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A fascinating difference in cell membranes is to be found in the archaea (Figure 5.8). This domain of microorganisms is discussed in more detail in the chapters on life in extreme environments (Chapter 7) and phylogenetic trees (Chapter 8). In the archaea, the lipids that make up the membranes are different from those of the bacteria and eukaryotes, one of the characteristics that put them into a separate domain of life. Instead of ester groups within the hydrophilic head of their lipids, they have ether groups, which are more resistant to a variety of chemicals and could play a role in their resistance to extreme environments. The long tails of the lipids are also different. They are called terpenoids (or isoprenoids) and have methyl side groups, in contrast to bacteria where the lipid tails are long and simple (fatty acids). These side groups might make the membranes less leaky and more resistant to extreme conditions. Even more strangely, in some archaea the bilayer is replaced by a monolayer where the tails from two lipids are fused together (Figure 5.8). This is the case in Ferroplasma, an archaeon (member of the archaea) that lives in acidic environments. It is thought that this adaptation might make the membranes more resistant to extreme conditions, preventing the membranes from falling apart.

Figure 5.8 The structure of archaeal cell membrane lipids compared to bacterial lipids. The lower diagram also shows how, in some organisms, archaeal lipids can be joined in the middle.

These membrane differences between archaea and bacteria have astrobiological significance. Preserved membrane lipids can be used to tell the identity of long-dead organisms. For example, archaea are often the dominant microorganisms in salty environments. Membrane isoprenoids preserved in salts are diagnostic of archaea and can be used to infer information on the microbial communities that once lived in the salts. Different membrane lipids preserve differently. In other words, their taphonomic potential is different. To be able to interpret which organisms lived in ancient environments, preserved in salt or rock, we need to know about cell membrane components, their ability to be preserved, and which ones might be more quickly degraded, leaving no trace.