Deciphering the biosynthetic pathway of branched glycerol dialkyl glycerol tetraethers occurring in the cell membrane of bacteria.

"Cell membranes protect the cell from the environment and modulate enzymatic activities and therefore are crucial to all life. They are mainly made from lipids. These lipids can be incredibly diverse, ranging from fatty acids that form bilayers to membrane spanning tetraether lipids.

Branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs) are membrane-spanning lipids occurring in the cell membrane of certain bacteria. So far, the biosynthetic pathway of brGDGTs as well as the substrate, intermediates and involved enzymes, are largely unknown. Currently, two competing hypothesis exists for the synthesis of brGDGTs in bacteria, either a pathway based on 'iso-diabolic acids' (iDAs) known as 'iso-DA pathway' or one based on an initial condensation of two iso-C15:0 fatty acids known as the 'diether pathway'.

Understanding the biosynthetic pathway of brGDGTs and identifying their source organism(s) is of great importance due to their crucial role in reconstructing parameters such as temperature and pH of the terrestrial environment in the geological past.

i) They are the only known membrane-spanning tetraether lipids produced by bacteria, such lipids are normally only found in archaea. They are therefore of interest to evolutionary biologists, providing insight into the diversity of the biosynthetic capabilities of microbial life, the origin of the domains of life, and the membrane transition in eukaryogenesis.

ii) They are widely distributed and abundant in the natural world, indicating that the source organism(s) play an important ecological role across mesophilic environments.

iii) The distribution of brGDGTs is dependent on temperature (and pH). As a result, they are widely used in paleoclimate reconstructions and are one of the few methods that allow scientists to quantify past terrestrial temperature across the last 100 million years and drug delivery. Thus, brGDGTs are a unique type of lipid that is at the forefront of research across disciplines."