The Formation of Sedimentary Alkylbiphenyls from Algal Lipid Precursors

Introduction

Fatty acids are by far the most abundant lipids occurring in bacteria, microalgae, higher plants and marine fauna. Fatty acids occur predominantly as glyco- and phospho-lipids, although other unsaturated long-chain alkene precursors have been identified in the cell membranes of algae. These include: (i) a range of long chain unsaturated alkenes reported in bacillariophyceae (diatoms) of which C21:6 alkene is reported to be one of the most abundant hydrocarbons of the total hydrocarbon fraction in cultures (ii) C25-27 n-polyenes and C30 and C34 n-alkenes also found in diatoms (iii) C37-C39 alkenones biosynthesised by haptophyceae and bacillariophyceae (diatoms) (iii) C27-C31 alkadienes and alkatrienes ultimately biosynthesised by the race A of the freshwater alga Botryococcus braunii (Chlorophyceae) (iv) hexatriaconta-9,15,22-triene reported in microbial mats (v) long chain unsaturated alcohols (1,15 C32:1 and 1,19 C36:1) reported in Nannochloropsis salinia.

Biphenyl and alkylbiphenyls derived from a range of unsaturated straight chain lipid algal precursors

In the present study, unsaturated lipid precursors have been isolated from six different cultured microalgae and one field sample, a macroalgae (Ecklonia radiata), in order to identify those that give rise to the formation of biphenyl following pyrolysis of lipid extracts. We have established that the biphenyl and alkylbiphenyl GC-MS distributions in different algal extracts exhibit a carbon number distribution consistent with an origin from unsaturated fatty acids, alkenones, unsaturated diols and polyenes. As an extension of this study, d13C values for biphenyl present in some immature sediments have been determined and found to be predominantly derived from an algal source in these sediments, having a d13C composition consistent with pristane, phytane and n-C15 and n-C17 alkanes which have been attributed to an origin mainly phytoplanktonic.

Cyclisation of unsaturated fatty acids

Biphenyl and n-alkylbiphenyls substituted in the ortho position were formed from the pyrolysis of unsaturated fatty acids and algal extracts. Alkylbenzenes and alkylbiphenyls were found to be major aromatic pyrolysis products of a polyunsaturated fatty acid (linolenic acid) and an algal extract (Ecklonia radiata). Pyrolysis on charcoal or Pt/C produced identical C5-alkylbiphenyl distributions for both linolenic acid and the algal extract, suggesting that in the algal extract fatty acids are also involved in the formation of these products. The alkylbiphenyls formed in these experiments are all ortho substituted, which is consistent with their formation from the cyclisation, aromatisation and decarboxylation of fatty acids. Sedimentary samples derived from algal sources show high relative abundances of alkylbiphenyls and anomalously high 2-methylbiphenyl. The occurrence of biphenyl and 2-methylbiphenyl in high relative abundance in sedimentary material may indicate an algal origin.

Formation of C5-alkylbiphenyl through cyclisation, aromatisation and decarboxylation of linolenic acid

Personnel

B. van Aarssen, T. Bastow, K. Grice, R. Alexander, R. Kagi

Funding

Centre of Excellence in Mass Spectrometry, Australian Petroleum CRC, MERIWA