Microalgal biomarkers: A review of recent research developments

Abstract

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 C₂₈ polyunsaturated fatty acids in some dinoflagellates 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-methylenecholesterol in sediments is probably derived in most cases from diatoms. High contents of C₂₅ highly branched isoprenoid (HBI) alkenes have been identified in the diatom Haslea ostrearia and both C₂₅ and C₃₀ 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.

Introduction

Over recent years, many new compounds have been identified in sediments deposited in marine and lacustrine environments. Despite the fact that our knowledge of algal lipids is still far from comprehensive, microalgal sources have now been identified for many of the lipids that are widely distributed in Recent sediments (e.g. Boon et al., 1979; Albaiges et al., 1984; Volkman, 1986; ten Haven et al., 1987; Volkman et al., 1992, Volkman et al., 1994; Conte et al., 1994). Some of these compounds are only slowly degraded or are transformed to more stable chemical structures (e.g. Gagosian et al., 1980; Kohnen et al., 1990), and thus they can be used as biomarkers for assessing the sources of the organic matter in sediments. However, it is also apparent that some compounds are more widely distributed in the biosphere than previously thought and thus their value as specific fossil biomarkers has diminished. Sterols provide a good example of this; some sterols are found only in a few classes while others are now known to be quite widely distributed (Table 1; Volkman, 1986; Patterson, 1991). The identification of several distinctive types of aliphatic biopolymer in some classes of microalgae (Gelin et al., 1998; de Leeuw and Largeau, 1993) provides a new type of biomarker and highlights the value of studies of the bound lipids and macromolecular organic matter in organisms and sediments. This paper provides a review of some of these new findings on microalgal lipid compositions and their relevance to organic geochemical studies.