Microalgal biomarkers: A review of recent research developments

Microalgae are major sources of lipids in lacustrine and marine environment s. This paper provides a review of some recent advances in our knowledge of the wide variety of lipid types that have been isolated from microalgae wi th an emphasis on those likely to be useful biomarkers for identifying sour ces of organic matter in sediments. Extensive data are now available on the fatty acids in all of the major classes of microalgae and some useful char acteristic features have been observed in the abundance of particular polyu nsaturated fatty acids. Despite several decades of study, it is now apparen t that some of the biosynthetic steps leading to the formation of these uns aturated fatty acids are still not known with certainty as shown by the occ urrence of C-28 polyunsaturated fatty acids in some dino-flagellates and th e likely involvement of chain-shortening reactions. Considerable data have also been obtained on the sterols in microalgae, but some classes of organi sms are still not well documented (e.g. cryptomonads, eustigmatophytes, xan thophytes and raphidophytes). Diatoms show a great variety of sterol compos itions and no sterol appears to be either unique or representative. However , 24-methylene-cholesterol in sediments is probably derived in most cases f rom diatoms. High contents of C-25 highly branched isoprenoid (HBI) alkenes have been identified in the diatom Haslea ostrearia and both C-25 and C-30 HBI alkenes have been found in diatom strains thought to be Rhizosolenia s etigera. Genetic and environmental factors appear to be important controls on the relative abundances of the various homologues identified. Microalgae are also suspected to be a source of long-chain saturated Fatty acids havi ng an even carbon number predominance and of long-chain alkanes with no odd over even carbon number predominance, although the available data are not conclusive. An exciting development in recent years is the identification o f highly aliphatic biopolymers (algaenans) in some species of marine and fr eshwater green algae and eustigmatophytes. This material persists in sedime nts and may be a source of the alkyl chains in ancient kerogens and crude o il constituents. Algaenans do not occur in all algal species and may be abs ent from some classes, such as diatoms. This implies that the organic matte r preserved in sediments is strongly influenced by a subset of the microalg al contributors of organic matter. Although reasonable sources have been id entified for many of the lipids in sediments, there are still many gaps in our knowledge and further studies are clearly required. (C) 1998 Elsevier S cience Ltd. All rights reserved.