Microalgal biomarkers: A review of recent research developments

Microalgae are major sources of lipids in lacustrine and marine environments. 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 with an emphasis on those likely to be useful biomarkers for identifying sources 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 characteristic features have been observed in the abundance of particular polyunsaturated fatty acids. Despite several decades of study, it is now apparent that some of the biosynthetic steps leading to the formation of these unsaturated fatty acids are still not known with certainty as shown by the occurrence of C(28) polyunsaturated fatty acids in some dino-flagellates and the likely involvement of chain-shortening reactions. Considerable data have also been obtained on the sterols in microalgae, but some classes of organisms are still not well documented (e.g. cryptomonads, eustigmatophytes, xanthophytes and raphidophytes). Diatoms show a great variety of sterol compositions and no sterol appears to be either unique or representative. However, 24-methylene-cholesterol in sediments is probably derived in most cases from 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 setigera. 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 having 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 of highly aliphatic biopolymers (algaenans) in some species of marine and freshwater green algae and eustigmatophytes. This material persists in sediments and may be a source of the alkyl chains in ancient kerogens and crude oil constituents. Algaenans do not occur in all algal species and may be absent from some classes, such as diatoms. This implies that the organic matter preserved in sediments is strongly influenced by a subset of the microalgal contributors of organic matter. Although reasonable sources have been identified for many of the lipids in sediments, there are still many gaps in our knowledge and further studies are clearly required. (C) 1998 Elsevier Science Ltd. All rights reserved.