Examples of source indicators in Recent sediments

In this section two of the most intensively studied classes of biomarkers are examined fatty acids and sterols.The units from which polysaccharides and lignins are formed (i.e. monosaccharides and phenolic compounds, respectively) can be analysed as readily as biomarkers and the source-related information that their distributions can convey is also considered. 

There is biological preference for the cis configuration at C=C bonds (although some clay-catalysed isomeriza- 

Not all monounsaturated fatty acids are biosynthesized as cis isomers with even numbers of C atoms. For example, trans-16 1(07 and trans-18 1(07 may be bacterial markers, whereas trans-16 1(013 is produced by photosynthetic bacteria and some phytoplankton but not cyanobacteria (Johns et al. 1979).The fatty acid distributions of cyanobacteria can be quite variable, with some exhibiting major 16 0 and 16 1(07, while others have abundant 18 lff)9.The odd-numbered acids 15 1 and 17 1 with C06 or (08 C=C bonds are bacterial markers, produced by the anaerobic biosynthetic route. Among the sulphate-reducing bacteria, 17 1(08 appears to be characteristic of Desulfohulhus (Taylor Parkes 1983). 

These branched acids can be source-specific and they are rarely unsaturated. They are formed by the incorporation of branched amino acids into the biosynthetic pathway, yielding iso and anteiso acids, as noted in Section 5.1.3. Iso and anteiso saturated fatty acids are found in fungi, molluscs and phytoplankton, but they are generally in higher levels in bacteria and are often observed in the C13—C17 range (Harwood Russell 1984).The C15 isomers (Fig. 5.1) are usually particularly abundant in bacteria and the ratio (iso + anteiso)/normal derived from C15 components can be used as an indication of relative bacterial contributions (Parkes Taylor 1983). Similarly, the (08 isomers of /. - 5 1 and iso-17 1 are bacterial markers (Perry et al. 1979), and iso-17 1(07 is characteristically a major fatty acid in the sulphate-reducing bacteria Desulfovibrio desulfuricans (Taylor Parkes 1983). 

Internally branched acids occur naturally, such as the 10-methyl (numbering from the acid-group end) isomers of 16 0 and 18 0 found in fungi and bacteria. For example, 10-methyl 16 0 (Fig. 5.1) is characteristic of Desulfobacter (Taylor Parkes 1983). There are also cycloalkyl acids, such as 17 0 and 19 0 cyclopropyl moieties. Cyclopropyl acids are more common in eubacteria than other organisms, but the position of the cyclopropyl group is important. Out of the more commonly occurring cw-11,12 and as-9,10 isomers of 19 0 and the as-9,10 isomer of 17 0, only the as-11,12 iso- 

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