An example of oxidative polymerization of unsaturated fatty acids as a preservation pathway for dinoflagellate organic matter

A palynologically monotypic assemblage from the Eocene Jatta Gypsum Formation has (Pakistan) been analyzed microscopically and chemically. Scanning electron microscopy shows that the palynomorphs are solid to spongeous dinoflagellate (Division Dinophyta) remains embedded in an amorphous ground mass. Unlike dinoflagellate resting and temporary cysts, a resistant outer wall is absent and the fossils are presently interpreted to be the internal contents (“dinocasts”) of a motile thecate dinoflagellate. Chemical analysis shows that the microfossils are highly aliphatic and consist of polymerized free lipids, likely to have been predominantly C18 and C16 fatty acids with a predominantly C9 mid-chain functionality. We suggest that these “dinocasts” have been formed post-mortem by oxidative polymerization of these lipids, derived from cellular membranes and storage vesicles. Additionally, lipids from outside may have contributed, during or after polymerization of the cell contents. Our hypothesis is supported by chemical analysis of autoxidized vegetable oil and contrasts with the generally accepted opinion that algal remains are preserved via the so-called selective preservation pathway only. In the present case, the unique morphological preservation of the “dinocasts” enabled linking of the post-mortem polymerized free lipids to their microalgal source. However, upon regular, less ideal, morphological preservation conditions such linkage is impossible since “amorphous” particles would result. We propose, therefore, that oxidative polymerization of lipids may be responsible for much more of the amorphous marine organic matter in sediments than presently acknowledged.