Figure 1.

Structures of known and novel tetraether membrane lipids identified in different sediments (see Table 1). The basic structure of a GDGT is composed of two glycerol moieties linked by two carbon chains, here denoted X and Y, whereby X and Y may be interchanged. The table gives the structures of X and Y for the different GDGT isomers. For the basic structure of a GDGT, both the parallel and anti-parallel configurations are drawn, as this has not been established. However, chemical degradation experiments on GDGT I from archaeal cell material indicate a racemic mixture (26).