Bacterial Markers
The term 'bacteria', used in a loose sense, covers both the Archaea (archaebacteria) and the Bacteria (or eubacteria). Archaebacteria (methanogens, halophiles, acidophiles, thermophiles etc.) are the most ancient form of life on Earth, and tend to occur in extreme environments that are inhospitable for other forms of life. The eubacteria include heterotrophic bacteria and cyanobacteria.
The amount of bacterial biomass contributing to sedimentary organic matter is difficult to determine, as the biomarkers used only constitute a very small fraction of the organic matter and may not be representative.
Hopanes
The hopanoids encountered in geological samples are derived from functionalised C30 and C35 biological precursors, the wider range of carbon numbered hopanes being obtained through diagenetic reactions which involve side-chain shortening in association with defunctionalisation and later generation of bound hopanoids from the kerogen. Whilst C30 biohopanoids have been found in several types of organisms including lichens, mosses and protozoa, the C35 (or extended) biohopanoids (e.g. bacteriohopanetetrol) are only produced by eubacteria. To date, such hopanoids have been found in a broad range of bacterial groups and over 50% of tested eubacteria produce hopanoids (, 1998; , 2007).
Hopanoid diagenesis.
Although different bacteria produce hopanoids with different functional groups on the side chain, these distinctions are lost during diagenesis, such that the same hopane products tend to be produced from the different precursors. For this reason, to date no bacterial source information has been derived from the carbon number distribution of the regular hopanes in oils or source rocks (although more unusual [e.g. methylated] hopane-type molecules have been used as source markers; see below).
Methylhopanes
Hopanoids methylated at the C-2 or C-3 positions have been reported in several classes of bacteria and cyanobacteria. 2-Methylhopanes in geological samples are most likely derived from cyanobacteria (, 1999; , 2008), although they have also been reported in some other bacteria. 3-Methylhopanoids have been found in methanotrophic bacteria, and others, and may be less specific markers, although (2004) report a possible relationship with modern alkaline saline lakes and have successfully employed them in oil-oil correlations.
Acyclic Isoprenoids
Although many isoprenoids, especially those with the regular 'head-to-tail' linking of their constituent isoprene units (e.g. pristane and phytane), are not specific to bacteria, some others are useful markers for archaebacteria. 'Head-to-head' acyclic isoprenoids are indicative of archaebacterial origin, being derived from the biphytanyl ether lipids (see , 1993, and refs. therein). Monitored in the m/z 183 mass chromatogram, they have proven useful in oil correlations (, 1981; , 2005).
C40 acyclic isoprenoids.
The 'tail-to-tail' isoprenoid, lycopane (C40), which is rarely reported in oils, may also be archaebacterial in origin (, 1981), although other inputs have been reported. Much more specific markers for methanogenic archaeobacteria are the C20 and C25 tail-to-tail isoprenoids 2,6,11,15-tetramethylhexadecane (crocetane) and 2,6,10,15,19-pentamethylicosane (PMI; , 2005). Although crocetane is difficult to measure due to coelution with phytane, PMI resolves and can be found in the m/z 183 mass chromatogram.
References
Brassell, S.C., Wardroper, A.M.K., Thomson, I.D., Maxwell, J.D. & Eglinton, G. (1981). Specific acyclic isoprenoids as biological markers of methanogenic bacteria in marine sediments. In: Nature vol. 290 pp. 693-696.
Farrimond, P., Fox, P.A., Innes, H.E., Miskin, I.P. & Head, I.M. (1998). Bacterial sources of hopanoids in Recent sediments: improving our understanding of ancient hopane biomarkers. In: Ancient Biomolecules vol. 2 pp. 147-166.
Farrimond, P., Talbot, H.M., Watson, D.F., Schulz, L.K. & Wilhelms, A. (2004). Methylhopanoids: Molecular indicators of ancient bacteria and a petroleum correlation tool. In: Geochimica et Cosmochimica Acta vol. 68 pp. 3873-3882.
Peters, K.E. & Moldowan, J.M. (1993). The biomarker guide., Prentice-Hall Inc. p. 363 ISBN: 0-13-086752-7.
Peters, K.E., Walters, C.C. & Moldowan, J.M. (2005). The biomarker guide., Cambridge University Press p. 1155 ISBN: 0 521 83763 4.
Seifert, W.K. & Moldowan, J.M. (1981). Paleoreconstruction by biological markers. In: Geochimica et Cosmochimica Acta vol. 45 pp. 783-794.
Summons, R.E., Jahnke, L.L., Hope, J.M. & logan, G.A. (1999). 2-Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. In: Nature vol. 400 pp. 554-557.
Talbot, H.M. & Farrimond, P. (2007). Bacterial populations recorded in diverse sedimentary biohopanoid distributions. In: Organic Geochemistry vol. 38 pp. 1212-1225.
Talbot, H.M., Summons, R.E., Jahnke, L.L., Cockell, C.S., Rohmer, M. & Farrimond, P. (2008). Cyanobacterial bacteriohopanepolyol signatures from cultures and natural environmental settings. In: Organic Geochemistry vol. 39 pp. 232-263.
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