Algal Markers

Many algal lipids have structures that are non-specific, being found in other types of organisms, including sterols. However, the steranes derived from sterols can be used as general algal markers, as outlined below, and there are some compounds that are quite specific.

Steroids

Sterols are produced by many different organisms, including algae, fungi, land plants and animals, although they are very rare in bacteria. Some of the structural specificity of sterols in organisms is in the number and position of double bonds in the structure, and this information is lost during burial and diagenesis. However, the structure of the side chain (differing for C27, C28, C29 and C30 steranes) is influenced to some extent by their biological source, and is preserved in the geological record. In general, sterols having 27 or 28 carbon atoms are most commonly derived from algae, whilst most land plant sterols have 29 carbon atoms. This is a generalisation that is complicated by the occurrence of C29 sterols in some algae (Volkman, 1986), and the production of some C27 and C28 sterols by some plants. Nevertheless, a triangular plot of sterane composition shows some relationship to organic matter type, distinguishing algal vs. higher plant inputs, and is a useful tool in oil-oil and oil-source correlation.

The C30 steranes (both 24-n-propyl and 24-isopropyl cholestanes) are derived from marine algae (Moldowan et al., 1985). They can be abundant constituents of the sterane distribution, possibly reflecting a dominant input of certain marine algae (Rogers et al., 1999).  

260.1

The structures of C30 steranes.

The absence of C30 steranes generally indicates a non-marine oil. However, care must be taken in determining C30 steranes from the m/z 217 mass chromatogram, due to potential co-elution of 4-methylsteranes.

4-methylsteranes have been used as markers of dinoflagellates (e.g. Robinson et al., 1984 and 1987). However, their precursor 4-methylsterols have also been reported in some prymnesiophyte algae (Volkman et al., 1990) and certain bacteria (Bird et al., 1971). Dinosteranes can be more confidently assigned to an origin from dinoflagellates, being derived from dinosterol or dinostanol (Withers, 1983). Dinosteranes contain 23,24-dimethyl side chains which lead to an m/z 98 ion in their mass spectra; consequently they can be most specifically monitored by GC-MS-MS using the m/z 414 → 98 transition. 

Botryococcane

This compound is an unusual irregularly branched acyclic isoprenoid that can be detected in the m/z 183 mass chromatogram. Its only known source is Botryococcus braunii, a fresh-brackish water green alga, which contains its precursor, botryococcene (Maxwell et al., 1968). Consequently, it is a highly specific biomarker, but one that is only rarely found in oils and source rocks.

260.2 Structures of two algal acyclic isoprenoids

Highly Branched Isoprenoids (HBIs)

The highly branched isoprenoids have been ascribed to a diatom source (Volkman et al., 1994), where they occur as unsaturated hydrocarbons. They have been found in a wide range of modern and ancient sediments, but more rarely in oils. The C20 alkane 2,6,10-trimethyl-7-(3-methylbutyl)-dodecane (initially referred to as 'Gx' in some older literature) is probably the most common HBI in oils, but it can co-elute with pristane and be overlooked. Unusually, it is the second most abundant alkane in Rozel Point crude oil (Volkman & Maxwell, 1986). 

References

Bird, C.W., Lynch, J.M., Pirt, F.J., Reid, W.W., Brooks, C.J.W. & Middleditch, B.S. (1971). Steroids and Squalene in Methylococcus capsulatus grown on Methane. In: Nature vol. 230 pp. 473-474 ISBN: 0028-0836.

Maxwell, J. R., Douglas, A.G., Eglinton, G. & McCormick, A. (1968). The Botryococcenes-hydrocarbons of novel structure from the alga Botryococcus braunii, Kützing. In: Phytochemistry vol. 7 pp. 2157-2171.

Moldowan, J.M., Seifert, W.K. & Gallegos, E.J. (1985). Relationship between petroleum composition and depositional environment of petroleum source rocks. In: American Association of Petroleum Geologists Bulletin vol. 69 pp. 1255-1268.

Robinson, N., Cranwell, P.A., Eglinton, G. & Jaworski, G.H.M. (1987). Lipids of four species of freshwater dinoflagellates. In: Phytochemistry vol. 26 pp. 411-421.

Robinson, N., Eglinton, G., Brassell, S.C. & Cranwell, P.A. (1984). Dinoflagellate origin for sedimentary 4 alpha methylsteroids and 5 alpha (H) stanols. In: Nature vol. 308 pp. 439-442.

Rogers, K.M., Collen, J.D., Johnston, J.H. & Elgar, N.E. (1999). A geochemical appraisal of oils seeps from the East Coast Basin, New Zealand. In: Organic Geochemistry vol. 30 pp. 593-605.

Volkman, J.K. & Maxwell, J.R. (1986). Acyclic isoprenoids as biological markers. In: Biological Markers in the Sedimentary Record. Methods in Geochemistry and Geophysics, 24 , Elsevier Science Publishers BV. pp. 1-42 ISBN: 0-444-42598-5.

Volkman, J.K., Barrett, S.M. & Dunstan, G.A. (1994). C25 and C30 highly branched isoprenoid alkenes in laboratory cultures of two marine diatoms. In: Organic Geochemistry vol. 21 pp. 407-413.

Volkman, J.K., Kearney, P. & Jeffrey, S.W. (1990). A new source of 4-methyl sterols and 5α(H)-stanols in sediments: prymnesiophyte microalgae of the genus Pavlova. In: Organic Geochemistry vol. 15 pp. 489-497.

Volkman, J.K. (1986). A review of sterol markers for marine and terrigenous organic matter. In: Organic Geochemistry vol. 9 pp. 83-99.

Withers, N. (1983). Dinoflagellate Sterols. In: Marine Natural Products: Chemical and Biological Perspectives: 5 , Academic Press Inc. pp. 87-130 ISBN: 0126240051.

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