Why cysts of Alexandrium catenella and/or A. pacificum (Gonyaulacales, Dinophyceae) do not remain in sediments as fossils?

The dinoflagellate genus Alexandrium contains a number of species that produce paralytic shellfish toxins and have been the focus of attention as toxic plankton for harmless algal studies. Among Alexandrium species, A. catenella and A. pacificum form ellipsoidal-shaped resting cysts, which are preserved in marine sediments, and have attracted attention as potential seeds for future proliferation after favorable environmental conditions environmental improvement. However, although these cysts are preserved in surface of marine sediments, there is no record of their occurrence from solidified sediments as fossils. In order to clarify the reason for this, we investigated the differences in the chemical composition of cyst walls between colorless cyst of Alexandrium catenella/pacificum and Lingulodinium machaerophorum, Polysphaeridium zoharyi, Spiniferites spp. by measuring the thickness of cyst walls and using Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy. The results showed that the cyst wall of A. catenella/pacificum and L. machaerophorum were all composed of cellulosic organic matter. However, A. catenella/pacificum have the higher percentage of α−/β-glucosidic linkages and that the thickness of their cyst walls is about one-third of that of L. machaerophorum. Therefore, these are reasons for the cysts of A. catenella/pacificum being more easily degraded in the sediment. •Cysts of Alexandrium catenella and/or A. pacificum are ellipsoidal in shape.•Cyst wall of Alexandrium catenella/pacificum is composed of cellulose.•Due to the higher percentage of 1,4-β-glucosidic linkages in the cyst wall, the cysts of A. catenella/pacificum are more easily degraded in the sediment.