The bottom of Earth’s oceans may as well be another planet. The temperatures are incredibly cold, there is almost zero sunlight, and all of that pressure from the water above could crush a person like a soda can. Some species including some octopus, squid, and ctenophores–aka comb jellies–still manage to live in this real-life twilight zone. A study published June 27 in the journal Science found that comb jellies have developed a unique structure to their cell membranes that allow them to live under such intense pressure.
Comb jellies and cell membranes
While comb jellies may look like jellyfish, they are not closely related. They are in the phylum Ctenophora, while jellyfish are in the phylum Cnidaria. Comb jellies are found at various depths oceans all around the world and have right rows of small comb-like plates that they use to move through the water. They primarily survive on other ctenophores and small marine invertebrates like salps and siphonophores.
In the study, the team of scientists from across the United States, looked to their cell membranes for clues to how these funky invertebrates have adapted to their environment. Cell membranes have thick sheets of lipids and proteins that must keep certain properties in order to work properly. Scientists have known for several years that some organisms have made changes to their lipids to maintain fluidity in extreme cold–a process known as homeoviscous adaptation. What was not known was how those living in the deep sea have adapted to intense pressure or if the adaptations for cold were the same as pressure.
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Study co-author and University of California, San Diego biochemist Itay Budin was studying homeoviscous adaptation in E. coli bacteria when co-author Steven Haddock from the Monterey Bay Aquarium Research Institute (MBARI) posed a question. Do ctenophores have the same homeoviscous adaptation to compensate for extreme pressure?
Looking to lipids
These cold adaptations often come down to lipids–or fats. The more complex the organism, the greater the variety of lipids. For example, human cells have thousands of different types. The heart’s lipids are different from the lungs, which are different from those found in the skin. These lipids also come in different shapes, from cylindrical to cone shape.
To see if pressure and cold adaptations were the same, the team needed to control the temperature variable in their experiments. Study co-author and MBARI and UC San Diego biochemist Jacob Winnikoff analyzed ctenophores that were collected from across the northern hemisphere. These included some that live at the bottom of the ocean in California, where the temperatures are cold and the water pressure is high. They also looked from some from the surface of the Arctic Ocean, which has cold temperatures but the pressure is not quite as high.
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“It turns out that ctenophores have developed unique lipid structures to compensate for the intense pressure that are separate from the ones that compensate for intense cold,” Budin said in a statement. “So much so that the pressure is actually what’s holding their cell membranes together.”
They call this adaptation “homeocurvature,” since the curve-forming shape of the lipids has adapted to the comb jelly’s unique deep-sea habitat. Here, their cone-shaped lipids have evolved into more exaggerated cone shapes. The intense pressure of the ocean counteracts with the exaggeration, so this makes the lipid shape more normal, but only at such extreme pressures. When deep-sea ctenophores are brought to the surface, the exaggerated cone shape returns. The membranes then completely split apart and the animals disintegrate.
According to the study, the molecules with an exaggerated cone shape are a type of phospholipid called plasmalogens, which are abundant in human brains. Declining plasmalogens often accompanies diminishing brain function and even neurodegenerative illnesses like Alzheimer’s Disease.
“One of the reasons we chose to study ctenophores is because their lipid metabolism is similar to humans,” said Budin. “And while I wasn’t surprised to find plasmalogens, I was shocked to see that they make up as much as three-quarters of a deep-sea ctenophore’s lipid count.”
Back to bacteria
To test this further, the team looked at E. coli. They conducted two experiments with the bacteria in high pressure chambers. One test used unaltered bacteria and the second used bacteria that had been bioengineered to synthesize plasmalogens. The uncharted E. coli died off, but the E. coli strain with plasmalogens thrived.
Budin hopes that this discovery will lead to more investigations into the role plasmalogens play in brain health and disease in the future.
“I think the research shows that plasmalogens have really unique biophysical properties,” Budin said. “So now the question is, how are those properties important for the function of our own cells? I think that’s one takeaway message.”