Sterols of Amphidinium species in the Operculatum Clade: Predominance of cholesterol instead of Δ sterols previously considered Amphidinium-specific biomarkers.

The dinoflagellates Amphidinium carterae and Amphidinium corpulentum have been previously characterized as having Δ -nuclear unsaturated 4α-methyl-5α-cholest-8(14)-en-3β-ol (C ) and 4α-methyl-5α-ergosta-8(14),24(28)-dien-3β-ol (amphisterol; C ) as predominant sterols, where they comprise approximately 80% of the total sterol composition. These two sterols have hence been considered as possible major sterol biomarkers for the genus. Here, we have examined the sterols of four recently identified species of Amphidinium (Amphidinium fijiense, Amphidinium magnum, Amphidinium theodori, and Amphidinium tomasii) that are closely related to Amphidinium operculatum as part of what is termed the Operculatum Clade to show that each species has its sterol composition dominated by the common dinoflagellate sterol cholesterol (cholest-5-en-3β-ol; C ), which is found in many other dinoflagellate genera, rather than Δ sterols. While the Δ sterols 4α-methyl-5α-cholest-8(14)-en-3β-ol and 4α,23,24-trimethyl-5α-cholest-8(14),22E-dien-3β-ol (C ) were present as minor sterols along with another common dinoflagellate sterol, 4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol (dinosterol; C ), in some of these four species, amphisterol was not conclusively observed. From a chemotaxonomic perspective, while this does reinforce the genus Amphidinium's ability to produce Δ sterols, albeit here as minor sterols, these results demonstrate that caution should be used when considering Δ sterols, especially amphisterol, as Amphidinium-specific biomarkers within these species where cholesterol is the predominant sterol.