Neutrons help uncover the secrets of life adaptation mechanisms to extreme conditions on Earth
Scientists at Institut Laue Langevin (ILL) have helped take a key step towards understanding the development of life on Earth.
The team, led by Dr Philippe Oger, a scientist from the University of Lyon – INSA Lyon, used organic molecules known as lipids and a chemical called squalane, to create a model of the cell membranes of single-celled organisms called archaea. Archaea are a class of organisms, alongside bacteria and eukaryotic cells, that form the three main lineages of current life on Earth.
The scientists discovered that squalane, a chemical that may serve a similar function to cholesterol in human cells, led to the formation of clear internal structures – the first time such regions have been seen in archaea cell membranes composed by a lipid bilayer.
“If this model is proved, then it means the presence of membrane domains predates the separation of the two prokaryotic groups,” says Marta Salvador-Castell, a former PhD student from the University of Lyon – INSA Lyon, who helped carry out the study, adding that Archaea and Bacteria separated as lineages of living organisms billions of years ago.
She believes this knowledge will help scientists to understand the origins of life.
Dr Salvador-Castell and her colleagues had set out to study how archaea survive extreme conditions. Archaea were first found in salt lakes and in volcanic hot springs, such as Grand Prismatic Spring in Yellowstone National Park. And were, for a long time, considered to be mainly extremophiles – able to survive in some of the harshest environments on Earth.
“There are archaea living in habitats suitable for humans too,” explains Dr Salvador-Castell. “An interesting, not well-known, fact is that we even have archaea in our guts, along with bacteria, so they are much closer to us than we ever imagined!”
The team suspected that a type of organic chemical, called apolar isoprenoids, might have a crucial role to play in helping archaea survive extreme conditions. To study this, they added squalane, a type of apolar isoprenoid made up of carbon and hydrogen, to a simple cell membrane model made up of archaeal lipids.
“When we study lipids in archaea, we usually get rid of the apolar molecules,” explains Marta Salvador-Castell. “But previously, our lab had shown that apolar isoprenoids might have an impact on the ability of archaea to adapt to temperature, pressure and other environmental conditions.”
The team used neutron diffraction at ILL to study the cell membrane model. “Neutrons are good for studying biological samples because they are non-destructive, and also the scattering of the neutrons changes as a function of the scattering density of hydrogen isotopes, which means we could localise the apolar molecules in the membranes,” she says.
To build up a picture of the location and function of apolar molecules in the cell membrane under different temperatures (25-85 degrees C) and pressures (1-1,000 bar), the team ran neutron experiments with different concentrations of heavy water. They were able to perform their experiments at ILL using the D16 beamline neutron diffractometer.
They discovered that adding apolar molecules improved the stability of the lipid membrane at high temperatures and pressures. They also helped solve another puzzle – why some archaea have single-layered membranes while others can survive in extreme environments with bilayers.
“One of next steps is to add monolayer type of lipids to the model,” says Dr Salvador-Castell, whose PhD formed part of the research. The rest of the team are working on complexifying the model by adding different lipids and apolar molecules. They hope, in the future, to also add proteins.
Re.: Salvador-Castell, M., Golub, M., Erwin, N. et al. Characterisation of a synthetic Archeal membrane reveals a possible new adaptation route to extreme conditions. Commun Biol 4, 653 (2021).
ILL Instrument: Diffraction patterns were measured on the D16 small-momentum transfer diffractometer at ILL