Rewriting cell membrane theory

Until recently, it was thought that all cells were surrounded by membranes containing molecules called phospholipids — oily compounds that contain phosphorus, as well as other basic biochemical nutrients including nitrogen.

However, researchers from the Woods Hole Oceanographic Institution and others have found phytoplankton in the Sargasso Sea that make their cell membranes without using phospholipids, using phosphorus-free “substitute lipids” instead.

These substitute lipids were once regarded as merely a molecular peculiarity of phytoplankton grown in the laboratory, but are now recognised to be used by phytoplankton in the wild. It is now thought that substitute lipids are the most abundant membrane molecules in the sea, despite being essentially unknown until recently.

The Sargasso Sea is naturally short in the supply of phosphorus and nitrogen. A molecule of phosphorus dissolved there remains for perhaps an hour or two before a phosphorus-starved cell absorbs it. By comparison, in the Pacific Ocean phosphorus may linger for nearly a year before being used by plankton.

Despite this, phytoplankton live well in the Sargasso Sea. In particular, small photosynthetic bacteria called cyanobacteria flourish in places where phosphorus is scarce. They do this by synthesising a membrane lipid called sulfoquinovosyldiacyglycerol (SQDG), a molecule based on sulphur rather than phosphorus.

Further research has revealed that more complex eukaryotic plankton also have phosphate-free lipids in their cell membrane, but these require nitrogen and form bipolar betaine molecules, whose structures resemble amino acids.

Chinese researchers have demonstrated that SQDG possesses antiviral activity against herpes simplex viruses type 1 and type 2, using hot water extracts of a green alga, Caulerpa racemosa, collected from the South China Sea.

Japanese in vitro studies using a chemically synthesised form of SQDG have demonstrated an inhibitory effect on DNA-polymerases, suggesting possible uses as an immunosuppressive and anticancer chemotherapy agent.