J.K. Volkman, S.M. Barrett, S.I. Blackburn, G.A. Dunstan.
Studies of the lipid composition of marine microalgae over the past decade have revealed a surprisingly diverse range of lipid compositions. Many new compounds have been identified and some of these appear to have a very restricted species distribution and thus show promise as chemotaxonomic markers. For example, each class of microalgae has a distinctive distribution of unsaturated fatty acids and some are excellent sources of certain essential polyunsaturated fatty acids required in the diet of many marine animals. A diversity of sterol patterns are also found ranging from the predominance of a single sterol in eustigmatophytes and some diatoms to mixtures of 20 or more sterols found in some species of dinoflagellates. The possible bioactivity of most of these unusual compounds remains to be assessed. Some of the new compounds identified in our CSIRO studies and their applications to chemotaxonomy, mariculture nutrition, paleoceanography and marine chemistry will be described. These include C37-C39 unsaturated ketones (alkenones) and hydrocarbons in prymnesiophyte algae such as Emiliania and Gephyrocapsa. The ratio of the alkenones in sediments provides a sensitive record of paleo-oceanic sea surface temperatures over the past 100000 years or more. Unusual highly branched unsaturated C20, C25 and C30 alkenes are commonly observed in marine sediments. Recently, we identified these compounds in the diatoms Haslea ostrearia and Rhizosolenia setigera confirming that diatoms are the most likely sources of these compounds in marine environments. Novel dihydroxy 4-methylsterols have been identified in species of Pavlova and these compounds appear to be useful chemotaxonomic markers for the Pavlovales. We have also identified unsaturated C30-C32 alcohols and diols in eustigmatophytes from the genus Nannochloropsis. Evidence is now accumulating that these diols are the building blocks for recently discovered novel highly aliphatic biopolymers produced by these microalgae. These species also contain long-chain fatty acids having 1 or 2 mid-chain hydroxyl groups which may also have a role in the biopolymer structure.
(CSIRO Division of Marine Research, GPO Box 1538, Hobart. Tasmania, 7001, Australia)