By Michael Le Page
Membrane-bound structures similar to those that enclose living cells have been shown to spontaneously self-assemble on micrometeorites, offering a tantalising hint that dust strewn across planets could play a role in the development of life.
鈥淚f we can show that protocells are forming on micrometeorites on Earth, then it鈥檚 evident that this could happen on other habitable planets,鈥 says Irep G枚zen at GOMOD, a research and education company in Sweden. 鈥淭hat鈥檚 why this is very exciting to me.鈥
Some fatty molecules called lipids can spontaneously form membrane-bound spheres, which are sometimes called protocells because they are thought to resemble the precursors to living cells. While this can happen in solution, G枚zen has been studying how some surfaces trigger protocell formation where it wouldn鈥檛 otherwise happen.
Surfaces have intrinsic energy that can power this kind of transformation because the atoms on their exposed sides don鈥檛 have a complete set of bonds, she says. 鈥淭he moment you create a surface, it will have this excess energy that it wants to get rid of.鈥
After recently studying a Martian meteorite, G枚zen realised that the rough, grainy surfaces of meteorites might be favourable for protocell formation. So she and her colleagues placed three kinds of micrometeorites in dishes containing suspensions of various kinds of lipids. They left the samples overnight and then examined them under a microscope, finding that protocells had indeed formed. They were particularly prolific in the samples that contained the same lipids as the membranes of simple cells called archaea.
There are hundreds of types of micrometeorites, says G枚zen, so this study is just an initial proof-of-concept. The findings don鈥檛 show that this is how the first living cells formed, of course, but G枚zen thinks they are intriguing given that simple organic compounds have often been found on meteorites and that micrometeorites are likely to be widely distributed over the surface of planets everywhere. 鈥淵ou have a little reactor that carries around interesting prebiotic organic matter,鈥 she says. 鈥淭hey鈥檙e raining down on almost all the planets. It鈥檚 all packaged in one particle.鈥
鈥淚 think it鈥檚 exciting that micrometeorites have sufficient surface energy to drive the [protocell] formation mechanism,鈥 says Anna Wang at the University of New South Wales Sydney in Australia. 鈥淭he physics was not a given.鈥
G枚zen says the protocells that form on surfaces also have some interesting properties. 鈥淭here鈥檚 a huge difference in terms of what happens on surfaces versus in solution,鈥 she says. 鈥淭hey form these interesting networks of protocells with little nanotubular connections in between, so they can actually transfer their contents. They can do a very rudimentary form of signalling.鈥