Nearly all living organisms on Earth have water-based structures around their cells called phospholipid bilayers. These keep water in (or out), and they shelter the insides of our cells from the rest of the world. Now, according to a study based entirely on computer modeling, small molecules in oxygen-free environments may be capable of forming compartments that resemble these lipid membranes. The findings, published in Science Advances last week, suggests that life—but “not as we know it”—is possible on worlds without oxygen. They just have to be comprised of methane-based cells.
Astronomers looking for signs of extraterrestrial life (and places where mankind might colonize one day) focus on the habitable zone, a narrow area around the sun where liquid water can exist. However, if cells weren't based on water but on methane—which has a much lower freezing point—could “life” exist in extremely cold worlds like Saturn’s moon Titan? The giant moon is spotted with seas of liquid methane and has no oxygen available for the formation of a lipid bilayer membrane.
Using computer simulations, a Cornell team led by Paulette Clancy has modeled a methane-based, oxygen-free template for life. Vesicles made from phospholipid bilayer membranes are called liposomes, from the Greek "lipos" and "soma" meaning "lipid body." So, the team created a theorized cell membrane they call an "azotosome," after the French word for nitrogen. Azotosomes are made from nitrogen, carbon, and hydrogen molecules known to exist in Titan’s seas, and they show the same stability and flexibility that liposomes do, with one major exception: Their cell membrane is capable of functioning in liquid methane temperatures of 292 degrees below zero.
When the team screened for candidate compounds from those present on Titan, they found the most promising one to be acrylonitrile—a colorless, poisonous, liquid organic compound used to make acrylic fibers, resins, and thermoplastics. An acrylonitrile azotosome provides good stability, a strong barrier to decomposition, and a flexibility similar to that our phospholipid membranes.
"We're not biologists, and we're not astronomers, but we had the right tools," Clancy explains in a news release. "Perhaps it helped, because we didn't come in with any preconceptions about what should be in a membrane and what shouldn't. We just worked with the compounds that we knew were there and asked, 'If this was your palette, what can you make out of that?'"
Inspired by Isaac Asimov, study co-author James Stevenson of Cornell says: "Ours is the first concrete blueprint of life not as we know it." Their next step is to demonstrate what reproduction and metabolism might look like with oxygen-free, methane-based cells.
