Novel polyunsaturated n-alkenes in the marine diatom Rhizosolenia setigera

Four previously unknown n-C-25 and n-C-27 heptaenes of the marine diatom Rhizosolenia setigera were isolated and identified using NMR spectroscopy. They possess six methylene interrupted (Z)-double bonds starting at C-3 and an additional terminal or n-2 Q-double bond. Structural and stable carbon isotopic evidence suggests that these polyenes are biosynthesized by chain elongation of the C22:6n-3 fatty acid, followed by decarboxylation and introduction of double bonds at specific positions.</p

Des-A-lupane in an East African lake sedimentary record as a new proxy for the stable carbon isotopic composition of C₃ plants

We studied the high-resolution and well-dated 25,000 year sedimentary record of Lake Challa, a deeptropical crater lake in equatorial East Africa, to explore new proxies for paleoenvironmental and paleohydrologicalchange. Sedimentary biomarker analysis revealed the presence of des-A-triterpenoids witholeanane, ursane and lupane carbon skeletons, microbial degradation products of angiosperm planttriterpenoids. Their increased influx from 16,000 years ago corresponds with previously documentedchanges in the terrestrial vegetation of the Lake Challa basin during postglacial warming, in particularthe relative increase in C3/C4 plant ratio inferred from the stable carbon isotopic signature (d13C) of sedimentaryn-alkanes derived from plant leaf waxes. In contrast to this n-alkane d13C signature, the d13C ofdes-A-lupane maintains a constant value of ?27.4 ± 1.1‰ across the glacial–interglacial transition. Sincedes-A-lupane is derived from C3 plants, its d13C signature is here proposed to represent a novel and independentproxy for the time-variable carbon isotopic composition of local terrestrial C3 plants, which canimprove estimates of the C3/C4 plant ratio based on two-end member mixing models of n-alkane d13Cvalues.