Sterol Synthesis and Distribution and Algal Phylogeny

Abstract

The wide range of structural types of sterols in algae has been long recognized. Along with the possibility of several biosynthetic pathways, especially in the side chain biosynthesis, these different structures in the various algal taxa provide strong evidence for particular phylogenetic affiliations. As with other photosynthetic plants, all algae examined synthesize sterols through the cycloartenol pathway. During side chain alkylation, however, occurrence of both the 24 (28) pathway and the 25 (27) pathway have been demonstrated. Unlike in higher plants, 24-beta alkyl sterols are dominant in algae, although 24-alpha alkyl sterols are known in some diatoms. Most of the algal divisions can be easily characterized on the basis of their sterol composition For instance, the red algae are unique in containing primarily cholesterol or related C-27 sterols. Blue green algae either contain no sterols or contain only small quantities of sterols. Brown algae contain almost exclusively fucosterol. Green algae are perhaps the most diverse with a very wide range of structural types that suggest phylogenetic relationships. Pyrrophyta contain sterols with an extra side chain methyl group not found in other algae. Chrysophyta appear to be a diverse group with many internal consistencies in sterol composition. As data become more accurate and more plentiful, it is clear that sterols can play an important role in determining the relationships between algae and in their evolutionary relationships with other organisms.