Under pressure: How comb jellies adapted to life on the ocean floor

The ocean floor is not hospitable. There is no light; the temperature is freezing; and the pressure of all the water above will literally crush you. Animals that live at this depth have developed biophysical adaptations that allow them to survive in these harsh conditions. What are these adaptations and how did they develop?

Itay Budin, an assistant professor of chemistry and biochemistry at the University of California, San Diego, teamed up with researchers from across the country to study the cell membranes of ctenophores (“comb jellies”) and discovered that they have unique lipid structures that allow them to live under intense pressure. Their work is published in Science.

Adapting to the environment

First of all, although ctenophores look like jellyfish, they are not closely related. Ctenophores are part of the phylum Ctenophora (pronounced te-no-pour-a). They are predators that can grow to the size of a volleyball and live in oceans all over the world and at various depths, from the surface to the deep sea.

Cell membranes contain thin sheets of lipids and proteins that must maintain certain properties for cells to function properly. Although it has been known for decades that some organisms have adapted their lipids to maintain fluidity in extremely cold temperatures (homeoviscous adaptation), it is not known how organisms living in the deep sea have adapted to extreme pressure, nor if adapting to pressure was the best solution. just like adaptation to the cold.

Budin was studying homeo-viscous adaptation in E. coli bacteria, but when Steven Haddock, a senior scientist at the Monterey Bay Aquarium Research Institute (MBARI), asked if ctenophores had the same homeo-viscous adaptation to compensate for extreme pressure, Budin was intrigued.

Complex organisms have different types of lipids. Humans have thousands of them: the heart has different ones, the lungs have different ones, the skin has different ones, and so on. They also have different shapes; some are cylindrical and others are cone-shaped.

To determine whether ctenophores adapted to cold and pressure through the same mechanism, the team needed to control the temperature variable. Jacob Winnikoff, lead author of the study who worked at both MBARI and UC San Diego, analyzed ctenophores collected from across the Northern Hemisphere, including those that lived on the ocean floor in California (cold, high pressure) and those on the surface of the Arctic Ocean (cold, not high pressure).

“It turns out that ctenophores have evolved unique lipid structures to compensate for intense pressure, distinct from those that compensate for intense cold,” Budin said, “so much so that pressure is actually what holds their cell membranes together.”

The researchers call this adaptation “homeocurvature” because the curved shape of the lipids has adapted to the unique habitat of ctenophores. In the deep sea, cone-shaped lipids evolved into exaggerated cone shapes. Ocean pressure counteracts this exaggeration, so the shape of the lipids is normal, but only at these extreme pressures. When deep-sea ctenophores come to the surface, the exaggerated cone shape returns, the membranes rupture, and the animals disintegrate.

The exaggerated cone-shaped molecules are a type of phospholipid called plasmalogen. Plasmalogens are abundant in the human brain and their decreasing abundance often accompanies decreased brain function and even neurodegenerative diseases such as Alzheimer’s disease. This makes them very interesting to scientists and medical researchers.

“One of the reasons we chose to study ctenophores is that their lipid metabolism is similar to that of humans,” Budin said. “And while I wasn’t surprised to find plasmalogens, I was shocked to see that they make up three-quarters of the lipid content of a deep-sea ctenophore.”

To further test this finding, the team returned to E. coli, conducting two experiments in high-pressure chambers: one with unchanged bacteria and a second with bacteria that had been bioengineered to synthesize plasmalogens. While the unchanged E. coli died, the E. coli strain containing plasmalogens thrived.

These experiments were carried out over several years and with collaborators from several institutions and disciplines. At UC San Diego, in addition to Budin, whose group conducted the biophysics and microbiology experiments, the laboratory of Distinguished Professor of Chemistry and Biochemistry Edward Dennis performed lipid analysis by mass spectrometry. MBARI marine biologists collected ctenophores to study, while University of Delaware physicists performed computer simulations to validate the membranes’ behaviors at different pressures.

Budin, who is interested in how cells regulate lipid production, hopes the discovery will lead to new research into the role of plasmalogens in brain health and disease.

“I think the research shows that plasmalogens have really unique biophysical properties,” he said. “So now the question is, how important are these properties to the functioning of our own cells? I think that’s a take-home message.”