.
But a few, such as tube worms, were more enterprising, says Tunnicliffe.
“Instead of hunting for bacteria, they provided homes on their surfaces or integrated them into their bodies,” says Tunnicliffe. “They have no mouth, no gut, no way of gathering food directly. Instead, they’re sustained by the bacteria. So now we have animals sort of acting as plants, with symbiotic bacteria acting as chloroplasts in plants which capture photons from the Sun.”
As oxygen grew in abundance, and photosynthetic life began to evolve, deep sea vent life remained undisturbed. That changed with the Cambrian explosion when life really took off, says Tunnicliffe.
“We do not have the same arrangement at hydrothermal vents today as 500 million years ago,” she says. “Some organisms came down from shallower water and realized there’s a food source. So many animals at vents today are creatures that, over many millions of years, were able to adapt to the nasty conditions.
“And vents are nasty,” Tunnicliffe continues. “You’ve got heat that’s swirling around like mad, there are toxic metals along with hydrogen sulphide and methane — which are both nasty poisons. So you have to have evolved the protection systems to live there.”
The animals that are relatively recent incursions (clams, barnacles, shrimp, mussels and such) are maybe 35 million years old, says Tunnicliffe. But some creatures are thought to be hangovers from the Mesozoic Era 200 million years ago.
The plate tectonics that produce the vents also help shape their ecosystems, says Tunnicliffe.
“Our whole globe has been reorganized many different times,” she says. “The Juan de Fuca ridge off Canada’s west coast is a product of that process. The fauna there is found only in our area, but its past history connects it to hydrothermal vents near the equator.”
While scientists are busy studying the fascinating life at the vents and speculating on what that might mean for the possibility of alien life in our solar system [see sidebar], resource-hungry countries are eyeing the vents for a different reason — the metals and minerals such as iron, zinc, copper, lead and cobalt at the chimneys.
“A few countries have metal rich deposits in their territorial waters, but there’s also an international agency to sign licenses for the high seas,” says Tunnicliffe. “They are exploration contracts at the moment, mining is not yet fully approved, but it’s getting close.”
Concerned about the catastrophic loss of biodiversity should mining go ahead, scientists have pushed for strong regulations, Tunnicliffe says. “They would protect active hydrothermal vents, so if mining does happen, it will only be in inactive sites. The problem is it’s hard for the miners to find those vent deposits if there isn’t a nice, active signal.”
In 2003, Canada established the Endeavour Hydrothermal Vents Marine Protected Area in the Juan de Fuca ridge system about 250 km southwest of Vancouver Island. Mining — which would target and destroy the chimneys — is not allowed.
“Vent sites along the ridges are the size of an auditorium, or at most a soccer field,” says Tunnicliffe. “So they’re tiny areas, and they foster amazing groups of organisms that are only found at those vents.
“It’s such an iconic system, both for the history it represents on Earth, and how much it’s taught us about the evolution of life and ocean chemistry,” says Tunnicliffe.
“Even if you don’t care about preserving biodiversity there’s something really deep in our origins about this system.”
Life After Mars
Mars has long been regarded as the top candidate for alien life in the solar system. While that question remains unresolved, several icy moons at Jupiter (Europa, Callisto, Ganymede) and Saturn (Titan, Enceladus) are also possibilities.
Seems unlikely, doesn’t it? The moons are impossibly remote from the Sun, and caked in ice like giant snowballs. How could life exist there?
The hydrothermal vent ecosystems on Earth offer a clue, says University of Regina astronomer Samantha Lawler.
“Astrobiologists are pretty sure that one thing you need for life is liquid water. Every type of life on Earth needs that. You need a solvent of some sort that can dissolve nutrients and help things move around inside cells.
“Methanol could maybe work, and there are other possibilities,” she adds. “But some are only liquid at colder or hotter temperatures, so that limits the reactions they can have. With water’s temperature range, reactions can happen at a good speed. Then you need carbon and other heavier elements.”
Scientists know from Galileo, Juno, Cassini and other space probes that some icy moons have subsurface oceans. They’re the by-product of a gravitational tug of war the moons are caught in with their host gas giant and other moons in the system.
The final requirement for life is energy. Photosynthesis is out of the question. But just like Earth, the moons are known or suspected to have undersea volcanic vents.
“If Europa wasn’t in orbit at Jupiter, it would freeze solid,” Lawler says. “But because it’s in orbit and gets gravitational kicks from Jupiter’s other big moons, it has tides which keep it from freezing solid. That could possibly create volcanic activity that serves as an energy source for life.”
There are lots of ifs — a big one being that scientists aren’t sure how long oceans on icy moons stay liquid before orbital conditions perhaps change and they refreeze. If it’s only a few thousand (or even million) years, life wouldn’t have time to evolve.
Still, the prospect is tantalizing enough that NASA, ESA and other space agencies have exploratory missions in the works. One is the Europa Clipper, which will launch for Jupiter in 2024. Upon arrival in 2030, it will make 44 passes of Europa, including searching for organic chemicals in warm water geysers.
Dragonfly is scheduled to launch in 2027, and will visit Saturn’s moon Titan, where it will dispatch a robocraft to study the moon’s exotic surface of hydrocarbon lakes, cryovolcanoes and methane rain and snow.
On Earth, life thrives (or at least survives) practically everywhere. Yes, it took billions of years to evolve. But the Sun is only halfway through its 10-billion-year lifespan. And over the next two billion years, it will expand and grow hotter — eventually becoming a red giant, says Lawler.
“We’re not sure exactly how big it will get. It could expand all the way to Earth’s orbit. It definitely will expand past Mercury and Venus. Then those outer layers will slowly float away and we’ll be left with a white dwarf as the only remnant of the Sun.”
Any life that still existed on Earth would be toast. As for planets further out, astronomers aren’t sure how they would fare, says Lawler. “Planets have been found around white dwarfs. But Jupiter and Saturn would suddenly be plowing through a bunch of hydrogen gas. Would that heat them up through friction, or would they be fine? We don’t know.”
But if the gas giants (and their icy moons) were able to survive, it raises an interesting possibility, says Lawler.
“What if there is life on Europa that’s evolved in a sun-free environment? As the Sun warms up, does Europa become warm enough that all the ice melts and it has liquid water on the surface? Does the life evolve to be able to make use of the sunlight that’s suddenly available? We’ll never find out for sure. But it’s sure fun to think about.”