Specific acyclic isoprenoids as biological markers of methanogenic bacteria in marine sediments

The widespread occurrence of extended hopanoids in sediments and petroleums illustrates the importance of bacterial lipid contributions to geological materials¹. In archaebacteria, however, hopanoids are absent¹; their role as structural components of biomembranes is fulfilled by acyclic isoprenoids^1-5. Recent studies^2-6 of the lipid constituents of archaebacteria have greatly extended the range of acyclic isoprenoid skeletons known in organisms (Fig. 1). In particular, isoprenoids with head-to-head linkages have been identified^3-5, and such compounds (for example, 3,7,11,15,18,22,26,30-octamethyldotriacontane, I) have been recognized in petroleums^7,8 and as degradation products of Messel shale kerogen^9-11. Here we report the first recognition of 2,6,10,15,19-pentamethyleicosane (II), a known component of methanogens^5,6, in marine sediments of Recent to Cretaceous age (Table 1) and suggest that it and certain other acyclic isoprenoids may be used as biological markers for methanogens.