Halogenated monoterpenes from Namibian Plocamium rigidum: Accurate quantification and chemotaxonomic relevance

Abstract

Recently, a Namibian Plocamium specimen that shares the characteristics of both P. suhrii and P. rigidum was described in terms of the chemistry of its secondary metabolite constituents. Three variants or forms of this species with similarities and subtle differences in terms of morphology were found in the intertidal zone of Henties Bay, Namibia. Without DNA analysis, there is a degree of uncertainty regarding which Plocamium species these samples belong to. However, tentative morphological identification of the samples suggests P. rigidum. Hence, detailed chemical profiles of these variants were investigated in order to determine if their chemical compositions are in agreement with their morphological and taxonomic similarities and differences. Twelve halogenated monoterpenes previously identified in Namibian Plocamium species were mostly found to be present in the crude MeOH/DCM extracts of the three variants in this study. The major compound, 1E,3R,4S,5E,7Z-1-bromo-3,4,8-trichloro-7-(dichloromethyl)-3-methylocta-1,5,7-triene, was previously identified as the major metabolite of P. suhrii, the Namibian Plocamium species and also mostly likely P. rigidum. Gas chromatography with flame ionisation detection (GC-FID) was used for the accurate determination of the absolute and relative quantities of the twelve metabolites in the extracts. This data revealed that two of the variants were chemically similar, while the quantities of the metabolites present in the remaining variant showed large variations. In particular, significant variations in the concentration of the major metabolite was observed in the latter variant. However, the relative abundance of this metabolite in the former variants was almost 90%, which is consistent with what was previously reported and hence these two variants most probably belong to the same Namibian Plocamium rigidum species. Hence it was concluded that the remaining sample could be an intraspecies variant, or even another species of Plocamium.